Cocculiniformia

(Deep-sea limpets)

Phylum Mollusca

Class Gastropoda

Order Cocculiniformia

Number of families 2

Thumbnail description
Small white limpets that occur on waterlogged wood, whale bone, and cephalopod beaks in the deep sea

Evolution and systematics

All living Cocculiniformia have cap-shaped shells. The earliest cocculiniform limpets are found in Tertiary sediments of New Zealand where they are associated with fossilized wood. They are likely descended from coiled snails, like other living limpets, but like the patellogastropods, possible ancestors have not yet been identified in the fossil record.

Living cocculiniform limpets were unknown until the advent of deep-sea exploration in the seventeenth century. At the time, a variety of substrates were recovered by bottom dredges, including cephalopod beaks, waterlogged wood, fish and whale bones, and the egg cases of sharks and skates; all of these substrates were found to have small white limpets living on their surfaces. Almost every species was placed in different genera and families based on their gill morphology and digestive systems, which were also loosely correlated with the different substrates from which these limpets came. In the late twentieth century, these families were grouped into the Coccuiliniformia. However, subsequent phylogenetic analyses based on morphology and molecules showed that there were actually two convergent groups represented. This resulted in the Cocculiniformia being restricted to the Cocculinoidea, while the Lepetelloidea were transferred to the Vetigastropoda, where they represent an early branch in that taxon. However, the placement of the Cocculinoidea among other gastropods remains problematic. Some workers would place it near the base of the gastropod tree, thereby representing an early offshoot in gastropod evolution, while other placements include a possible relationship to the Neritopsina.

The order Cocculiniformia includes two families: the Cocculinidae and the Bathysciadiidae.

Physical characteristics

All living cocculiniforms have cap-shaped shells. The apex of the shell is typically situated at the center or nearer the posterior end of the shell. Shells are sculpted with concentric growth lines, and in some species, additional fine radial threads or beads extend from the apex to the shell margin, while in other species, the surface appears cancellate. The shell aperture is typically oval. The inner surface of the shell bears a horseshoe-shaped muscle scar that opens anteriorly where the head is located. The head is not very flexible and the mouth opens ventrally for grazing on the substrate. Copulatory organs are present and are typically part of the right cephalic tentacle. The Cocculiniform radula is rhipdoglossate, as in the Vetigastropoda and Neritopsina. The Cocculiniform gill consists of a vestigial pseudoplicate structure that does not resemble the typical molluscan gill or ctendium.

Cocculiniformia range in size from about 0.2–0.6 in (5–15 mm) in length, and are white in color and covered with a periostracum.

Distribution

Members of the Cocculiniformia are distributed throughout the world's oceans at bathyal and abyssal depths.

Habitat

Cocculiniformia are associated with three substrates in the deep sea: cephalopod beaks, waterlogged wood, and whale bone.

Behavior

Because of the deep-sea habitat of members of this group, little is known of their behavior. The presence of copulatory structures in these simultaneous hermaphrodites suggests a mating system for reciprocal cross-fertilization, which in other gastropods (such as the Heterobranchia) occurs in conjunction with ritualized courting behavior.

Feeding ecology and diet

Cocculiniformia that live on cephalopod beaks are unlikely to be receiving nutrition directly from the beak's chitin substrate. It is more likely that they are feeding and utilizing nutrients from microbes and fungi that are breaking down the chitin on the sea bottom. A similar situation is found in the wood-dwelling cocculiniforms that are found on decaying rather than fresh wood, which is also rich in microbes. Cocculiniforms associated with whale bones are feeding on the sulfate-reducing bacteria that oxidized the lipids found in the bones.

Predators of cocculiniform limpets are not known.

Reproductive biology

All members of the Cocculinoidea studied thus far have been found to be simultaneous hermaphrodites with distinct regions of the gonad producing eggs and sperm. The eggs are rich in yolk and development is thought to be lecithotrophic as in other basal gastropods such as the Patellogastropoda and the Vetigastropoda. Because of the ephemeral nature of the substrates used by members of the Cocculinifomia, larval duration must be sufficient for the larvae to mature and locate suitable habitat on the deep-sea bottom. Such a strategy often incorporates relatively rapid maturation, followed by an extended period of larval competence for locating suitable substrates. This strategy is also often associated with year-round reproduction to maximize the probability of locating patchy habitats. Moreover, in the deep sea, seasonal clues such as day length, water temperature, storm periods, food availability, etc. are nonexistent.

Conservation status

Conservation status of members of the Cocculinifomia is not known. While their habitat at first estimation would appear to be incredibly sparse and rare given the vastness of the deep-sea bottom, the fact that another group of gastropods (the Lepetelloidea in the Vetigastropoda) has also evolved to utilize similar substrates in the deep sea suggests that this first impression is incorrect. However, human interactions with the substrates may pose a serious threat as well as enhancement to these species. For example, increased logging and near-shore construction have undoubtedly increased the amount of wood in the ocean, and the end of commercial whaling will provide an increase in the amount of whale falls in the deep sea. Such an increase has likely increased the amount of substrate available to the Cocculinoidea. In contrast, fisheries' over-exploitation of squid stocks could jeopardize the habitat of the Bathysciadiidae by reducing the number of squid beaks that accumulate on the bottom.

No Cocculiniformia species are listed on the IUCN Red List.

Significance to humans

Because of their occurrence in the deep sea, the Cocculinoidea have no significant interaction with humans.

Species accounts

List of Species

Japanese deep-sea limpet

Cocculina japonica

family

Cocculliidae

taxonomy

Cocculina japonica Dall, 1907, Sea of Japan.

other common names

None known.

physical characteristics

Shell aperture oval with apex close to center; shell sculpture of concentric growth lines with radial punctuations extending from apex to shell margin. Shell white-gray covered with thin periostracum. Interior of shell whitish with inconspicuous muscle scar. Length 0.15–0.31 in (4–8 mm).

distribution

Found at 164–2,460 ft (50–750 m). Northwestern Pacific Ocean and Sea of Japan.

habitat

Waterlogged wood.

behavior

Nothing is known.

feeding ecology and diet

Feeds on microbes associated with the breaking down of wood on the deep-sea bottom.

reproductive biology

Simultaneous hermaphrodite with copulatory structure adjacent to right cephalic tentacle.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Resources

Books

Haszprunar, G. "Comparative Anatomy of Cocculiniform Gastropods and Its Bearing on Archaeogastropod Systematics." In Prosobranch Phylogeny. Malacacogical Review, Supplement 4, edited by W. F. Ponder. New York: Academic Press, 1988.

Periodicals

Marshall, B. A. "Recent and Tertiary Cocculinidae and Pseudococculinidae (Mollusca: Gastropoda) from New Zealand and New South Wales." New Zealand Journal of Zoology 12 (1985): 505–546.

Strong, E. A., M. G. Harasewych, and G. Haszprunar. "Phylogeny of the Coccilinoidea (Mollusca, Gastropoda)." Invertebrate Biology 122 (2003): 114–125.

David Lindberg, PhD