Aplacophora

(Aplacophorans)

Phylum Mollusca

Class Aplacophora

Number of families 30

Thumbnail description
Vermiform (worm-shaped) marine mollusks lacking shells and living in the zone between the seashore and the edge of the continental shelf

Evolution and systematics

The class Aplacophora contains two subclasses: Neomeniomorpha (also called Solenogastres) and Chaetodermomorpha (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 Aplacophora 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 copulatory 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.

Species accounts

List of Species

No common name

Epimenia australis

order

Cavibelonia

family

Epimeniidae

taxonomy

Epimenia australis Thiele, 1897, Timor Sea, at a depth of 590 ft (180 m).

other common names

None known.

physical characteristics

Epimenia australis may grow as long as 4.33 in (11 cm) and as wide as 0.19–0.23 in (5–6 mm). There are irregular iridescent bright green or blue patches against a reddish brown background along the upper surface and sides of the organism, with raised reticulate (netlike) ridges and verrucae (wartlike projections) between the patches. Epimenia australis has spicules lying parallel to the body; the spicules cross each other running from the front of the lower surface to the rear of the upper surface and from the rear of the lower surface to the front of the upper surface. The cuticle is thicker within the ridges and verrucae, and the spicules stand entirely upright, extending in thick clumps beyond the cuticle. The head end is narrower than the posterior end when the organism is viewed from above; the posterior end is somewhat flattened dorsoventrally and is usually bent downward into a hook. The part of the head in front of the somewhat protruded pedal pit is lifted off the substrate, with the vestibule held open. The animals are stiff to handle. Epimenia australis has a radula with about 200 rows of teeth on a distinct pedestal; a pedal pit with two joined lobes; and several footfolds toward the front, decreasing to three and then to one toward the rear of the animal.

distribution

Found throughout the Indo-Pacific tropics and subtropics.

habitat

Epimenia australis is found close to shore with its cnidarian prey on the undersurface of sheltered coral slabs. It usually forms tightly tangled masses of as many as 11 individuals.

behavior

Nothing is known.

feeding ecology and diet

Epimenia australis feeds on cnidarians.

reproductive biology

Individual members of this species are hermaphrodites. Fertilization takes place internally.

conservation status

Not listed by the IUCN.

significance to humans

Useful in scientific research.

No common name

Helicoradomenia juani

order

Cavibelonia

taxonomy

Helicoradomenia juani Scheltema and Kuzirian, 1991, Endeavour Segment, Juan de Fuca Ridge, 47°57′N, 129°04′W, at a depth of 7,382 ft (2,250 m).

other common names

None known.

physical characteristics

The body shape is somewhat elongated, with a fuzzy appearance. The organism may grow as long as 0.2 in (5 mm). It is narrowest toward its front end, with a sensory pit on the upper surface; its spicules are widest at the base and point backward, varying in shape from short, wide, and recurved, to long, slender and curved. There are two spicules in the pocket of each copulatory spicule. The mouth lies at the closer end of the vestibule; the pedal pit is large and often protrudes. The cuticle is thin. H. juani has a large radula that makes a single turn into its ventral pockets. The radula has 34–35 rows of teeth with five or six denticles (small toothlike projections).

distribution

Explorer, Juan de Fuca, and Gorda Ridges, at a depth of 5,906–10,732 ft (1,800–3,271 m).

habitat

Hydrothermal vents.

behavior

Nothing is known.

feeding ecology and diet

Helicoradomenia juani differs from most neomenioids in that it does not eat cnidarians; it feeds instead on organic matter.

reproductive biology

Adult individuals of H. juani are hermaphrodites.

conservation status

Not listed by the IUCN.

significance to humans

Scientific research.

No common name

Spiomenia spiculata

order

Cavibelonia

family

Simrothiellidae

taxonomy

Spiomenia spiculata Arnofsky, 2000, West European Basin, 55°7.7 min N, 12°52.5 min W, at a depth of 9,505 ft (2,897 m).

other common names

None known.

physical characteristics

Spiomenia spiculata has a curved body that tapers slightly toward the posterior end; it is usually widest at its midsection. It has a dorsofrontal sensory pit, dorsoterminal sense organ, and a mantle cavity opening partly closed off by spicules. The longest epidermal spicules on S. spiculata are found toward the rear of the organism near the opening to the mantle cavity. There are nine different types of spicules, including two types of copulatory spicules that occur in paired groups protruding through the mantle cavity. Several accessory copulatory spicules are grouped near the opening to the mantle cavity. The pedal groove contains three types of solid spicules. The radula of S. spiculata makes a single turn into paired anteroventral radular pockets; there are 22–25 teeth with 22–23 denticles per tooth. In addition, this aplacophoran has unusually large paired salivary glands on its upper surface that empty into the esophagus where it joins the radular sac.

distribution

West European Basin at a depth of 6,560–13,120 ft (2,000–4,000 m).

habitat

Nothing is known.

behavior

Nothing is known.

feeding ecology and diet

Spiomenia spiculata does not appear to depend on cnidarians as a food source. It feeds on diatoms and unidentified spicules that resemble those found in sponges.

reproductive biology

Members of this species are hermaphrodites.

conservation status

Not listed by the IUCN.

significance to humans

Spiomenia spiculata is used in scientific research.

No common name

Chaetoderma argenteum

order

Chaetodermatida

family

Chaetodermatidae

taxonomy

Chaetoderma argenteum Heath, 1911, southwestern Vancouver Island, British Columbia, 328–656 ft (100–200 m).

other common names

None known.

physical characteristics

This species has four distinct body regions: the anterium (with the oral shield), the anterior trunk, the posterior trunk, and the

posterium. This species typically has an anterior trunk either equal in diameter to, or narrower than, the neck and longer than the posterior trunk. Dense erect spicules on anterior trunk; more sparse flat spicules against posterior trunk. The oral shield is unpaired and cleft dorsally. Posteriorly the dorsoterminal sense organ is obvious and about 0.04 in (1 mm) in length in large specimens. The spicules of the posterium do not form a terminal ring. Greatest length to more than 1.6 in (40 mm). Radula with a large, cone-shaped cuticular piece and a single pair of denticles. Denticles are rather small. The cuticular dome extends proximally one-half the length of cone.

distribution

Found in the northeast Pacific, south of Monterey Bay in the Santa Maria Basin.

habitat

In sediment of fine silt and clay at depths between 328 ft (100m) and 1,969 ft (600 m) in the sea.

behavior

Nothing is known.

feeding ecology and diet

Nothing is known.

reproductive biology

Nothing is known.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Chevroderma turnerae

order

Chaetodermatida

family

Prochaetodermatidae

taxonomy

Chevroderma turnerae Scheltema, 1985, North American Basin, 35°50.0′N, 64°57.5′W, 15,857 ft (4,833 m).

other common names

None known.

physical characteristics

Chevroderma turnerae is large for a prochaetodermatid species, with total body length of 0.11–0.20 in (2.8–5 mm). Body has three distinct regions reflecting internal anatomy: the anterium, the trunk, and the posterium. Trunk diameter averages 0.02 in (0.5 mm), with greatest diameter of 0.03 in (0.8 mm). The posterium is long, from two-fifth to nearly one-half the body length; it averages 0.04–0.06 in (1.1–1.6 mm) in length, with greatest length of 0.11 in (2.9 mm), and 0.01 in (0.3 mm) in diameter, with greatest diameter of 0.02 in (0.5 mm). The spicules of the trunk meet at a distinct angle along the dorsal midline. The long spicule blades of the posterium extend out from the body. There are two rows of prominent oral shield spicules; the oral shield is distinctively large and is divided into two lateral parts. The margin of the cloaca in lateral view is slanted. Radula and jaws large, teeth up to 0.00551 in (140 µm) long, jaws up to 0.0276 in (700 µm) long, and central plate long, up to 0.00197 in (50 µm), narrow and curved.

distribution

Chevroderma turnerae is a cosmopolitan abyssal species of the Atlantic basins, absent only in Guyana and Iberian Basins. Its depth range is also great, from a little over 6,890 ft (2,100 m) to 17,090 ft (5,208 m). It is most commonly found at depths greater than 9,840 ft (3,000 m) except in the Argentine basin, where it was taken in largest numbers at depths less than 9,840 ft (3,000 m).

habitat

Muddy surfaces at great depths in the sea.

behavior

Nothing is known.

feeding ecology and diet

Thought to be omnivores, feeding on a wide variety of organic material.

reproductive biology

Nothing is known.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Prochaetoderma yongei

order

Chaetodermatida

family

Prochaetodermatidae

taxonomy

Prochaetoderma yongei Scheltema, 1985, North American Basin, 39°46.5′N, 70°43.3′W, 4,364–4,823 ft (1,330–1,470 m).

other common names

None known.

physical characteristics

Prochaetoderma yongei is a small, slender, translucent species with flat-lying spicules oriented anterior-posterior except where they diverge along the ventral midline. Body has three distinct regions reflecting the internal anatomy: the anterium (with the oral shield), the trunk, and the posterium. Total body length averages 0.06–0.11 in (1.5–2.8 mm). Trunk diameter averages 0.01–0.02 in (0.3–0.4 mm), with greatest diameter of 0.02 in (0.6 mm). The posterium is about one-quarter the total length. An opaque, thickened patch of cuticle at the ventral junction of the trunk and posterium is characteristic of the species. Oral shield is small, and is divided into two lateral shields with indistinct spicules. The cloaca is rounded. Solid spicules are flat with the base shorter than the blade; blade is broad and triangular with median keel and sharp distal point. There is a pair of large, cutilar jaws and a small distichous radula. The central radula plate is short.

distribution

Very widely distributed on the continental slope between 2,625 ft (800 m) and 6,560 ft (2,000 m) in the northwestern and eastern Atlantic.

habitat

Muddy surfaces at great depths in the sea.

behavior

Nothing is known.

feeding ecology and diet

Thought to be omnivores, feeding on a wide variety of organic material.

reproductive biology

Nothing is known.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Resources

Books

Barnes, Robert D. Invertebrate Zoology. 4th ed. Philadelphia: Saunders College Publishing, 1980.

Beesley, Pamela L., Graham J. B. Ross, and Alice Wells. Mollusca—The Southern Synthesis, Part A. Canberra: Australian Biological Resources Study, 1998.

Geise, Arthur C., and John S. Pearse. Reproduction of Marine Invertebrates. Vol. V, Mollusks: Pelecypods and Lesser Classes. London: Academic Press, 1975.

Purchon, R. D. The Biology of Mollusca. Oxford: Pergamon Press, Ltd., 1968.

Periodicals

Arnofsky, Pamela. "Spiomenia spiculata, Gen. et sp. nov (Aplacophora: Neomeniomorpha) Collected from the Deep Water of the West European Basin." The Veliger 43, no. 2 (2000): 110–117.

Scheltema, Amélie H. "The Aplacophoran Family Procahetodermatidae in the North American Basin, including Chevroderma n.g. and Spathoderma n. g. (Mollusca: Chaetodermomorpha)." Biology Bulletin 169, no. 2 (1985): 484–529.

Scheltema, Amélia H., John Buckland-Nicks, and Fu-Chiang Chia. "Chaetoderma argenteum Heath, a Northeastern Pacific Aplacophoran Mollusc Redescribed (Chaetodermomorpha: Chaetodermatidae)." The Veliger 34, no. 2 (1998): 204–213.

Scheltema, Amélie H., and M. Jebb. "Natural History of a Solenogaster Mollusc from Papua New Guinea, Epimenia australis (Thiele) (Aplacophora: Neomeniomorpha." Journal of Natural History 28 (1994): 1297–1318.

Scheltema, Amélie H., and Alan M. Kuzirian. "Helicoradomenia juanigen, Gen. et sp. nov., a Pacific Hydrothermal Vent Aplacophoran (Mollusca: Neomeniomorpha)." The Veliger 34, no. 2 (1991): 195–203.

Scheltema, Amélie H., and C. Schander. "Discrimination and Phylogeny of Solenogaster Species Through the Morphology of Hard Parts (Mollusca, Aplacophora, Neomeniomorpha)." Biology Bulletin 198 (2000): 121–151.

Treece, Granvil D. "Four New Records of Aplacophorous Mollusks from the Gulf of Mexico." Bulletin of Marine Science 29, no. 3 (1979): 344–364.

Tatiana Amabile de Campos, MSc