Aculifera: Solenogastres

Taxa on This Page

1. Aplacophora
2. Solenogastres

Introduction

Neomenia, showing mouth (top), pedal groove, and cloaca. Image source unknown: possibly from Invertebrate Zoology by Edward E. Ruppert and Robert D. Barnes, reproduced at Man and Mollusc.

The Solenogastres are mostly small (less than 5 cm long] worm-like mollusks that live symbiotically (or feed upon) cnidarians. As with the Caudofoveata, many of the typical molluscan characters are absent. They have no shell, eyes, or tentacles. The mantle cavity is rudimentary, and instead of the standard molluscan flattened foot there is a pedal groove, which the animal uses to creep along the bottom. They are hermaphrodites and lack ctenidia (gills) in the mantle cavity. As with the Caudofoveata, the integument contains layers of embedded calcareous spicules, possibly a link to early Cambrian coelomates. The worm-shaped body is derived from the inrolling of the mantle margins.

There are about 250 described species, probably with many more awaiting discovery. They are found primarily below 200 meters and are sometimes quite abundant in deep-sea epifauna or infauna. Many reside and feed upon cnidarians (hydroids and corals).

Despite being an important part of the deep-sea benthos, neomeniomorphs are poorly known, mainly because the difficulty of studying creatures that live in very deep water.

The Solenogastres have in the past been combined with the Caudofoveata to form the class Aplacophora. In some classifications the terms Aplacophora and Solenogastres seem to be synonymous. However the tendency now is to see them as distinct classes of mollusks.

Evolutionary Relationships

Differentiation of the mantle cover in just metamorphosed Nematomenia banyulensis (Solengastres - A and B) and Middendorffia caprearum (Polyplacophora - C and D). Pl shell plates in formation through the coalescence of the scaly bodies Sp arranged in seven transverse rows. Caption and figure from Salvini-Plawen 1980 p.252

Whilst adult physiology may differ greatly between different groups of organisms, embryonic ontogeny may show evolutionary relationships, a fact discovered by the Darwinian philosopher Ernst Haeckel in the 19th century. Here larval ontology indicates the relation between Solengastres and Polyplacophora on the one hand, and sclerite-bearing "molluscomorphs" on the other. Salvini-Plawen 1980 provides an illustration (below) showing the similarity between larval forms of each class.

Similarly, a recent discovery of a neomenioid postlarva (Scheltema & Ivanov, 2002) shows it has six iterated, transverse groups of spicules and seven regions devoid of spicules. These resemble the shell fields in developing polyplacophorans, and spicule arrangement is compared to sclerite arrangement on the Cambrian fossils Wiwaxia corrugata and Halkieria evangelista and to the spines and shell plates of the Silurian Acaenoplax hayae. Such iterative morphogenesis was probably a common theme in earliest Cambrian halwaxiid animals and shows how these groups are related

It is still unclear to what extent the Solenogastres and Caudofoveata are specialized and to what extent they are primitive, even "living fossils". Both these groups have been hypothesized to be ancient, pre-Tethyan deep-sea forms, perhaps relics of the original Cambrian metazoan radiation (Dzik (1993), in contrast, suggests they developed from aberrant chitons like the Silurian Carnicoelus gazdzickii (id. at p.368, fig.12), but there is no evidence a shell was ever present. Barnes 1980). Salvini-Plawen (1980) sees Solenogastres as the sister group to all shelled mollusks (Testaria), while a cladistic study by Haszprunar 2000) indicates that Solenogasters are even more underived than Caudofoveata.

Descriptions

Aplacophora

Phylogeny: either paraphyletic or polyphyletic, not a clade as currently defined, evolved from Paleozoic Heloplacidae

Comments: laterally-compressed marine worm-like shellless molluscs. Some burrow in the mud or live in interstitial spaces. Many aplacophorans live among corals and feed on them. Mantle covers almost all of the body except for a ventral groove, secretes several layers of calcareous bodies. They have a preoral sense organ and a subterminal ventral mouth. They do not have ctenidia, but sometimes do have secondary gills. There are no specialized excretory organs. Some of the aplacophorans do not have a radula. They have ganglia that are fused with both ventral and dorsolateral longitudinal nerve cords. - Jack R. Holt

Links the Aplacophora Home Page - the only site dedicated to these two little-known molluscan groups, the Caudofoveata and Solenogastres; Aplacophoran good coverage; Two Aplacophoran Subclasses - short intro and illustration; The Class Neomeniomorpha and The Class Chaetodermomorpha - short intro; checked ATW050731

Solenogastres

Synonym: Neomeniomorpha

Phylogeny: Heloplacidae ::: *

Comments: Included with the Caudofoveata under the paraphyletic or, in view of current phylogenies, probably polyphyletic Aplacophora. No gills, foot is present but highly reduced; hermaphroditic (Jack R. Holt)