Two new species of Solenogastre from the Gulf of Mexico.

Solenogastres (Aplacophora) are a unique group of Molluscs which have vermiform bodies (i.e. are 'worm-shaped'), lack shells (although some have calcareous sclerites), and have a greatly reduced mantle and foot. This anatomy long led evolutionary biologists to conclude that Solenogastres represent a 'primitive' state for Molluscs, and they were interpreted as an early-branching clade with a sister-group relationship to all other Molluscs. Modern genetics-based taxonomy has reviewed this slightly, showing that the Solenogastres are the sister group to the Chitons (Polyplacophora), a superficially Gastropod-like group which have segmented shells and lack internal torsion, with these two combined forming the out-group to all other Molluscs. There are currently about 300 species of described Solenogastres, although it is thought that their true diversity is much higher. Solenogastres are notoriously difficult to locate and identify, particularly for non-specialists. Most species are less than 5 mm in length, many live infaunally in marine sediments, some of them on the oceans deep-abyssal plains, while others live epifaunally or on the bodies of Corals of Hydrozoans.

In a paper published in the journal ZooKeys on 31 September 2024, Carmen Cobo of the Department of Invertebrate Zoology at the Smithsonian National Museum of Natural History, William Farris, Chandler Olsen, and Emily McLaughlin, of the Department of Biological Sciences at the University of Alabama, and Kevin Kocot, also of the Department of Biological Sciences at the University of Alabama, and of the Alabama Museum of Natural History, describe two new species of Solenogastre from the Gulf of Mexico, collected from Autonomous Reef Monitoring Structures.

Autonomous Reef Monitoring Structures are standardised structures used to sample marine life on reefs. They are made up of multiple stacked flat plates which mimic a complex reef-environment. These structures are screwed onto a reef-surface, where they attract settling benthic organisms, and can be collected at a later date, without damaging the overall reef-structure. The Autonomous Reef Monitoring Structures examined by Cobo et al. where placed on reefs in the Gulf of Mexico by the Research Vessel Point Sur in May 2019, and collected in August 2021.

One of the Autonomous Reef Monitoring Structures deployed in the Gulf of Mexico from which Solenogasters were collected. Cobo et al. (2024).

The first new species described by Cobo et al. is placed within the genus Dondersia and given the specific name tweedtae, in honour of marine biologist Sarah Tweedt, who collected the specimens used in the study, for her outstanding work studying invertebrate biodiversity using Autonomous Reef Monitoring Structures. It is described from a single specimen from an Autonomous Reef Monitoring Structure deployed at a depth of 82 m.

The single known specimen of Dondersia tweedtae is about 14 mm in length and 0.55 mm wide at the mid-body, although in life the Animal expanded and contracted its body as it moved. It is bright pink in colour, with a dorsal keel made up of seventeen distinct bright yellow lobes. It has a smooth appearance, with a covering of scale-like sclerites, of which there are three different types. 

Habitus of Dondersia tweedtae. (A) Field photographs of the holotype showing the contractions and extension range (usnm 1718003). (B) Photograph of the holotype preserved in ethanol B’ detail of the lobes of the dorsal keel. Star indicates the anterior end of the Animal. Cobo et al. (2024).

The second new species is placed within the genus Eleutheromenia and given the specific name bullescens, where 'bullescens' derives from the Latin 'bullescere', meaning 'to bubble', in reference to the protrusions on the dorsal keel of the Animal. The species is described from two specimens collected from an Autonomous Reef Monitoring Structure placed at a depth of 82 m within the Gulf of Mexico, about 100 km to the west of the structure from which Dondersia tweedtae was collected.

The specimens of Eleutheromenia bullescens are 10-12 mm in length and 0.5-1.0 mm in width at their midsections, although again, in life these Animals expanded and contracted their bodies as they moved. They were light orange in colour, with a dorsal keel made up of numerous lobes, and a covering of spines formed from hollow sclerites, some of which are hook- or harpoon-shaped.

Habitus of Eleutheromenia bullescens. (A), (A’) Field images of the Holotype (USNM 1718004) (B), (B’) Field images of the paratype (USNM 1718005). (C( Paratype in 95% ethanol. (D) Detail of the dorsal lobes in the holotype (decalcified mid-body region). Images were captured using Olympus DSX100 optical microscope with anti-halation and fast HDR adjustments; brightness 0016 to 0022, texture 50-71, contrast 36-50. Star indicates the anterior end of the animal. Arrow indicates detached lobes and their 'pedunculi'. Cobo et al. (2024).

Both Dondersia tweedtae and Eleutheromenia bullescens have distict dorsal keels, something not generally found in Solenogastres, combined with brighter colouration than is typical for the group. Despite this, they were confirmed by both morphological and genetic analysis to be members of distantly related groups. Furthermore, the structures of the dorsal keels are quite different in the two species, strongly suggesting that this is a result of convergent evolution, rather then recent common ancestry. the reason for this is unclear, though Cobo et al. note that in the absence of shells, Solanogastres have adopted a range of other defence strategies, including mimicry, crypsis, autotomy, production of defensive chemicals, or the retention of exogenous biochemically active compounds and cnidocytes (the stinging cells of Cnidarians) from their prey. They further note that the lobes which make up the dorsal keel of Eleutheromenia bullescens contained a number of cells which appeared to be derived from another organism, including one possible cnidocyte.

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