Aplacophorans

Photo: A caudofoveatan (species unknown) seen in the Antarctic. (Photo by S. Piraino [University of Lecce]. Reproduced by permission.)

Evolution and systematics

The class Aplacophora contains two subclasses: Neome-niomorpha (also called Solenogastres) and Chaetodermo-morpha (also called Caudofoveata). Most neomenioids creep by means of a narrow foot with a ventral groove that begins as a pedal pit toward the front of the animal. They have a sensory vestibule above the mouth; a single midgut organ combining stomach and digestive gland; and serial sets of muscle bands running along their sides and lower surface. Neomenioids are simultaneous hermaphrodites; they also lack ctenidia in their mantle cavities. A ctenidium is a finger-shaped or comblike structure that functions in respiration. The subclass of Neomeniomorpha comprises three orders (Pholidoskepia, Neomeniomorpha, and Cavibelonia); 24 families; 75 genera; and fewer than 250 species. The subclass Chaetodermomorpha contains six families and 15 genera. Aplacophorans in this subclass have a midgut separated into a stomach and a digestive organ, and one pair of ctenidia in their mantle cavities.

It is uncertain to what extent aplacophorans are specialized and to what extent they are primitive mollusks, but there is no evidence that they ever had shells. Their specialized features include the reduction or loss of the foot; the absence of a shell; sometimes the lack of a radula (a specialized organ unique to mollusks that allows them to scrape food from the ocean floor); and modifications of the nephridia (simple organs for excreting wastes) in certain genera to form accessory sexual organs. The possession of well-defined cerebral and pleural ganglia (groups of nerve cells) indicates that the Apla-cophora are more advanced than the Polyplacophora mollusks in this respect at least. It seems probable that aplacophorans represent a secondary simplification of an ancestral form. If this hypothesis is accurate, however, the form and location of the mantle cavity in present-day aplacophorans tells researchers little about the condition of the remote ancestor. Aplacophorans have little in common with chitons (small armor-plated mollusks), although in the past the two groups were placed together in the class Amphineura.

Physical characteristics

Aplacophorans, which are also called solenogasters, are worm-shaped mollusks covered with spicules or sharp needle-like projections. The body shape varies from almost spherical to elongated and slender. These mollusks are usually less than 2 in (5 cm) in length, but adult individuals may vary from 0.039-0.078 in (1-2 mm) to 3.9 in (10 cm) or more in length.

The exterior of an aplacophoran may be spiny, smooth, or rough. The head is poorly developed, and the typical mollusk shell and foot are absent. The exoskeleton is represented only by a cuticular (horny secreted) layer that bears spicules in a variety of forms. The spicules and integument (covering) together form a character that links genera or families in this class to one another. Most aplacophorans have some specialized spicules at the entrance to the mantle cavity; these are presumably used in copulation. The cuticle and epidermis may be either thick or thin relative to the size of the species: a thick cuticle may occur together with a thin epidermis; a thin cuticle may occur with a thick epidermis; or they may be the same thickness. Glandular cells on the epidermis known as papillae may have either long stalks or no stalks at all.

Aplacophorans have a midventral longitudinal groove containing one or more ridges, which are similar in structure to the foot of other mollusks. The mantle covers the upper surface, the sides, and the greater part of the lower surface of the animal. A large gland that secretes mucus opens into the groove toward the front.

The mouth at the front of the animal opens into a muscular pharynx lined by a thick cuticle. The pharynx typically receives the products of one or two pairs of salivary glands and the radula sac. Some genera lack salivary glands. Neomenioid species creep by ciliary action of the "foot" along a sticky track of mucus produced from the ciliated, eversible pedal pit at the anterior end of the pedal groove. Both the pedal groove and the pedal pit are supplied by many mucus-secreting glands. The radula is highly variable in form. It is situated where the pharynx joins the midgut unless an esophagus is present; it may have two teeth per row, one tooth per row, or many teeth per row. The radula is lacking in 20% of known species.

The posterior end of the body contains a cavity into which one or two gametopores open as well as the anus, the copu-latory spicule sacs, and the folds of respiratory tissue or papillae. The posterior cavity is believed to represent a mantle cavity. Burrowing species have a pair of gills. In Neomenia and in several other genera there is a circlet of laminar gills in the mantle cavity; in other genera, however, there are no gills.

Distribution

Aplacophorans are found in all oceans of the world; some genera have worldwide distribution. Although they have been sampled at depths ranging from 16 to 17,390 ft (5-5,300 m), the greatest diversity of species occurs at depths greater than 656 ft (200 m).

Habitat

Neomenioids live on hydroids, corals, or surface sediment. Caudofoveatans construct burrows in marine sediments, which they inhabit head downward.

Behavior

Nothing is known about the behavior of aplacophorans.

Feeding ecology and diet

Neomenioids feed on cnidarians—stony and soft corals, hydrozoans, zooantharians, or gorgonians. Some species prey only on specific cnidarians. Caudofoveatans ingest sediment or may be selective carnivores or scavengers. They feed mostly on faraminifera.

Reproductive biology

Aplacophorans are hermaphrodites and have paired gonads. Copulation probably occurs in those with the former condition, and spawning in the latter. Researchers have inferred from the presence of seminal receptacles, the structure of introsperm (sperm that never contact the water), and observation of living specimens of Epimenia australis that fertilization takes place internally.

Conservation status

No species are listed by the IUCN.

Significance to humans

Aplacophorans are used in scientific research, especially research into the evolutionary origins of mollusks.

Aplacophorans

1. Prochaetoderma yongei; 2. Chaetoderma argenteum 3. Chevroderma turnerae; 4. Spiomenia spiculata; 5. Helicoradomeria juani; 6. Epimenia australis. (Illustration by Bruce Worden and John Megahan)

No common name

Epimenia australis

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