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Coracuta obliquata n. gen. (Chaster, 1897) (Bivalvia: Montacutidae) - First British record for 100 years


Abstract and Figures

Coracuta obliquata n. gen. is recorded from Carmarthen Bay in the Bristol Channel, providing the first British record in 100 years and the first live record in British waters. Its previous generic placements are discussed and hinge structure compared with similar galeommatids. A full description of the species and of the new genus are given and a lectotype is designated.
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In July 2003 benthic sampling was carried out as
part of the Outer Bristol Channel (OBC) Marine
Habitat Study led by the National Museum of
Wales (NMW) and the British Geological Survey
(BGS), and supported by major grants from the
Aggregates Levy Sustainability Fund (ALSF)
for Wales and England. The survey, research-
ing macrobenthic invertebrate communities and
sediment characterisation in the outer Bristol
Channel, collected grab samples from 148 sites.
In six of these sites the identification team of
Jennifer Gallichan & Harriet Wood isolated a tiny
bivalve which they identified as “galeommatid
sp.” This material was subsequently identified
as Neolepton’ obliquatum Chaster, 1897 a species
described from off Northern Ireland but which
had not been subsequently recorded from British
or Irish waters.
This paper re-describes this species, selects a
lectotype from the Chaster collection, and creates
a new generic name.
The material was collected using a 92 kg 0.1 m2
modified Van Veen grab. The animal fraction was
floated off each sample before sieving through a
0.5mm mesh sieve. All material was fixed in for-
maldehyde and subsequently preserved in 70%
Shells were prepared for imaging by soften-
ing the tissue in a detergent, removing the soft
parts mechanically and drying in air. The light
photography was made using a digital macro-
scopic system with computer enhancement via
AutoMontage™ software. Scanning electron
micrographs were made following gold coating
and back-scattered imaging. The anatomical
photograph was made from specimens stained
in Rose Bengal and photographed with the
AutoMontage™ system. Details and interpre-
tation of the anatomy came from histological
preparations made by Jorgen Lützen, Institute of
Zoology, Copenhagen.
Coracuta n.gen.
Type species Neolepton obliquatum Chaster, 1897
Diagnosis Shell very small, obliquely oval in
outline, inequilateral, beaks towards the poste-
rior. Sculpture of concentric raised threads some
dichotomizing. Ligament large, internal; resilifer
shallow, slightly posterior to beaks. Right valve
with single anterior and posterior lateral teeth;
Left valve with marginal flanges, single small
tubercle beneath and slightly to the anterior of
the beak. Ctenidium with single demibranchs,
seminal receptacles present.
Species included Monotypic.
Derivation of name An allusion to Montacuta but
for Cora, daughter of AM Holmes.
Remarks Chaster (1897) placed his new species
in the genus ‘Neolepton’ based on valves from
1 Dept BioSyB, National Museum Wales, Cathays Park, Cardiff CF10 3NP
Abstract Coracuta obliquata n. gen. is recorded from Carmarthen Bay in the Bristol Channel, providing the first British
record in 100 years and the first live record in British waters. Its previous generic placements are discussed and hinge struc-
ture compared with similar galeommatids. A full description of the species and of the new genus are given and a lectotype is
Key words Coracuta obliquata, Galeommatoidea, Britain
Contact author :
Rathlin Island, Ireland. This generic placing was
retained by Smith & Heppell (1991) and van
Aartsen (1996a, b), although the latter stated that
this was very doubtful. Salas (1996) subsequently
redesignated it Mysella obliquatum, based on
comparisons of the hinge structure with Mysella
bidentata (Montagu, 1803), and CLEMAM now
consider this the valid name of the species.
Comparisons with the type species of Neolepton
(Lepton sulcatulum Jeffreys, 1859) (Figs 1-3) clearly
confirm the erroneous original placement.
However, many authors agree (Gofas, pers
comm; Van Aartsen, 1996b; Kallonas, Zenetos &
Gofas 1999) , as do we, that the generic place-
ment is still in question.
Salas (1996) transferred Chaster’s species to
Mysella based on comparisons with M. bidentata.
A problem arises here from the likely erroneous
generic placement of the widespread and com-
mon species universally referred to as Mysella
bidentata (see figs 4-6) (Middlefart, in prep). We
present images of the hinge of the type specimen
of the type species of Mysella, Mysella anomala
Angas, 1877 from south-east Australia, along
with those of M. bidentata (see figs 7, 7a, 8, 8a).
The hinges differ markedly with the hinge plate
closing off the umbonal cavity in M. anomala but
absent in M. bidentata. This results in a visible
resilifer in M. anomala while in M. bidentata the
ligament attaches deep in the umbonal cavity.
In our opinion neither of these hinge forms are
seen in ‘M.’ obliquata where the resilifer is slightly
to the posterior, where the right valve anterior
lateral is elongate and not projecting and where
there is a tubercle in the left valve.
Recent literature (Coan, Scott & Bernard, 2000:
Scott, 1998) applies the generic name Rochefortia
Velain, 1878 to species with a Mysella bidentata
type of hinge. We examined digital images and
original line drawings (from MNHN, Paris)
of the type specimen of the type of the genus,
Rochefortia australis (Velain, 1878). The pres-
ence of a hinge plate and two equal-sized teeth
confirms that Rochefortia is similar to Mysella
and therefore that ‘M.’ obliquata should not be
assigned to Rochefortia.
A number of genera share hinge characteristics
with Mysella notably Montacutona, Curvemysella
and Nipponomysella and a wide-ranging review
is required to assess the relationships and distinc-
tiveness of these taxa. Montacutona species have
a hinge similar to ‘M.’ bidentata but differ in hav-
ing a ctenidium with both demibranchs (Morton
& Scott, 1989) and are therefore unlike Coracuta.
The hinge of Curvemysella is again similar to ‘M’.
bidentata and the shell is very distinctive with its
sickle shaped outline. We have not been able to
examine the type species of Nipponomysella but
Figs 1-3 Neolepton sulcatulum, Guernsey, Marshall coll. NMW.1953.183.101 Fig. 1 hinge of left valve Fig. 2 hinge of
right valve Fig. 3 external of right valve. Figs 4-6 Mysella bidentata, Irish Sea, NMW.Z.1991.076. 535 Fig. 4 hinge
of left valve Fig. 5 hinge of right valve Fig. 6 external of right valve.
the illustrations in Yokoyama (1922) indicate a
form very close to ‘M.’ bidentata with the diverg-
ing teeth and smooth shell. The Nipponomysella
subtruncata (Yokoyama 1927) studied by Lützen,
Takahashi & Yamaguchi (2001) is doubtfully con-
generic with the type species of Nipponomysella,
Montacuta oblongata Yokoyama 1922. In N. oblon-
gata there is a large gap between the teeth, as
in ‘M.’ bidentata, indicating the deep, umbonal
attachment of the ligament. This contrasts with
the N. subtruncata where ligament is attached to
a hinge plate between the teeth.
The anatomy, especially the presence of
seminal receptacles, links C. obliquata to the
Galeommatoidea and Montacutidae in particular
(Jespersen & Lützen, 2000). However, they are
present in a number of nominal genera includ-
ing Mysella, Montacuta, Potidoma, Nipponomysella
and Jousseaumiella (Jespersen & Lützen, 2000)
and their presence is therefore not helpful for
defining genera.
Despite the number of nominal genera availa-
ble within the Montacutidae there is no adequate
generic placement for this species and thus the
new genus is proposed.
Coracuta obliquata n. gen. (Chaster, 1897)
Figs 9-13, 14-16, 17
Neolepton obliquatum Monterosato 1875: Conchigle
Mediterranee Vol. v pp 1-50.
Neolepton obliquatum Chaster, 1897: The Irish
Naturalist (for 1897): 186.
Neolepton obliquatum Chaster – Carrozza 1983:
Boll. Malac. 19: 70.
Neolepton obliquatum Chaster Cianfanelli &
Talenti 1987: Boll. Malac. 23: 266.
Neolepton obliquatum Chaster- Smith & Heppell
1991: Checklist of British Marine Mollusca p. 66.
“Neolepton” obliquatum Chaster- van Aartsen
1996: La Conchiglia n. 281 p.38.
Mysella obliquata (Chaster)- Salas 1996: Haliotis
25: 62.
“Neolepton” obliquatum Chaster- Kallonas,
Zenetos & Gofas 1999: La Conchiglia n. 291 p.18.
Mysella obliquata (Chaster)- CLEMAM 2005
‘Mysella’ obliquata (Chaster)- Mackie et al (in
Type material Lectotype herein designated,
1 lv., Church Bay, Rathlin Island, Antrim
County (sea area 29a), G.W. Chaster colln
Paralectotype herein designated, 1 lv., as lecto-
type NMW.1910.029.01827.
Other material 3v. Mouth of Kenmare
River, between Bull Rk and Skerriff 44-47fm
G.W. Chaster colln NMW.1910.029.01570.
6 spec. Carmarthen Bay (OBC survey),
Stn 55B, 51°27.674’N 04°47.382’W, 58.5m,
NMW.Z.2003.048.1; 10 spec. Carmarthen Bay
(OBC survey), Stn 96A, 51°29.070’N 04°41.291’W,
Figs 7, 7a Mysella bidentata SEM of hinge details, Irish Sea, NMW.Z.1991.076. 535 Figs 8, 8a Mysella anomala Angas,
Syntype, Port Jackson, New South Wales, Australia, BM(NH) 1877.5.12.108 SEM of hinge.
52.6m, NMW.Z.2003.048.2; 21 spec. Carmarthen
Bay (OBC survey) Stn 97A, 51°28.436’N
04°44.024’W, 55.9m, NMW.Z.2003.048.3;
14 spec. Carmarthen Bay (OBC survey),
Stn 97B, 51°28.371’N 04°44.252’W, 55.9m,
NMW.Z.2003.048.4; 32 spec. Carmarthen Bay
(OBC survey), Stn 98B, 51°24.581’N 04°46.757’W,
60.7m, NMW.Z.2003.048.5; 3 spec. Carmarthen
Bay (OBC survey), Stn 134A, 51°28.161’N
04°51.471’W, 53.8m, NMW.Z.2003.048.6; 1
spec. Carmarthen Bay (OBC survey), Stn
144A, 51°26.628’N 04°52.319’W, 59.0m,
NMW.Z.2003.048.7; 3 spec. Carmarthen Bay (OBC
survey), Stn 144 B, 51°26.628’N 04°52.319’W,
59.0m, NMW.Z.2003.048.8.
Measurements Lectotype L 2.69mm, H 1.81mm;
Paralectotype L 1.69mm, H 1.31mm.
Shell (Figs 9-13a) Length to 2.7mm, rela-
tively flattened, equivalve, strongly inequilat-
eral, beaks in the posterior two-thirds of the
shell. Umbos prominent, rounded. Outline
obliquely subovate, transversely tear-drop in
form. Dorsal-anterior (lunule) margin promi-
nent, straight, gently sloping, slightly angled
at junction with anterior margin; lunule
elongate, narrow, not sunken but defined by
a weak ridge. Anterior margin rounded; ven-
tral margin gently curved; posterior margin
subtruncate, junctions with ventral and pos-
terior dorsal margins weakly angulate; poste-
rior dorsal margin short, straight to slightly
convex, steeply angled.
Ligament large, internal, on a shallow resilifer,
slightly posterior to the beaks. Hinge in right
valve with single anterior and posterior lateral
Figs 9-13 Coracuta obliquata n.gen. (Chaster,1897) Figs 9-10a, Lectotype, left valve, NMW.1910.1826 Fig.11
Paralectotype, left valve, NMW.1910.1827 Figs 12, 12a, left valve, Carmarthen Bay, NMW.Z.2003.048.2. Fig 13
right valve, Carmarthen Bay, NMW.Z.2003.048.1.
teeth; anterior lateral longer than posterior at a
ratio of 2.7:1, parallel and equal to the length of
the lunule margin, which creates a shelf to form a
socket; posterior lateral short subparallel to dor-
sal margin and forming a small v-shaped socket
at its posterior extremity.
Left valve (Figs 14, 14a) without lateral teeth
but dorsal margins slightly extended as marginal
flanges that fit into right valve sockets. A small
tubercle is present beneath and slightly to the
anterior of the beak.
The prodissoconch (Fig. 16) is smooth, bearing
no sculpture and there is a suggestion of a PdI at
79µm in length; PdII is 344µm.
Sculpture (Fig. 16) concentric, of fine, sharp,
concentric ridges these irregulary undulating
and occasionally dichotomizing. Shell colour
cream to very light brown, glossy.
Figs 14-16 Coracuta obliquata n.gen., Carmarthen Bay, NMW.Z.2003.048.3 Figs 14, 14a left valve and detail of hinge
Figs 15, 15a right valve and detail of hinge Fig 16 external of right valve.
Figs 17a, b Coracuta obliquata n.gen. , Gross anatomy after removal of shell and right mantle, photograph after
staining in Rose Bengal, Carmarthen Bay, NMW.Z.2003.048.
AA anterior adductor; CT ctenidium, F(toe) toe of foot, GA/ME junction of gill axis and mantle edge, LP labial
palps, ME mantle edge, PA posterior adductor, PPR posterior pedal retractor.
Anatomy (Fig. 17) Mantle edge un-fused except
at junction with gill axis. Ctenidium of single
demibranchs only, filaments reflexed. Labial
palps small with a prominent oral fold. Foot axe
shaped. Adductor muscles of equal size, anterior
oval, posterior subcircular. Females with paired
seminal receptacles. Eggs brooded to larval
Habitat In the Bristol Channel living specimens
were collected from between 50 to 60m water
depth and from a variety of substrate types (Table
1). Sediments were typically sandy but with vary-
ing amounts of mud and gravel. Off the Iberian
coast (Salas, 1996) C. obliquata has been collected
in depths of 22-60m in gravel, mud, muddy
sand and gravel and rocks. In the Mediterranean
(Kallonas et al., 1999) the species was recorded at
depths of 4-85m in sandy mud and silt.
Geographical distribution In the Atlantic Ocean it
is known from off northern and western Ireland,
the Bristol Channel (this paper) and the Alboran
Sea (South Iberia off Cadiz and off Huelva)
(Salas, 1996). In the Mediterranean from theTyr-
rhenian, Ligurian, Adriatic, Ionian and Aegean
Seas (Kallonas et al, 1999; Monterosato, 1875).
The lack of records in the UK could be
explained by the very small size of C. obliquata
and probably sporadic distribution as evidenced
in the Outer Bristol Channel. Tebble (1976) men-
tioned the Rathlin Island record but considered
it a ‘doubtful British record’ although Smith and
Heppell (1991) did include it in their checklist.
The similarity of C. obliquata to Mysella bidentata
juveniles could have lead to misidentifications in
the past as our samples contained both species.
Fig. 18 Coracuta obliquata n. gen., transverse section, Carmarthen Bay. K kidney, R rectum, SR seminal recepta-
cles, PPR posterior pedal retractor, VG visceral ganglion, L larvae, P periostracum, MM mantle margin..Fig. 19
Coracuta obliquata n. gen., longitudinal section, Carmarthen Bay. AA anterior adductor, DG digestive gland, I
intestine, O ovary, OE oesophagus, PA posterior adductor, SR receptables & duct opening.
The Galeommatoidea and in particular the
Montacutidae are known to have strong associa-
tions with other marine invertebrates. This can
be a precise commensal relationship such as
between Montacuta substriata and the echinoid
Spatangus (Tebble, 1966) and Montacutona mutsu-
wanensis and the Cnidarian Cerianthus filiformis
and phoronid Phoronis australis (Morton & Scott,
1989; Morton, 1989) or may be less restricted
such as the association of ‘Mysella’ bidentata with
both ophuroids and sipunculans (Ockelmann &
Muus, 1978). The taxonomic range of associates
is wide, frequently with echinoderms and sipun-
culans but also with polychaetes, phoronids,
crustaceans and cnidarians (Morton & Scott,
We have no direct evidence of an association
between Coracuta obliquata and any invertebrate
but the extensive community data from the
Outer Bristol Channel survey (Mackie et al., in
press) allows us to look for possible associations.
Coracuta obliquata was found at only six of the 148
stations sampled and these six are geographi-
cally clustered. From analysis of the overall
faunal assemblages, all but one of these stations
is represented by community IId (Mackie et al,
in prep). On analysis of the molluscs alone, all
our stations group together in a single com-
munity. This total fauna assemblage clusters
geographically and the five stations are in the
southern, deeper half of the area occupied by
community IId. Some 320 species were recorded
from this community with the top ten dominant
being Scalibregma inflatum, Spiophanes bombyx,
Ampharete lindstroemi, Abra alba, Lagis koreni,
Mysella bidentata, Mediomastus fragilis, Phoronis
spp indet., Ampharete sp. and Ampelisca spinipes
(in descending order of dominance). Polychaetes
dominate the community, which also contains a
phoronid, an amphopod and two bivalve spe-
cies. Trawl samples in the area recorded large
numbers of brittle stars - Ophiura albida and O.
ophiura - and the bristle worm Aphrodita aculeata.
There are a number of possible associates within
this assemblage list including ophuroids, pho-
ronids, polychaetes and crustaceans. A cluster
analysis of all species and all stations using
R mode shows C. obliquata clustering at 34%
with Ophiura ophiura and Eudorella truncatula
(Cumacean). Sixty percent of the distribution of
O. ophiura is found in another 2 communities as
well as IId. Eudorella truncatula is found most
frequently in community IId, although two of
its stations overlap with an adjacent community.
Amphiura filiformis is found at all six of our C.
obliquata stations along with Mysella bidentata but
both are found throughout all four communities
and are known to be associated with each other.
No one species stands out from the analysis as a
certain associate and until further studies are car-
ried out it is still unknown as to whether or not
C. obliquata has any symbiotic association.
This paper was made possible through financial
support provided to the Outer Bristol Channel
Marine Habitat Study by the Aggregates Levy
Sustainability Fund (ALSF) for Wales, the
Sustainable Land Won and Marine Dredged
Aggregate Minerals Programme (SAMP) in
England, the National Museum of Wales,
the Natural Environment Research Council
(NERC)/British Geological Survey (BGS) and the
Table 1 Sediment characterisation for stations at which Coracuta obliquata was found in the Bristol
Station % sand % mud % gravel SUM
55 93.62 5.79 0.59 Sandy
96 72.28 26.82 0.9 Muddy sand
97 63.75 16.89 19.36 Muddy gravelly sand
98 41.9 1.14 56.96 Sandy gravel
134 95.11 2.98 1.91 Sandy
144 96.03 3.97 0 Sandy
British Marine Aggregate Producers Association
We are indebted to Jorgen Lützen for his advice
and examination of the anatomy; to Graham
Oliver for his support and to Teresa Darbyshire
for sediment data and information on the associ-
ated fauna.
Thanks also to A. Zenetos and S. Gofas for the
loans of N. obliquatum material; to Virginie Heros
for the images of Rochefortia and to John Taylor
and the Natural History Museum for the images
of Mysella anomola.
We would also like to acknowledge the Crew
of the RV Prince Madog without whom the mate-
rial would not have been collected.
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... A number of species were found to be unique to the tunnel-valleys (Table 42.3). The most notable is the small bivalve mollusk, Coracuta obliquata, which has only been recorded once in UK waters in the last 100 years [23]. This bivalve appears to have very specific environmental requirements, having been recorded only in samples taken from the floor of the Sole Pit. ...
... This bivalve appears to have very specific environmental requirements, having been recorded only in samples taken from the floor of the Sole Pit. Holmes et al. [23] observed C. obliquata in sediments ranging from sand to sandy gravel, which corresponds well with our observations in the Sole Pit where this mollusk was observed in gravelly sand. A BIO-ENV analysis carried out on C. obliquata abundance, and the full range of environmental variables listed in Table 42.1, yielded a maximum correlation of 0.341 with geomorphic feature (Table 42.4A). ...
Tunnel-valleys are prominent features of the southern North Sea, where they form actuate and linear seabed depressions with a limited sediment infill. In the outer Humber area, the valleys attain water depths up to 100 m and radiate outward from the mapped palaeo-ice margin of the Devensian (Weischelian) glaciations. The location and orientation of the valleys indicates that their origin is associated with the ice sheet, although the precise process of formation is uncertain. It is likely that the valleys were formed by either subglacial fluvial activity or ice-damn breakouts. Extensive areas of Sabellaria spinulosa reefs have been identified on the western flanks of the Silver Pit that extend down to the valley floor, representing a resource of significant conservation interest. The eastern flanks were found to support a diverse faunal assemblage with widespread hydroid and bryozoans turfs and abundant ascidians. The seabed of the Sole Pit is characterized by much sandier deposits than that of the Silver Pit, and it supports an abundant bivalve community, dominated by Abra alba. This bivalve assemblage also contains Coarcuta obliquata, which has only been recorded once before in the UK. A multivariate analysis of similarity (ANOSIM) test was used to investigate differences between the macrofauna associated with the tunnel-valleys and macrofauna recorded across the wider Humber study area. The ANOSIM test is analogous to a univariate analysis of variance (ANOVA) test based on permutations, or randomization, of the data, making minimal assumptions about the distribution of that data.
... The Galeommatida are cosmopolitan in distribution, with many species inhabiting European waters [11,12], but our overall knowledge of them remains fragmentary. The first modern revisions were made by van Aartsen [13,14], followed by additional taxonomic descriptions and anatomical remarks [15][16][17][18][19][20][21][22][23][24]. In this context, Oliver and Lützen [25] described the new genus Draculamya, from deep waters in the north-western Atlantic Ocean, characterized by peculiar anatomical features. ...
Full-text available
A new Galeommatid bivalve is described for the Mediterranean Sea, tentatively assigned to the elusive genus Draculamya Oliver and Lützen, 2011. “Draculamya” uraniae n. sp is described upon a number of dead but fresh and articulated specimens, plus many loose valves. Its distribution is almost basin-wide in the Mediterranean, and it possibly occurs in the adjacent Gulf of Cadiz. As for many members in Galeommatida, we hypothesize that “Draculamya” uraniae lives as commensal upon a still-unknown host. The possible co-identity of the extant genus Draculamya with the morphologically similar Pliocene Glibertia Van der Meulen, 1951, is discussed, although the lack of anatomical and genetic support leaves the problem open.
... A further clue to considering the teeth in K. bidentata as laterals is that cardinals normally cluster anteriorly to the chondrophore (e.g. in Mactridae); we do not know of any heterodont bivalve where cardinals would be situated on either side of an internal ligament. Our view agrees with Tebble (1966) and Chavan in Moore (1969), but conflicts with the interpretation as cardinal teeth by Holmes, Gallichan & Wood (2006) for the present species, by Morton & Scott (1989) for Mysella anomala, and by Coan et al. (2000) and Boyko & Mikkelsen (2002) for similar West American species. ...
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The type species of the genera Mysella Angas, 1877, Rochefortia Velain, 1878, Rochefortula Finlay, 1927 and Altenaeum Spaink, 1972 are illustrated and discussed. A new genus Kurtiella is introduced for 'Mysella' bidentata ( Montagu, 1803) and other European species currently placed in Mysella. These species display a unique diagnostic character state in having the cardinal platform deeply receding to house the internal ligament, along with an array of more general montacutid traits. At the species level, diagnostic characters are given for the type species K. bidentata, and for other included species K. ovata (Jeffreys, 1881), K. pellucida (Jeffreys, 1881), K. triangularis (Watson, 1897) and K. tumidula ( Jeffreys, 1866), based mostly on characters of the prodissoconch and on shell outline. Montacuta simillima Smith, 1892, from St Helena, is considered a junior synonym of K. pellucida. The loss of labial palps in K. tumidula is interpreted as a further derived character, which does not conflict with the generic placement in Kurtiella.
Mysella gregaria new species (Bivalvia: Galeommatoidea: Montacutidae) is described from Wrightsville Beach, North Carolina, USA. Several individuals were collected from the body column of an intertidal burrowing unidentified actinian. The morphology of the shell and soft parts is described and compared with other species of the genus from the W. Atlantic Ocean and with other bivalves associated with solitary anthozoans. Mysella gregaria is a ctenidial brooder and specimens are either males or females; no true hermaphrodites were found. Contrary to many less social commensal bivalves, reproduction in M. gregaria does not involve sperm storage. We suggest that this may be correlated with the species' gregariousness.
Abstract Jespersen, Å. and Lützen, J. 2009. Structure of sperm, spermatozeugmata and ‘lateral organs’ in the bivalve Arthritica (Galeommatoidea: Leptonidae). —Acta Zoologica (Stockholm) 90: 51–67 The position and structure of paired ‘lateral organs’ in the foot of Arthritica semen and Arthritica bifurca might indicate a chemosensory function. In both species part of the organ is also glandular. In A. semen the glandular epithelium is detached piecemeal and, probably by means of the foot, is moved to and grafted upon the gills of the same individual. The transferred epithelia appear as disk-shaped actively secretory ‘gill bodies’ which, attached to the abfrontal side of the inner demibranch, replace the ordinary unciliated gill epithelium. The secretion is liberated into the suprabranchial chamber, which serves as a marsupium, but its function is uncertain. Arthritica semen is a protandric hermaphrodite and produces very large ova that undergo a direct development that results in a non-planktonic lecithotrophic crawling juvenile stage. The sperm cells have filiform nuclei that are straight in the euspermatozoa and more or less helicoidal in what is considered to represent paraspermatozoa. By a process of aggregation, spermatozeugmata are formed which consist exclusively either of euspermatozoa or paraspermatozoa. Spermatozoa are stored in the oviduct in A. semen but in paired seminal receptacles in A. bifurca.
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The Bivalves (239 species) obtained during the French cruise "Balgim" (1984) and the Spanish cruise "Fauna 1" (1989) in the Alboran Sea, Strait of Gibraltar and Ibero-Moroccan Gulf, are listed with detail of material examined. The Nuculidae, Nuculanidae and other Protobranchs, Limopsidae and Cuspidariidae are well represented and are given particular attention, with most of the species illustrated. Taxonomic or nomenclatural decisions herein: Bathyarca pectunculoides (Scacchi, 1834) restored as valid name (ICZN application pending); L. angusta (Jeffreys, 1879) recognized as valid and distinct from L. minuta (Philippi, 1836); Limaria tuberculata (Olivi, 1792) used as the valid name for "Limaria inflata (Chemnitz)"; Thyasira perplicata new name for Cryptodon plicatus Verrill, 1885 non Adams, 1862; Neolepton obliquatum Chaster, 1897 transferred to Mysella (Montacutidae); Cuspidaria striolata Locard, 1898 synonymized with C. costellata (Deshayes, 1835). There is a definite influence of the Mediterranean outflow on the distribution of Bivalves: it is a barrier which keeps out the deep water Atlantic fauna. The suite of species affected is found along the Moroccan slope as shallow as 700 m, and in the axis of the Strait at similar depths below the Mediterranean water, but is restricted to depths below ca. 1300 m off Huelva. Off Cape St. Vincent, the incidence of the Mediterranean outflow is very attenuated, and practically does not affect the bathymetric distribution of the Bivalves.
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Twenty-two species of galeommatacean bivalves from four families and nine genera are described, compared and illustrated. The invertebrate hosts of many of these commensal species are reported for the first time. Six new species are described, belonging to the genera Pseudopythina (4), Barrimysia (1) and Scintillona (1). Of the known species, eleven have been recorded previously from Japan, two from the Philippines, Ephippondonta oedipus only from Hong Kong and Lasaea rubra from shores virtually world-wide. The generic and familial placement of many of the species discussed are revised.
Mysella bidentata is a euryvalent species. Its distribution in the N. Øresund 3-4 months after settling is positively correlated with the density of adult Amphiura filiformis.In the laboratory, Mysella prefers to live in the oxydized layers around the burrow of Amphiura. Dense Amphiura populations enlarge the water-sediment interface and thereby the oxydized layers and the microbial activity. This benefits Mysella which obtains protection from surface-hunting predators and additional supply of food.In their second year the bivalves are functional males, and when three years old and 3 mm long they are hermaphrodites. Males produce fertile and oligopyrene sperm simultaneously. Sperm transfer involves sperm bags and receptacula seminis, and is thus time-independent of egg-spawning. Mysella reacts to gravitation by positive or negative taxes, and to direction of water flow by positive tropo-taxis or compass reactions. Chemical stimuli from a predator (Astropecten) evoke direct orthokinesis. Stimuli from a host (Amphiura) evoke direct orthokinesis and direct klinokinesis as well.A model links the natural hierarchy of responses and behavioural patterns in Mysella to fundamental physiological functions, survival mechanisms, site selection and intraspecific regulation in consistent order. The model may be adapted to fit other species for various purposes.Within the Montacutidae, changes in host selection and behaviour have been more decisive for evolution and speciation than changes on other organismic levels.Earlier descriptions of growth, biology and behaviour of A. filiformis and A. chiajei are revised.
It is argued that the paired structures in adult Montacuta phascolionis previously claimed to be testes are seminal receptacles. The receptacles in 3.0mm long females are filled with sperm cells or spermatids. A 2.0-mm long specimen had a well-developed testes exhibiting ripe sperm. It is concluded that the species is a protandrous hermaphrodite and that the previously described dwarf males are rather complemental males. The ultrastructure of the sperm cells is described and compared with that of the sperm of related species.
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