European Journal of Taxonomy 12: 1-24

http://dx.doi.org/10.5852/ejt.2012.12

This work is licensed under a Creative Commons Attribution 3.0 License.

ISSN 2118-9773

www. europeanj ournaloftaxonomy. eu

2012 • P. Graham Oliver

Research article

Taxonomy of some Galeommatoidea (Mollusca, Bivalvia)

associated with deep-sea echinoids:

A reassessment of the bivalve genera Axinodon Verrill & Bush, 1898

and Kelliola Dali, 1899 with descriptions of new genera

Syssitomya gen. nov. and Ptilomyax gen. nov.

P. Graham OLIVER

Dept, of Biodiversity & Systematic Biology, National Museum of Wales,

Cathays Park, Cardiff, Wales, UK.

E-mail: [email protected]

Abstract. The type species of Axinodon ellipticus Verrill & Bush, 1898 and Kellia symmetros Jeffreys,

1876 are re-described. It is concluded that the two species are not conspecific and that K. symmetros

cannot be placed in the genus Axinodon. The family affinity of Axinodon is not resolved, although it

is probable that this genus belongs to the Thyasiridae. Kellia symmetros is the type species of Kelliola

and is placed in the Montacutidae. Kelliola symmetros is most probably associated with the echinoid

Aeropsis rostrata and is not the species previously recorded from North Atlantic Pourtalesia echinoids

under the name of Axinodon symmetros. This commensal associated with the North Atlantic Pourtalesia

is here described as new and placed in the new genus as Syssitomyapourtalesiana gen. nov. sp. nov.,

Syssitomya gen. nov. differs from all other genera in the Montacutidae by having laminar gill filaments

modified for harbouring symbiotic bacteria and it is thus assumed to be chemosymbiotic. A montacutid

associated with the hadal Pourtalesia heptneri is described as Ptilomyax hadalis gen. nov. sp. nov.

Key words. Galeommatoidea, deep-sea, echinoids, commensal, new genera.

Oliver P.G. 2012. Taxonomy of some Galeommatoidea (Mollusca, Bivalvia) associated with deep-sea echinoids:

A reassessment of the bivalve genera Axinodon Verrill & Bush, 1898 and Kelliola Dali, 1899 with descriptions of

new genera Syssitomya gen. nov. and Ptilomyax gen. nov. European Journal of Taxonomy 12: 1-24. http://dx.doi.

org/10.5852/eit.2012.12 .

Introduction

The bivalve currently known as Axinodon symmetros (Jeffreys, 1876), a commensal associated with the

deep-sea echinoid Pourtalesia A. Agassiz, 1869, has been found to have a gill structure similar to that of

the Thyasiridae Dali, 1900, and to harbour symbiotic bacteria suggesting that it may be chemosymbiotic

(Oliver, Southward & Dando in press). In that study, some key taxonomic issues arose that now require

resolution before publication of that study can proceed.

European Journal of Taxonomy 12: 1-24 (2012)

Firstly, there has been debate about the family placement of Axinodon Verrill & Bush, 1898. Originally,

it was placed in the Thyasiridae (Verrill & Bush 1898) and this was followed by Aartsen (1996) and

Coan et al. (2000). Ockelmann (1965) placed it in the superfamily Galeommatoidea Gray, 1840 and

family Montacutidae Clark, 1855; this placement was maintained by Chavan (1969) and is followed

in the current European checklist (CLEMAM 2012). The implications of this debate are twofold:

• If Axinodon were a thyasirid then the chemosymbiosis might be expected, but the commensalism

would be a first for that family.

• If Axinodon were a montacutid then Axinodon would be the first of that family and the first

commensal bivalve shown to harbour symbiotic bacteria.

Secondly, it was also discovered that commensal bivalves were present on the echinoid Aeropsis

rostrata (Wyville Thomson, 1877), but although similar to that on Pourtalesia , they were not identical.

Eve Southward (pers. comm.) noted that the type locality and sample number for Aeropsis rostrata

is the same as that for Kellia symmetros Jeffreys, 1876, both from “Station 9” of the “Valorous

Expedition” in the Davis Straits. This brought into question the identity of K. symmetros and the

identity of the Pourtalesia commensal. This is further exacerbated by the conflicting descriptions of

Axinodon , some stating that it is edentulous (Aartsen 1996) other showing a distinct tooth in the right

valve (Bouchet & Waren 1979).

Bivalves belonging to the Galeommatoidea and the Montacutidae in particular have long been known

to be commensal with species of sea-urchins and some shallow water species have been studied in

considerable detail. There are numerous family and subfamily taxa proposed within the Galeommatoidea

(Bieler & Mikkelsen 2006) but their distinction and relationships remain controversial (Mikkelsen &

Bieler 2007) although the latter authors do recognise the Galeommatidae Gray, 1840 as distinct form

all others. Mikkelsen & Bieler (2007) and Carter et al. (2011) refer all non-galeommatids to the

Lasaeidae Gray, 1842, that being the earliest family name. Many other authors continue to follow the

classification of Chavan (1969) and adopt Montacutidae Clark, 1855 for those taxa morphologically

similar to Montacuta Turton, 1822 (Jespersen et al. 2004; Kamenev 2008; Gofas & Salas 2008).

Jespersen et al. (2004) argue for recognition of Montacutidae on grounds of hinge and anatomical

characters in that the Lasaeidae, Leptonidae and Kelliidae have both cardinal and lateral teeth, and

the presence of dimorphic sperm is peculiar to species placed in the Montacutidae. Unfortunately,

galeommatoids are poorly represented in molecular phylogenies (Taylor et al. 2007) and there is no

resolution at the family level. Here, Montacutidae is preferred over Lasaeidae, as the species under

consideration are morphologically most similar to Montacuta and other associated genera that have

been grouped in the Montacutidae.

Popham (1940) and Oldfield (1961) studied the morphology of Montacuta sub striata (Montagu,

1808) [associated with Spatangus purpureus (Muller, 1776)] and Tellimya ferruginosa (Montagu,

1808) [associated with Echinocardium cordatum (Pennant, 1777)]. Gage (1966 a, b, c) studied the

behaviour of these species. Ockelmann (1965) studied Montacuta ( Decipula ) tenella (Loven, 1846)

[now Tellimya tenella (Loven, 1846)] associated with Brissopsis lyrifera (Forbes, 1841). From South¬

east Asia, Jespersen, Liitzen & Nielsen (2004) studied the fine anatomy of three species all associated

with Brissus latecarinatus (Leske, 1778) and created two new genera Montacutella Jespersen, Liitzen

& Nielsen, 2004 and Brachiomya Jespersen, Liitzen & Nielsen, 2004. Table 1 gives a compilation of

galeommatids known to be co mm ensal with echinoids.

Associations with deep-sea echinoids are known, but taxonomic recognition has been restricted to a

single taxon associated with North Atlantic species of Pourtalesia. Currently this taxon is known as

OLIVER P.G., Galeommatid bivalves from deep-sea echinoids

Table 1 . Galeommatoid species attached to T or associated with w echinoids

Galeommatoid

Associated echinoid

Reference to

association

Distribution

Depth range

w TeUimya ferruginosa

(Montagu 1808).

Echinocardium cordatum

(Pennant, 1777)

Gage 1966a;

Oldfield 1961

NE Atlantic

Intertidal

Shelf

w TeUimya tenella

(Loven, 1846)

Brissopsis lyrifera

(Forbes, 1841)

Ockelmann 1965

NE Atlantic

Shelf

T Montacuta substriata

(Montagu, 1808)

Spatangus purpureus

(Muller, 1776)

Gage 1966a;

Oldfield 1961

NE Atlantic

Shelf

J Scintillona brissae

Morton & Scott, 1989

Brissus latecarinatus

(Leske, 1778)

Jespersen et al.

2004

SE Asia

Sublittoral

1 Montacutella echinophila

Jespersen et al., 2004

Brissus latecarinatus

Jespersen et al.

2004

SE Asia

Sublittoral

[ Brachiomya stigmata

(Pilsbry, 1920)

Brissus latecarinatus

Jespersen et al.

2004

SE Asia

Sublittoral

T TeUimya vitrea

(Hedley, 1907)

Brissusgigas (Fell, 1947)

Ponder 1968

S. Australia

Shallow shelf

T TeUimya vitrea aupouria

(Ponder, 1968)

Brissus gigas

Ponder 1968

New

Zealand

Shallow shelf

T Montacuta

echinocardiophila

(Habe, 1964)

Echinocardium

cordatum ; Lovenia

e/ongata (Gray, 1845)

Habe 1964:

Jespersen et al.

2004

Japan

Shallow shelf

Montacuta divaricata

Gould, 1861

Schizaster lacunosus

(Linnaeus, 1758)

Jespersen et al.

2004

Japan

Shallow shelf

1 Montacuta semiradiata

Tate, 1889

Echinocardium sp;

Spatangus sp.

Ponder 1968; Barel

& Kramers 1977

S. Australia

Shallow shelf

T Montacuta semiradiata

neozelanica (Dell, 1956)

Cycluster sp.

Dell 1963

New

Zealand

340-620 m

w Neaeromya compressa

(Dali, 1899)

? Brissaster latifrons

(Agassiz, 1898)

Coan et al. 2000

NE Pacific

10-700 m

T Kelliola symmetros

(Jeffreys, 1876)

Aeropsis rostrata

(Wyville Thomson, 1877)

This paper

N. Atlantic

3000 m

Syssitomya pourtalesiana

sp. nov.

Pourtalesia jeffreysi &

P. miranda.

Bouchet & Waren

1979

N. Atlantic

800-3000 m

l Ptilomyax hadalis

sp. nov.

Pourtalesia heptneri

Mironov, 1978

Mironov 1978

Banda

Trench

7340-7335 m

Unidentified

Sternospatangus sibogae

(de Meijere, 1904)

Mironov pers.

comm.

Deep-sea

Unidentified

Carnarechinus clypeatus

(Agassiz, 1879)

Mironov 1993

S. Atlantic

Deep-sea

Axinodon symmetros (Jeffreys, 1876) and is associated with Pourtalesia jeffreysi Wyville Thomson,

1877 and P. miranda Agassiz, 1869 (Bouchet & Waren 1979; Gage et al. 1985). The synonymy of

Kellia symmetros and Axinodon ellipticus Verrill & Bush, 1898 was first made by Ockelmann in Chavan

(1969). Dali (1899) noting discrepancies in Jeffreys original description of K. symmetros placed it in a

new genus Kelliola Dali, 1899; this was synonymised with Axinodon by Chavan (1969).

In the original descriptions of “ KellicT symmetros (Jeffreys 1876) and Axinodon ellipticus (Verrill

& Bush 1898) there is no mention of an association with echinoids. The first mention of an association

is by Ockelmann & Muus (1978) and is restated by Bouchet & Waren (1979), both giving Pourtalesia

as the host.

European Journal of Taxonomy 12: 1-24 (2012)

In 1978 Mironov described a hadal echinoid, Pourtalesia heptneri Mironov, 1978 from the Banda Trench

at a depth between 7340 and 7335 m. He noted that there were montacutid bivalves attached to the

spines and figured them (Mironov 1978: plate 1) but they have never been described. Alexandr Mironov

(pers. comm.) has also found bivalves attached to Sternospatangus sibogae (de Meijere, 1904) and

most interestingly internal commensals in intestines close to the peristome of Carnarechinus clypeatus

(Agassiz, 1879); neither are available for study at this time.

Resolution of the issues raised in this introduction requires examination of type materials but, although

available, are restricted to holotypes consisting of shells only. For this paper, the holotypes of the

species Kellia symmetros and Axinodon ellipticus are re-described and their supposed synonymy and

family affinities are analysed. This is followed by descriptions of the commensals from Pourtalesia and

Aeropsis Mortensen, 1907 and their relationship to previously described species is reviewed. Finally, the

specimens figured by Mironov (1978) are formally described.

Material and Methods

The materials used in this study are all from museum collections derived from late 19 th century

oceanographic explorations and from recent research studies by British and Norwegian deep water

programmes. The modern collections were made by grab or sledge hauls and the bivalves were found

attached to the spines of sea urchins. Details of each collection site are given for each specimen examined.

Specimens are of dry shells or whole animals previously fixed in formaldehyde and preserved in 70%

ethanol.

Stereo microscope observations were made with Teica M8 or M10 instruments and macrophotography

with the latter, attached to a digital camera system, employing enhancement via AutoMontage™

software. Scanning electron micrographs of type material held by the United States National Museum

were supplied by that institution and all others were made in the National Museum of Wales using a

Jeol Neoscope. Soft tissues were prepared by critical point drying and subsequent gold coating for SEM

examination.

Abbreviations used in figures

aa =

ad =

ct =

ga/me =

lig

mef =

mf =

pa =

rv =

anterior adductor muscle

anterior depression

byssus

cardinal peg

ctenidium

foot

mantle edge/gill axis junction

ligament

left valve

anterior mantle edge folds

marginal flange

posterior adductor muscle

right valve

OLIVER P.G., Galeommatid bivalves from deep-sea echinoids

Results

Phylum Mollusca Linnaeus, 1758

Class Bivalvia Linnaeus, 1758

Superfamily “uncertain”

Pamily “uncertain”

Genus Axinodon Verrill & Bush, 1898

Type species

Axinodon ellipticus Verrill & Bush, 1898.

Diagnosis (based on shell characters alone)

Shell small, thin. Equivalve. Inequilateral, beaks behind the midline. Umbos prominent, beaks

prosogyrate. Outline obliquely subcircular, distinctly expanded anteriorly. Sculpture of commarginal

ridges. Hinge plate narrow, ligament deeply sunken on a groove running from under the beaks posteriorly.

Right valve lacking any projecting teeth; left valve with a short, weak, marginal flange beneath the

lunule. Isomyarian. Pallial line entire.

Axinodon ellipticus Verrill & Bush, 1898

Axinodon ellipticus Verrill & Bush, 1898: 796, pi. XC figs 5, 6; pi. XCII, fig. 1.

Axinodon symmetros - Aartsen 1996: 30, fig. 5.

Material examined

Holotype

1 shell, North Atlantic, off New Jersey, United States Pish Commission Albatross, stn 2096, 39°22'20"N

70°52'20"W, 1451 fathoms (2864 m). United States National Museum- USNM35175.

Redescription of holotype (Fig. 1)

Shell small, length 3.5 mm, height 3.3 mm T hin , fragile. Equivalve. Inequilateral, beaks behind the

midline. Umbos prominent, beaks prosogyrate. Outline obliquely subcircular, distinctly expanded

anteriorly; anterior dorsal margin short, as a poorly defined lunule; posterior dorsal margin indistinct

sloping into broadly rounded posterior, anterior broadly rounded more so than posterior, ventral margin

broadly rounded no distinct junctions with lateral margins. Sculpture of dense, fine commarginal

ridges most obvious on margins. Prodissoconch II distinct, 754 pm across, with weak commarginal

lines (Fig. 1H). Hinge plate narrow, ligament deeply sunken on a groove running from under the beaks

posteriorly for about one-third of the posterior dorsal slope. Right valve lacking any projecting teeth,

hinge plate slightly and irregularly thickened below lunule. Left valve with a short, weak, marginal

flange beneath the lunule; a very weak protuberance is visible below the beak. Adductor scars oval,

roughly of equal size; pallial line entire. Interior with feeble radial striae.

Differential diagnosis

At the generic level Chavan (1969) lists Kelliola Dali, 1899 as congeneric with Axinodon and assigns

them to the Galeommatoidea in the family Montacutidae. This is despite Verrill & Bush (1898) describing

European Journal of Taxonomy 12: 1-24 (2012)

Axinodon as edentulous and Dali (1899) describing Kelliola with teeth. The hinge of A. ellipticus has

been examined carefully to ascertain if it is edentulous as stated by Aartsen (1996) or if a cardinal peg

was present but has been broken off The SEM images presented here (Fig. 1A) show no indication of

a broken tooth confirming the edentulous condition. Comparisons with figures in Chavan (1969) should

not be made as these incorrectly show a shell that is expanded posteriorly and teeth that are not shown

in the accurate figures made by Verrill & Bush (1898). From the description below, Kelliola is seen to

Fig. 1. Holotype of Axinodon symmetros Verrill & Bush, 1898, USNM 35175. A-B. SEM of hinges of

right and left valves. C-D. SEM of internal of right and left valves. E. Photo micrograph of internal of

right valve. F-G. SEM of external of right and left valves. H. SEM of prodissoconch.

OLIVER P.G., Galeommatid bivalves from deep-sea echinoids

have a cardinal peg in the right valve (Fig. 2A) and therefore Axinodon and Kelliola are not congeneric.

In outline, Axinodon looks like Kelliopsis Verrill & Bush, 1898 but the latter has small, distinct teeth

(Verrill & Bush 1898). Verrill & Bush (1898) placed Axinodon in the Thyasiridae as did Aartsen (1996)

and Coan et al. (2000), and there are similarities with the thyasirid genus Mendicula Iredale, 1924.

Without anatomical data, it is not possible to confirm the affinity with the Thyasiridae but it is, however,

very doubtful that Axinodon is a galeommatid as the latter all display some degree of dentition in the

right valve.

Superfamily Galeommatoidea Gray, 1840

Family Montacutidae Clark, 1855

Genus Kelliola Dali, 1899

Type species

Kellia symmetros Jeffreys, 1876 (OD).

Diagnosis

Small to minute shells, Equivalve. Slightly inequilateral, beaks behind the midline. Outline subovate,

anterior a little more expanded than posterior. Hinge with an anterior cardinal peg in the right valve,

an anterior marginal flange in the left valve, posterior teeth lacking; ligament internal on a recessed

resilifer beneath and posterior of the beaks. Sculpture weak primarily of commarginal lines, faint radial

striations may be present along with microscopic notches in the ventral margins. Adductor muscles of

approximately equal size; mantle edge with a large pedal gape, fused from mid point posteriorly with a

small exhalant aperture; foot with an active byssus. Ctenidia of single, un-reflected, demibranchs.

Kelliola symmetros (Jeffreys, 1876)

Kellia symmetros Jeffreys, 1876: 491.

Kellia (Kelliola) symmetros - Dali 1899: 890.

? Kellia symmetros -Fuels & Grieg 1901: 29.

Axinodon symmetros - Waren 1980: 47.

Not Kellia symmetros - Focard 1898: 297, pi.XIII, figs 18-20.

Not Axinodon symmetros - Bouchet & Waren 1979: 216-217, figs 3A-D. —Aartsen 1996: 30, fig. 5 (is

Axinodon ellipticus Verrill & Bush, 1898).

Not Montacuta (Axinodon) symmetros - Gage, Billett, Jensen & Tyler 1985: 189.

Not Axinodon sp.l - Olabarria 2005: 20 (is Mysella sp.)

Material examined

Holotype

1 shell, North Atlantic, SW of Godthaab, Davis Strait, Valorous stn 9, 59°10'N 50°25'W, 1750 fathoms

(3202 m), United States National Museum- USNM170626.

Other material

2 specimens, attached to Aeropsis rostrata , Bay of Biscay, Shackleton cruise 1977/5, stn D7, 47°29.7'N

09°33.3'W, 4250-4265 m, 30 Apr. 1977, leg. E. Southward, National Museum Wales, Zoology- NMW.Z.

2012.015.1; 4 specimens, attached to Aeropsis rostrata , Bay of Biscay, Shackleton cruise 1977/5, stn

European Journal of Taxonomy 12: 1-24 (2012)

Fig. 2. Holotype of Kellia symmetros Jeffreys, 1876, USNM 170626. A-B. SEM of hinges of right and

left valves. C-D. SEM of internal of right and left valves. E-F SEM of external of right and left valves.

G-H. photo micrographs of internal and external of right valve.

OLIVER P.G., Galeommatid bivalves from deep-sea echinoids

D2, 47°35.52'N 09°44.07'W, 4120-4165 m, 29 Apr. 1977, leg. E. Southward, National Museum Wales,

Zoology- NMW.Z. 2012.015.2.

Type locality

North Atlantic, Valorous St. 9, SW of Godthaab, Davis Strait, 59°10’N 50°25’W, 1750 fathoms (3202 m).

Redescription of the holotype (Fig. 2)

Shell minute, length 1.0 mm, height 0.77 mm. Thin, translucent. Equivalve. Weakly inequilateral, beaks

just behind the midline. Umbos prominent, beaks orthogyrate. Outline subovate, slightly extended

anteriorly; anterior dorsal margin sloping into broadly rounded anterior margin; posterior dorsal margin

indistinct sloping into broadly rounded posterior margin, this slightly less expanded than anterior margin;

ventral margin weakly curved. Sculpture weak, of indistinct commarginal lines most obvious on lateral

margins. Prodissoconch II distinct, 373 pm across, with weak commarginal lines. Hinge plate weak.

Ligament short, internal, attached to a shallow resilifer situated beneath and posterior of the beaks. Right

valve with a single, projecting, cardinal peg, immediately anterior to this tooth a slight depression. Left

valve with a short, weak, marginal flange in a posterior lateral position. Adductor scars oval, roughly of

equal size; pallial line entire.

Description (based on material from Bay of Biscay)

Shell. (Fig. 3) Minute, largest of length 1.3 mm, height 1.0 mm. Thin, fragile. Equivalve. Inequilateral,

beaks behind midline. Umbos wealdy inflated, beaks orthogyrate. Outline subovate, longer than high,

length to height ratio 1.3:1, slightly extended anteriorly; anterior dorsal margin sloping, rather straight

merging smoothly with rounded anterior; posterior dorsal margin shorter and sloping more steeply

than anterior, merging smoothly with rounded posterior margin; anterior slightly more expanded than

posterior; ventral margin gently curved. Sculpture weak almost smooth, of fine commarginal lines; radial

lines apparent under transmitted light (Fig. 3A) but these very faintly raised (Fig. 3J). Prodissoconch

II distinct, 380 pm in diameter sculptured with commarginal lines (Fig. 31); Prodissoconch I weakly

demarcated, 140 pm in diameter with a punctate micro-sculpture (Fig. 31). Ligament short, internal,

attached to a shallow resilifer situated beneath and posterior of the beaks. Right valve with a single,

projecting, cardinal peg, immediately anterior to this tooth a slight depression (Fig. 3D). Left valve with

a short, weak, marginal flange in a posterior lateral position (Fig. 3E). Adductor scars oval, roughly of

equal size; pallial line entire. Ventral margin dissected by minute transverse grooves (Fig. 3K).

Anatomy. (Fig. 4A) Mantle margin free for most of its length, joined and attached to terminal of gill axis,

anterior (pedal) aperture extensive, posterior aperture very small. Adductor muscles of approximately

equal size. Foot with a large toe and small heel, byssus functional producing a mass of threads arising

from a single stalk. Anterior pedal retractor inserted above the anterior adductor, posterior pedal retractor

above the posterior adductor. Ctenidium of a single demibranch, with nine non-reflected filaments in the

largest specimen. Filaments rod shaped, lacking abfrontal extension or harbouring symbiotic bacteria.

Labial palps small but projecting.

Association. Attached by byssus threads to the spines of the echinoid Aeropsis rostrata (Fig. 5).

Distribution

Kelliola symmetros is known only from the type locality and from the Bay of Biscay, at abyssal depths.

The host ec hin oid is widely distributed in the North Atlantic (Echinoid Directory 2012).

European Journal of Taxonomy 12: 1-24 (2012)

Fig. 3. Kelliola symmetros (Jeffreys, 1876) from Biscay. A-C. Photo micrographs a external of left

valve, internals of both valves. D-E. SEM of hinges of right and left valves. F-H. SEM of internal of

both valves and external of left valve. I. SEM of prodissoconch. J. SEM of anterior area showing weak

radial sculpture. K. SEM of margin showing transverse grooves.

OLIVER P.G., Galeommatid bivalves from deep-sea echinoids

Differential diagnosis

The hinges of K. symmetros and the Aeropsis commensal are almost identical, but K. symmetros has

slightly more prominent umbos and lacks the marginal transverse grooves. Given that K. symmetros

was taken in the same sample along with Aeropsis , but not attached to it, it is possible that the two are

associated. This suggests an ecological affinity with the Aeropsis commensal described here. Despite

the wide geographical separation of the samples considered here, Aeropsis rostrata is regarded as pan

Atlantic and having an abyssal bathymetric range (WoRMS 2012). With so few specimens at hand and

the poor condition of the holotype of K. symmetros , we have chosen to be conservative and regard the

Aeropsis commensal from Biscay conspecific with Kelliola symmetros.

Fig. 4. Scanning electron micrographs of anatomy. A. K. symmetros (Jeffreys, 1876) from Biscay. B.

Syssitomyapourtalesiana sp. nov. from Norwegian Sea. C. Montacuta substriata (Montagu, 1808) from

North Sea. D. Excised ctenidium ofM substriata. E. Excised piece of ctenidium from S. pourtalesiana

sp. nov.

European Journal of Taxonomy 12: 1-24 (2012)

Consequently, at the family level the hinge and anatomical characters of Kelliola are entirely in keeping

with the Montacutidae, consisting of a single cardinal peg in the right valve, a marginal flange in the left

valve and an internal ligament. The ligament is attached to an elongate shallow depression extending

below the beaks and is therefore most similar to Montacuta sensu stricto (Fig. 6). Montacuta sub striata,

Fig. 5. Aeropsis rostrata (Wyville Thomson, 1877) with Kelliola symmetros (Jeffreys, 1876) attached,

from Biscay.

Fig. 6. Montacuta substriata (Montagu, 1808), NMW.Z 2000.101.73. Gulfaks Oil Field, North Sea,

217 m. A-B. SEM of hinges of right and left valves. C. SEM of internal of right valve. D. SEM of

external of left valve.

OLIVER P.G., Galeommatid bivalves from deep-sea echinoids

the type species of Montacuta, has a longer cardinal tooth, has radial ridges and ovate in outline (Fig. 6).

Anatomically Kelliola is similar to many montacutids, where the ctenidium is reduced to a single

demibranch. However, in this genus, the filaments are very few and not reflected (Fig. 4A). This

condition could be due to the small size of the specimens.

Dali (1899) noted that Jeffreys’ (1876) description of the hinge of K. symmetros was incorrect and,

in re-describing it, created the new genus Kelliola for it. Dali did not make comparisons with other

genera but noted that it was similar to Aligena Fea, 1846. This is difficult to understand as Aligena

species have a cardinal peg in each valve (Harry 1969). Comparisons with other montacutid genera are

currently complex due to a lack of compatibility in descriptions of characters and the widely varying

use of generic names. Such difficulties were amply recognised by Gofas & Salas (2008) in their

review of My sella Angas, 1877 and consequent creation of the genus Kurtiella Gofas & Salas, 2008.

For the purposes of this paper, comparisons are restricted to genera that have a single cardinal peg

in the right valve only and left valve with varying degrees of pseudocardinal development. In hinge

characters, Kelliola is most similar to Montacuta sensu stricto (Fig. 6) and Neaeromya Gabb, 1873

(Coan et al. 2000) in that the development of the posterior teeth is limited to a marginal flange in the

left valve and the ligament is attached to a shallow depression beneath and posterior to the beaks. For

N. rngifera (Carpenter, 1864) Narchi (1969) states that there is a tooth in each valve, although it may

be reduced in the left valve (Paul Valentich-Scott pers comm); a further example of the contradictory

descriptions found for the montacutid species. In Tellimya T. Brown, 1827 the resilifer is developed

and the hinge plate thickened accordingly (see Ockelmann 1965, Fig. 2). In Montacutella the left

valve flange is developed as a small projection (Jespersen et al. 2004) and approaches the condition

seen in Aligena. The shell of Brachiomya is like that of Tellimya (Jespersen et al. 2004).

Kelliola is as different from Montacuta sensu stricto as are the other genera and a molecular study is

required to evaluate the significance of the morphological characters. Kelliola is retained here until

such a study is undertaken.

Species level comparisons are restricted to the few abyssal galeommatid species that have been

described and none other than that described below under Syssitomya pourtalesiana sp. nov. have

been found attached to echinoids. The shell of S. pourtalesiana sp. nov. is more expanded anteriorly,

has a more depressed lunule and lacks marginal notches. The ctenidium is highly modified with

laminar filaments whereas that of K. symmetros is not modified in this ma nn er Other described,

Atlantic, deep-sea, galeommatids have been assigned to the genera Mysella (now Kurtiella ) (Gofas

& Salas 2008), Epilepton Dali, 1899 (Allen 2007) or Draculamya Oliver & Fiitzen, 2011 (Oliver &

Ftitzen 2011) none having a dentition identical to Kelliola or Montacuta. Among ten undescribed

galeommatoids from the deep Atlantic, Allen (2008) lists two undescribed Montacuta species that

may or may not be similar to K. symmetros.

Genus Syssitomya gen. nov.

Type species

Syssitomya pourtalesiana sp. nov. (here designated).

Diagnosis

Shell small, thin. Equivalve. Inflated. Inequilateral, beaks behind the midline. Outline roundly subovate,

distinctly expanded anteriorly, lunule depression distinct. Hinge with an anterior cardinal peg in the

European Journal of Taxonomy 12: 1-24 (2012)

right valve, an anterior marginal flange in the left valve, posterior teeth lacking; ligament internal on a

recessed resilifer beneath and posterior of the beaks. Sculpture weak primarily of commarginal lines.

Ctenidium of single partly reflected demibranchs; filaments laminar (Fig. 4E), extended abfrontally,

abfrontal surfaces lined with bacteriocyte cells densely packed with symbiotic bacteria. A detailed

scanning electron and transmission electron microscopy study of the ctenidium is in preparation and is

beyond inclusion in this taxonomic paper (Oliver, Southward & Dando in press).

Etymology

From the Greek, syssitos a messmate and mya a clam, referring to the commensal habit and bacterial

symbiosis.

Syssitomyapourtalesiana sp. nov.

Figs 4B, E; 7, 8A-B

? Kellia symmetros - Locard 1898: 297, pi.XIII, figs 18-20.

? Kellia symmetros - Friele & Grieg 1901: 29.

Axinodon symmetros - Bouchet & Waren 1979: 216-217, fig. 3A-D.

Montacuta (Axinodon) symmetros — Gage et al. 1985: 189.

Not Axinodon symmetros - Waren 1980: 47.

Not Axinodon symmetros - Aartsen 1996: 30, fig. 5 (is Axinodon ellipticus Verrill & Bush, 1898).

Not Axinodon sp.l - Olabarria 2005: 20 (is Mysella sp.).

Etymology

After Pourtalesia, the host echinoid; and the Latin termination -iana , to denote belonging with.

Material examined

Holotype

1 specimen, Norwegian Sea, Ormen Lange gas field, off Sor-Trondelag, Central Norway, 63°47'N

03°35'E, 815-925 m, Swedish Museum of Natural History SMNH5566.

Paratypes

Same recolt data as for the holotype: 7 specimens, Swedish Museum of Natural History SMNH5567; 1

specimen + 2 shells, National Museum Wales, Zoology- NMW. Z.2012.014.

Other material

8 specimens, Rockall Trough, RRS Challenger , stn ES137, 54°40'N 12°19'W, 2900 m, 22 Feb. 1978,

leg. I.J. Killeen NMW.Z. 2009.045.2; 6 specimens, Rockall Trough, RRS Challenger, stn ES231,

54°42'N 12°12'W, 2898 m, 17 May 1983, leg. I.J. Killeen, National Museum wales, Zoology- NMW.Z.

2009.045.3.

Type locality

Norwegian Sea, Ormen Lange gas field, off Sor-Trondelag, Central Norway, 63°47’N 03°35’E, 815-

925 m, 2009. Swedish Museum of Natural History, det. Anders Waren.

OLIVER P.G., Galeommatid bivalves from deep-sea echinoids

Fig. 7. Syssitomya pourtalesiana sp. nov. from Norwegian Sea. A-C. Photo micrographs a external of

left valve, internals of both valves. D-E. SEM of hinges of right and left valves. F-G. SEM of internal of

both valves. H. SEM of prodissoconch. I. SEM of internal of right valve from NORBI cruise, abyssal.

Courtesy of Anders Waren.

European Journal of Taxonomy 12: 1-24 (2012)

Measurements (SMNH5566 and part of SMNH5567 paratypes)

Length (mm)

Height (mm)

L:H

Holotype

4.1

3.2

1.3:1

Paratype

3.0

2.3

1.3:1

Paratype

2.4

2.0

1.2:1

Paratype

1.3:1

Paratype

1.2:1

Paratype

1.2:1

Description based on specimens from the Norwegian Sea

Shell. (Figs 7, 8) Small, largest length 4.2 mm, height 3.2 mm, breadth 2.7 mm. Thin, fragile, translucent.

Equivalve. Inequilateral, beaks distinctly behind midline. Umbos moderately inflated, beaks orthogyrate

or marginally prosogyrate. Outline subovate, longer than high, length to height ratio 1.3:1, distinctly

expanded anteriorly becoming a little oblique; anterior dorsal margin short as a depressed but ill-

defined lunule(Fig. 8A 1 ); anterior broadly rounded; posterior dorsal margin sloping merging smoothly

with rounded posterior margin; anterior distinctly more rounded than posterior; ventral margin gently

Fig. 8. Computer enhanced photomicrographs of whole specimens of Syssitomya pourtalesiana sp. nov.

from the Norwegian Sea. A-A 2 . Holotype A. Exterior from right side. A 1 . Dorsal. A 2 . Ventral. B.

Paratypes, size series.

OLIVER P.G., Galeommatid bivalves from deep-sea echinoids

curved. Sculpture of dense, fine commarginal ridges. Prodissoconch II distinct, 390-410 pm in diameter,

sculptured with commarginal lines (Fig. 7H); Prodissoconch I wealdy demarcated, 148-150 pm in

diameter with a punctate micro-sculpture (Fig. 7H). Figament short, internal, attached to a shallow

resilifer situated beneath and posterior of the beaks (Fig. 7E). Right valve with a single, projecting,

cardinal peg, immediately anterior to this tooth a prominent depression (Fig. 7D). Feft valve with a

short, marginal flange in a posterior lateral position (Fig. 7E). Adductor scars oval, roughly of equal size;

pallial line entire. Margin entire.

Anatomy. (Figs 4B, E; 8A 2 ) Mantle margin free for most of its length, joined and attached to terminal

of gill axis, anterior (pedal) aperture extensive, posterior aperture very small (Fig. 8 A 2 ). Anterior mantle

edge thrown into folds (Fig. 4B). Adductor muscles of approximately equal size. Foot with a large toe

and small heel, byssus functional producing a mass of threads arising from a single stalk. Anterior pedal

retractor inserted above the anterior adductor, posterior pedal retractor above the posterior adductor.

Ctenidium of single demibranchs, each with up to 30 partly reflected filaments (Fig. 4B). Filaments

laminar, extended abfrontally with an extensive bacteriocyte zone (Fig. 4E). Fabial palps small but

projecting.

Specimens from abyssal depths in the Rockall Trough do not differ significantly from those from the

Norwegian Sea except for a large shell that shows greater anterior expansion. The ctenidia of this

specimen show the same laminar filaments with abfrontal extension. Specimens collected in the month

of February were gravid, the suprabranchial chamber holding hundreds of sub-triangular larvae, on

average 112 pm in diameter.

There is no record of any association with an echinoid but these specimens originate from the same

sampling programme reported upon by Gage et al. (1985) where they were attached to Pourtalesia

miranda.

Association. Attached by byssus threads to the spines of Pourtalesia jeffreysi and P. miranda (Fig. 9).

Distribution

Confirmed from the NE Atlantic, Norwegian Sea to Bay of Biscay at depths from 800-3617 m. Records

(Allen 2008) from the NW and SW Atlantic have not been confirmed although one of the host species P.

miranda has been recorded in these areas.

Pourtalesia jeffreysi has two recognised subspecies with both hosting Syssitomya gen. nov.: Pourtalesia

jeffreysi gibbosa Mironov, 1995 has a bathyal range while the subspecies lata Mironov, 1995 is abyssal.

Fig. 9. Pourtalesia miranda Agassiz, 1869 with Syssitomya pourtalesiana sp. nov. attached, from

Biscay. Courtesy of A.J. & E.C. Southward.

European Journal of Taxonomy 12: 1-24 (2012)

The known geographic range for P. jeffreysi is the Norwegian Sea and Russian Arctic Ocean (WoRMS

2012 ).

Differential diagnosis

The shell characters of Syssitomya gen. nov. are montacutid in all respects, notably the anterior expansion,

internal ligament and the hinge reduced to a single cardinal peg in the right valve and a marginal flange

in the left valve. In shell character, similarities are greatest with Kelliola but Syssitomya gen. nov.,

uniquely within the Galeommatoidea, has highly modified ctenidia with abfrontally extended, laminar

filaments.

At the species level, for comparison with Kelliola symmetros see above. The somewhat inflated,

anteriorly expanded form of S. pourtalesiana sp. nov. is rather distinctive and not like the form of other

deep-sea montacutids such as species of Kurtiella (Gofas & Salas 2008) or Epilepton (Allen 2007) that

are more ovate, compressed and have different dentition patterns. From external appearances it more

resembles some thyasirids notably Thyasira subovata (Jeffreys, 1881) (see Oliver et al. 2012) and if not

found attached to its host could easily be mistaken for a thyasirid.

Genus Ptilomyax gen. nov.

Type species

Ptilomyax hadalis sp. nov. (here designated).

Diagnosis

Very small, juvenile shell equivalve, subovate, slightly expanded anteriorly. Adult shell inequivalve

dorsal margin of left valve elevated and rolled over, lateral dorsal margins extended, alate. Ligament

small, posterior, deeply sunken; dorsal margin with thickened periostracal “ligament”. Hinge plate

not examined. Sculpture almost smooth with faint commarginal lines. Ctenidium of single reflected

demibranchs; byssus of multiple strands; pedal aperture large, exhalant aperture small.

Etymology

Ptilomyax , from the Greek, ptilo — winged and my ax — a small clam (gender masculine).

Ptilomyax hadalis sp. nov.

Montacutid Mironov, 1978: 722, pi. 1.

Etymology

hadalis — referring to the hadal zone of the deep ocean (deeper than 6000 m).

Material examined

Type series of 18 specimens Banda Trench, RV Vityaz cruise 57, stn 7271, 5°37'S 131°07.5'E, 7340-

7335 m, 21-22 Mar. 1975. Leg. A.N. Mironov.

OLIVER P.G., Galeommatid bivalves from deep-sea echinoids

Fig. 10. Ptilomyax hadalis sp. nov. A. Holotype attached to echinoid spines. B. Paratype, left side.

C. Juvenile paratype lacking “wings”. D. Anatomy of a paratype as viewed from the left side.

European Journal of Taxonomy 12: 1-24 (2012)

Holotype

1 specimen. Zoological Museum of Moscow State University, Moscow, ZMMU Ld-3043.

Paratypes

14 specimens. Zoological Museum of Moscow State University, Moscow, ZMMULd-3044. 2 specimens

(1 on SEM stub), National Museum Wales, Zoology- NMW.Z.2012.013.

The material has been poorly preserved, thus the shells are considerably softened, and the tissues are

rather contracted. No attempt has been made to examine the hinge, as the valves are so soft that they

do not survive being separated. However, the distinctive external form of the shell and the anatomy are

sufficient to describe it and place it at the family level.

Description

Shell. (Fig. 10A-C) Small, (largest specimens 2.5 mm L x 1.9 mm H). Inequivalve in adult, lateral

dorsal margins of right valve elevated and rolled over projecting beyond left valve. Outline slightly

inequilateral, beaks slightly prosogyrate, just behind the mid line, anterior more expanded than posterior;

dorsal margins more or less straight, those in adult right valve projecting laterally, becoming alate;

forming distinct lateral sinuses with adjacent anterior and posterior margins; lateral margins broadly

rounded, anterior more so; ventral margin curved. Ligament small, posterior, deeply sunken; dorsal

margin with thickened periostracal “ligament”. Hinge plate not examined. Sculpture almost smooth with

faint commarginal lines. Prodissoconch I, 105 pm; prodissoconch II, 265 pm, with fine commarginal

lines.

Anatomy. (Fig. 10D) Mantle edge mostly unfused with extensive anterior-ventral pedal aperture; fused

with terminal of gill axis; exhalant aperture small. Anterior-ventral inner margin raised into series of

prominent evenly spaced transverse ridges. Anterior and posterior adductor muscles of similar size,

oval in section. Posterior pedal retractor and anterior pedal muscles present. Ctenidium of a single

demibranch, ascending and descending lamellae of almost equal size. Labial palps small. Foot with

well developed toe, sole flattened and apparently with tissue projections (these may be an artefact of

preservation); heel distinct, byssus functional producing numerous thin filaments.

Differential diagnosis

The projecting and rolled over dorsal margins of the right valve are reminiscent of pteriids but Ptilomyax

gen. nov. is clearly allied with the galeommatids as evidenced by the anatomy. Ptilomyax gen. nov. shares

many common features with other montacutids associated with echinoids including: a large anterior

pedal aperture, mantle fusion limited to junction with gill axis, ctenidium with a single demibranch,

foot with prominent toe with a sole and with a functional byssus, ligament internal and weak hinge

with poorly developed teeth (Oldfield 1961; Ockelmann 1965; Ponder 1968; Jespersen et al. 2004). The

Galeommatoidea exhibit an extraordinary range of shell morphologies as can be seen in Chavan (1969)

but none have projecting dorsal margins. This unique feature is sufficient to warrant the creation of a

new genus and new species.

General discussion

This study has been necessary largely because of inadequate descriptions of small species based on

shells alone and from few specimens. Family placement of these taxa cannot be made on shell characters

alone and when anatomical data are lacking contemporary studies are compromised. Two key taxa based

OLIVER P.G., Galeommatid bivalves from deep-sea echinoids

on unique and poorly preserved shells have been the centre of this paper with the taxonomic crux being

their relationship to either of the co mm ensals fully described here from the ec hin oids Aeropsis and

Pourtalesia. This paper has excluded Axinodon but has not clarified the family affinity of that genus.

The shell characters of the presence of a single cardinal peg in the right valve, marginal flange in the

left valve and an internal ligament attached to a flat or grooved resilifer are shared by the commensals

and K. symmetros. Although this paper concludes that the Aeropsis commensal is K. symmetros there

was an option to regard K. symmetros as a nomen dubium because of the poor condition of the holotype

and the lack of anatomical data for further comparisons. Despite the holotype of K. symmetros being

very small and in poor condition, its almost symmetrical outline and lack of lunule distinguish it from

the Pourtalesia commensal. As a consequence of this decision the Pourtalesia commensal was left

without a species name and is named here as pourtalesiana. The shell characters suggest that it should

be placed in Kelliola but the highly modified gill and bacterial symbiosis warrant further distinction

and the new genus Syssitomya gen. nov. is erected for it. The use of anatomical characters to define

genera and species within the Galeommatoidea is not novel, most recently Jespersen et al. (2004) used

anatomical characters to define their montacutid genera Montacutella and Brachiomya and Oliver &

Tiitzen (2011) did likewise for Draculamya. It is accepted that using anatomical characters makes

systematic comparisons, with species known from shells alone, difficult but no more so than when

applying molecular data to morphological systems. In this case, the adaptive radiation from suspension

feeding to bacterial symbiosis represents a considerable change in both morphology and ecology. In my

view, this adaptation is of considerably more significance than small changes of shell morphology that

have traditionally been recognised by many bivalve taxonomists.

Acknowledgements

This paper would not have been possible without the enormous help given by Ellen Strong of the

Smithsonian Institution for the many scanning electron micrographs of the type specimens of Axinodon

ellipticus and Kellia symmetros. Many thanks to: Anders Waren of the Swedish Natural History Museum

for and his helpful correspondence and the specimens from the Norwegian Sea, which came via Barabra

Voegele of Alcvaplan-Niva, Tromso, Norway; to Eve Southward of the Marine Biological Association,

Plymouth, for the specimens of Aeropsis and the attached commensal bivalves; to Alexandr Mironov of

the P.P. Shirshov Institute of Oceanology for allowing me to examine and describe the bivalves from the

Banda Trench and his information on other co mm ensals: and finally to my colleague Ben Rowson for

reading the manuscript and to the two reviewers for their constructive suggestions.

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Manuscript received on: 16 November 2011

Manuscript accepted on: 19 March 2012

Published on: 25 April 2012

Topic editor: Rudy Jocque

Section editor: Kurt Jordaens

In compliance with the ICZN , printed versions of all papers are deposited in the libraries of the institutes

that are members of the EJT consortium: Museum national d’Histoire naturelle, Paris, France; National

Botanic Garden of Belgium, Meise, Belgium; Royal Museum for Central Africa, Tervuren, Belgium;

Natural History Museum, London, United Kingdom; Royal Belgian Institute of Natural Sciences,

Brussels, Belgium; Natural History Museum of Denmark, Copenhagen, Denmark.