Fissidentalium aurae: A new species of deepwater Anemone-associated Scaphopod from the Labrador Sea.

Scaphopods, or Tusk Shells, are a distinct group of Molluscs found exclusively living infaunally (i.e. beneath sediments) in marine environments, although they are found in all oceans from the shallow continental shelves to the deepest abyssal environments. Scaphopods have an elongate, tapering shell, between 0.5 and 18 cm in length, curved, but never coiled, with an opening at each end. The body of the Scaphopod is entirely contained within this shell, although it's foot can be extended from the larger opening, to drag the Animal through the sediment. Water is drawn in through the smaller opening, and passed over the mantle of the Scaphopod using cilia, with oxygen being absorbed across the mantle surface (there are no discrete gills). Food particles are extracted from the sediment by a series of tentacles surrounding the foot, and then carried to the mouth. Phylogenetic reconstructions suggest that the Scaphopods were one of the earliest-branching Mollusc groups, but their fossil record only goes back to the Early Carboniferous, making them the youngest class within the Mollusca. This could imply that earlier Scaphopod fossils simply haven't been found, or possibly that early ancestors of Scaphopods lacked shells, subsequently evolving these separately from other Molluscs.

Diagram of the scaphopod Rhabdus rectius. Wikimedia Commons.

In a paper published in the journal Marine Biology on 7 November 2024, Katrin Linse of the British Antarctic Survey and Jenny Neuhaus of the German Centre for Marine Biodiversity Research describe a  new species of deepwater Anemone-associated Scaphopod from the Labrador Sea. 

The Labrador Sea separates the northeast coast of Newfoundland from Greenland, and is an area of noted biodiversity, with an ecosystem driven by the admixture of warm waters from the Atlantic with cold waters from the Arctic. The presence of sea ice makes the Labrador Sea the least saline basin of the Atlantic Ocean, with this currently increasing due to the higher rate at which the sea ice is melting, something which impacts on both blooms of Phytoplankton and benthic organisms, and which has the potential to alter the mixing of the deep ocean currents which drive Atlantic circulation. The Labrador Sea can be divided into three bathymetric zones, a wide continental shelf on its southwestern side, the deep Labrador Basin in the centre, and a narrow, steep continental shelf on its northeastern side.

The new Scaphopod species is described on the basis of material collected by trawls of the Labrador Basin at depths of between 3380 and 3390 m, as well as observations of the seabed by remote operated vehicle. The new species is placed in the genus Fissidentalium, and given the specific name aurae, meaning 'breeze', in reference to the windy conditions in the area when the survey was being caried out, as well as to Briese Research, the company which operated the remote operated vehicle used in the research. Genetic analysis confirmed that Fissidentalium aurae is a new species.

Fissidentalium aurae; (a)–(b) Holotype SMF 366428, side and anterior view of shell and associated Anemone; (c)–(d) Paratype 2 NHMUK 20230932, side view of shell and Anemone and soft parts; (e) Paratype 3 NHMUK 20230933, side view of dead shell. Arrows indicate delineation of the Anemones. Linse & Neuhaus (2024).

Specimens of Fissidentalium aurae are between 15.52 and 63.63 mm in length, and an opaque white in colour, with longitudinal ribs on the shell. Of the 19 Scaphopod specimens collected,18 had an Anemone attached to the upper, convcave, side of their curved shells. Genetic analysis showed these to be members of the genus Maractis, which is otherwise associated with deep-sea hydrothermal vents. Video footage showed the Anemones producing a distinct trail as they were dragged through the sediment by the buried Scaphopods, and actively using their tentacles to acquire food particles from their forward size, reaching over the 'bow wave' of sediment produced by their movement. 

See also...

Borniopsis mortoni: A new species of Galeommatoid Bivalve from Shikoku Island, Japan.

Galeommatoid Bivalves are an extremely diverse group of bivalves from shallow and intertidal waters around the world. They are noted for the commensual relationships that many species form with other invertebrates, living either on the bodies of their partners or within their burrows. Members of the genus Borniopsis are known from around the coast of eastern Asia, where they have been found living on the bodies of Mantis Shrimps, Crabs, Holothurians (Sea Cucumbers), Sipunculid Worms, Echiurans (Spoon Worms) and probably Tanaid Crustaceans.

In a paper published in the journal ZooKeys on 7 September 2016 Ryutaro Goto of the Museum of Zoology and Department of Ecology and Evolutionary Biology at the University of Michigan and Hiroshi Ishikawa of Uwajima on Shikoku Island, Japan, describe a new species of Borniopsis from Sea Cucmbers found on mud flats at the mouth of the Souzu River on southwestern Shikoku Island.

The new species is named Borniopsis mortoni, in honour of Brian Morton of the Univeristy of Hong Kong for his contributions to marine biology, marine ecology and malacology, including the discovery of many Galeommatoid species. Borniopsis mortoni was found living on the bodies of the Sea Cucumber Patinapta ooplax, a Worm-like burrowing species, with about 70% of the Sea Cucumbers inspected hosting the bivalves and some having more than ten attached.

Patinapta ooplax with multiple individuals of Boriopsis mortoni attached. Goto & Ishikawa (2016).

The Bivalves are small, with the largest specimens reaching 4.1 mm by 2.8 mm, and have thin, elongate and slightly inflated shells. The periostracum (covering of the shell) is tan to brown, withblack markings. They attach to their hosts using both their foot and byssal threads (thick silky threads produced by some Bivalves as a way of attaching themselves to substrates).

A crawling individual of Boriopsis mortoni. Goto & Ishikawa (2016).

See also...

The evolution of Galeommatoid Bivalves. Galeommatoid Bivalves are a large group of Molluscs that inhabit a broad range of environments and often form commensal relationships with a broad range of other invertebrates. Their classification has...

Symbiosis and the success of Galeommatoid Bivalves.                                                       Galeommatoid Bivalves are small (under 20 mm, mostly under 10 mm) nondescript Clams found in both soft and hard bottomed marine environments (which is unusual, as most Bivalves...

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The evolution of Galeommatoid Bivalves.

Galeommatoid Bivalves are a large group of Molluscs that inhabit a broad range of environments and often form commensal relationships with a broad range of other invertebrates. Their classification has been somewhat uncertain, with the group divided into either two (Galeommatidae and Lasaeidae) or four (Galeommatidae, Lasaeidae, Kelliidae and Montacutidae) families.

In a paper published in the journal BMC Evolutionary Biology on 6 September 2012, a group of scientists led by Ryutaro Goto of the Graduate School of Human and Environmental Studies at Kyoto University and the Department of Marine Ecosystem Dynamics at the Atmosphere and Ocean Research Institute at The University of Tokyo, publish the results of a genetic study into relationships within the Galeommatoid Bivalves.

Goto et al. found that the Galeommatoid Bivalves are split into six clades (distinct evolutionary lineages), but that these bore no relationship to either previous classification of the group. 

Galeommatoid Bivalves are known to colonise a number of very different invertebrate hosts, notably Crustaceans, Sea Cucumbers, Spoon Worms, Sipunculan Worms, Brachiopods, Bryozoans, Annelids, and other Bivalves. Typically when epibiotic animals (animals that live on the surface of other organisms) switch hosts they do so between closely related species, with jumps between distantly related species being rare. However each clade of Galeommatoid Bivalves contained species which lived on very different hosts, with host species apparently being no guide to relationships within the Galeommatoid Bivalves. When parasites colonise new hosts they have to learn to get past the defences of the new organism, however Galeommatoid Bivalves are not true parasites; they live on the bodies of other invertebrates but gain nutrition by filter feeding from the water column, and few (if any) animals seem to have defences against this sort of colonisation, apparently making it easy for Galeommatoid Bivalves to switch between unrelated hosts.

The Galeommatoid Bivalve Arthritica japonica that attaches directly onto the body surface of intertidal Crabs. Goto et al. (2012).

Neaeromya rugifera that attaches onto the abdomen of Upogebid Shrimps. Goto et al. (2012).

Devonia semperi (top) and Anisodevonia ohshimai (bottom), which attach to the body surfaces of the burrowing Sea Cucumbers. Goto et al. (2012).

Byssobornia yamakawai on an Echiuran (Spoon) Worm. Goto et al. (2012).

Litigiella pacifica on the body of the Sipunculan worm, Siphonosoma cumanense. Goto et al. (2012).

Finally Goto et al. identified four ways in which Galeommatoid Bivalves colonised their hosts. Two of these, living inside shells used by Hermit Crabs and living inside the esophaguses of Sea Cucumbers, were utilised by single species, suggesting that these were unique evolutionary innovations, with little taxonomic significance. The remaining two methods, colonising the surface of the bodies of the host animals, and colonising the burrows of the hosts, were found in a variety of unrelated forms, suggesting that Galeommatoid Bivalves are also able to switch easily between these lifestyles.

The Galeommatoid Bivalve Ephippodonta gigas that lives in the burrows of Thalassinidean Shrimps. Goto et al. (2012).

Curvemysella paula, lives inside shells carried by Hermit Crabs. Goto et al. (2012).

This suggests that Galeommatoid Bivalves are extremely elastic in their ability to colonise new hosts and therefore new environments, which helps to explain the success of the group, even if it does make its taxonomy difficult to understand.

See also Symbiosis and the success of Galeommatoid Bivalves, The biology of pumice rafts, Deep-sea Gastropods from Miocene Cold Seeps and Whale-falls in Japan, Thirteen new species of interstitial Gastropods from New Zealand, and A new species of Scallop from Western Australia.

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