All known modern forms are shell-less: only some extinct primitive forms possessed
valves. The group comprises the two clades
Solenogastres (Neomeniomorpha) and
Caudofoveata (Chaetodermomorpha), which between them contain 28 families and about 320
species. The aplacophorans are traditionally considered ancestral to the other mollusc classes. However, the relationship between the two aplacophoran groups and to the other molluscan classes and to each other is as yet unclear.
Aplacophorans are cylindrical and worm-like in form, and most very small, being no longer than 5 centimetres (2 in); some species, however, can reach a length of 30 centimetres (12 in).
Habitat
Caudofoveates generally burrow into the
substrate while solenogasters are usually
epibenthic. Both taxa are most common in water regions deeper than 20 metres (66 ft) where some species may reach densities up to four or five specimens per square metre (three or four per square yard). Solenogasters are typically
carnivores feeding on
cnidarians or sometimes
annelids or other taxa while caudofoveates are mostly
detritovores or feed on
foraminiferans.[2]
Description
Aplacophorans are worm-like animals, with little resemblance to most other molluscs. They have no
shell, although small calcified
spicules are embedded in the skin; these spicules are occasionally coated with an organic pellicle that is presumably secreted by microvilli.[3] Caudofoveates lack a foot while solenogasters have a narrow foot which lacks intrinsic musculature. The
mantle cavity is reduced into a simple
cloaca, into which the
anus and excretory organs empty, and is located at the posterior of the animal. The head is rudimentary, and has no eyes or tentacles.[4] The cuticle of both subclasses is chitinous,[5] and has an irregular texture.[6] Spines bear an organic matrix.[7]Sclerites can be hollow or solid; the void within the sclerites of some species fills during growth.[6] Sclerites generally form within the cuticle, protruding through when they are fully grown.[6] They probably start life as amorphous calcium carbonate, which the organic matrix coaxes into an
aragonitic habit as the spines mature.[6] Sclerites can be shaped as simple spines, straight, curved, keeled, striated or hooked; or as cupped blades; more complex arrangements are common in copulatory spicules.[8][9] In several species of solenogaster, sclerites change morphology during growth; young specimens might bear flat, solid, scale-like sclerites, to be replaced with longer hollow spine-like sclerites in adults.[10]
The relationship with other molluscs, however, is apparent from some features of the digestive system; aplacophorans possess both a
radula and a
style. A variety of radular forms and functions exist.[11] Solenogasters are
hermaphroditic and assumed to have
internal fertilization, in contrast to caudofoveates which have two sexes, and reproduce by
external fertilization. During development, the mantle cavity of the larva curls up and closes, creating the worm-like form of the adult.[4] Caudofoveates also differ from Solenogasters in having a head shield and a body that is differentiated into three sections.[12]
Taxonomy and evolution
This class was once classified as
sea cucumbers in the
echinoderms. In 1987, they were officially recognized as molluscs and given their own class. It consists of two
clades: the
Solenogastres and the
Caudofoveata. It has been considered polyphyletic[13] but more recent molecular evidence supports it as a monophyletic clade.[14][15]
The affinities of Aplacophorans have long been uncertain. Molecular and fossil evidence seemed to put Aplacophorans in the clade
Aculifera, as a sister group to
Polyplacophora.[14][15][16][17][18] The discovery of Kulindroplax in 2012, a fossil aplacophoran with a polyplacophoran-like armour, strongly supports this hypothesis, and shows that aplacophorans evolved from progenitors that bore valves.[19]
^Scheltema, A. H.; Schander, C; Kocot, KM (2012). "Hard and soft anatomy in two genera of Dondersiidae (Mollusca, Aplacophora, Solenogastres)". The Biological Bulletin. 222 (3): 233–69.
doi:
10.1086/bblv222n3p233.
hdl:1912/5273.
PMID22815372.
S2CID28029055.
^Todt, C.; Von Salvini-Plawen, L. (2005). "The digestive tract of Helicoradomenia (Solenogastres, Mollusca), aplacophoran molluscs from the hydrothermal vents of the East Pacific Rise". Invertebrate Biology. 124 (3): 230–253.
doi:
10.1111/j.1744-7410.2005.00023.x.
^
abSmith, S. A.; Wilson, N. G.; Goetz, F. E.; Feehery, C.; Andrade, S. N. C. S.; Rouse, G. W.; Giribet, G.; Dunn, C. W. (2011). "Resolving the evolutionary relationships of molluscs with phylogenomic tools". Nature. 480 (7377): 364–367.
Bibcode:
2011Natur.480..364S.
doi:
10.1038/nature10526.
PMID22031330.
S2CID4321076.