Parasitic Mollusca
On some families with parasitic life-strategies in the phylum Mollusca

Modern parasitology can be divided in three mainstream disciplines, each of which have their starting point from the hosts involved: the first has a medical approach as it dwells upon those types of parasitism causing disease in humans. The others study those types of parasitism causing economical damage, that is disease to domestic animals and plants. The parasites involved are either Protozoans, worms in the widest sense (Plathelminthes, Nemathelminthes), and certain Arthropod families.

While there is a wide range of literature dealing with the life-strategies of these groups, no monographic work has ever been published on the Mollusca as a phylum bearing a wide range of parasitic species and an even wider range of strategies.

The phylum Mollusca consists of approximately 150.000 species. There are eight classes:

Caudofoedata and

Solenogastres (these two are also known as Worm-Molluscs)

Polyplacophora (Chitons)

Monoplacophora (a small group of archaic deep water snails of limpet-like shape)

Gastropoda (Snails)

Bivalves (Mussels)

Scaphopoda (Elephant-Tusks)

Cephalopoda (Squids and Nautilus).
The majority of species belong to the Gastropoda and the Bivalvia. These two classes are also those with a wide range of parasites.

The term parasitism is derived from the greek para (next to...) and the latin situs (site, position), referring to favourites lying next to their sponsors on grand dinner banquets, taking advantage from a position right beside their host. The latin word parasitus has two meanings: the table-guest, and the parasite.

Parasitism refers to a variety of life-strategies which may have developed differently. The parasite is the one taking advantage from the host, whose disadvantage defines the relationship as parasitism. Those relationships which are to the advantage of both are called symbiosis. Many types of parasitism have developed from symbiosis, whenever one of the partners would start to take profit to the discadvantage of the other.
Another relationship called phoresis occurs when an animal or plant uses another species as a mean of transportation. It is mostly temporary and not to the disadvantage of the "taxi". Parasitism may be temporary or permanent, it may be closely linked to a particular type of host (obligatory), one or many hosts may be involved. We are speaking of endoparasitism when the parasite lives permanently inside the host, or ectoparasitism, when the parasite sits somewhere outside, on the surface of the host. There are temporary types of parasitism called brood-parasitism, when the host is used only for the larval development of a species. An extreme case is the so called nurse-eggs, in which only one larva develops while eating all its sisters and brothers in the same capsule (larval cannibalism).

Most real-life types of parasitism are mixtures or combinations of these various theoretical models, and examples for all of them are found in the Mollusca. In the following, a few examples will be shown.

Predators on sponges
The most primitive type of parasitism is probably that performed by many molluscs feeding on sponges. The host is not killed (or eaten up) completely, but suffers severe damage. The ancient group of slit shells (Pleurotomariidae) all feed on sponges but may also be found feeding on crinoids and other animals. The parasite/host relationship is rather loose.


Perotrochus midas feeding on a sponge

In the Cowries (Cypraeidae) a number of genera feed on sponges, too. In some cases, the relationship to the sponge as food and shelter has become obligatory. In Zoila marginata an interesting transition between the southern and the northwestern subspecies can be observed. Whereas the southern marginata feeds on smaller sponges and also moves from sponge to sponge, the northwestern ketyana permanently lives on one sponge and eats cavities into its "tissue".


Zoila marginata marginata feeding on a small sponge


Zoila marginata ketyana eating itself into a sponge

On the photo it is apparent that the sponge forms a cavity around the posterior portion of the shell, hiding the produced lateral flanges typical in this subspecies. Single specimens of ketyana were reported to have eaten a tunnel deep into a sponge, into which the animal could retreat for shelter. This is the first step from a predatory animal to a parasite living in an association with a specific host.

In the South African Cypraeovula mikeharti another step towards endoparasitism may be observed. Some individuals live under and inside the black sponge Guitarra. Also the egg-capsules are placed into cavities of the host. The closely related Cypraeovula algoensis from the same geographic area merely feeds on sponges, but the relation to the host is less obligatory.


Cypraeovula mikeharti
Parasites on Tunicates
The species of the Triviidae and Lamellariidae comprise many species of obligatory parasites whose life-cycle is closely linked with a tunicate. In the genus Trivia, the female posesses a tube like structure called ovipositor. With the aid of this orgen, egg capsules are deposited into the openings of commensal tunicates. The larvae find shelter and food in their host. Adult Triviidae may feed on sponges, algae and also tunicates.


Trivia magnidentata in a colony of tunicates. The mantle perfectly camouflages the parasite animal

Parasites on and in corals
Most species of parasitic molluscs are found in association with corals. The family Ovulidae consists of hundreds of species with a different degree of host-specificy and various transitional stages from commensalism, via brood-parasitism and host-specific parasitism.

Simnia spelta on a host coral

But there are also transitions away from Parasitism to Symbiosis. This is very interesting, as the evolutionary way that has lead into parasitism is postulated to be irreversible by many biologists. A group of animals that has evolved into parasites has undergone a variety of morphological adaptations to the host organism. A reverse evolution appears unlikely. In Pedicularia this seems to be the case, however. The group has evolved from parasitic Ovulids some twenty million years ago and now occurs with a handful of species on stylasterine corals in all temperate seas. The shells are modified individually to fit exactly to one spot on the coral, which over the years forms a cavity for the mollusc and its spawn. The shell cannot move from this position but has a long proboscis to reach far around. The former parasite no longer feeds on the polyps of the coral itself, but on the mucus that is secreted by the polyps, subsequently "cleaning" the colony.


Pedicularia sp. on a stylasterine coral


Pedicularia elegantissima, living animal, removed from host


Cavity in the coral where a Pedicularia was sitting.

In the large family of the Coralliophilidae there are many examples of coral-parasites, some of which are well known for their most bizarre shells. The endoparasitic species are living more or less buried in the tissue of the coral. Some anthozoan colonies were reported to grow larger and denser when a particular Coralliophilid is present. Such cases suggest that also here the step from parasitism towards symbiosis was performed, as the anthozoans of such colonies were all undamaged. Also in these cases, the Coralliophilid seems to feed only on the anthozoan secretions, which are rich in protein and saccharids.


Rapa sp. in a soft coral


The same individual, removed from its host

Parasites on and in Echinoderms
There are many groups of ectoparasitic snails on starfish and Sea-urchins. Most of these belong to the Opisthobranchia, such as Ophioderma, a small snail that lives on the oral area and the arms of ophiurids, with their proboscis eaten into the tissue of the host.


Ophioderma sp. on its host

Other limpet like shells (Neritina) live half-buried in the arms of the blue Linckia-starfish.

But there are also species of uncertain position that live inside the tissue of Sea-cucumber. The veligers of the genera Enterocolax and Enteroxenon attach themselves to the skin of their host and actively drill into the flesh, leaving their shells. The adult parasite is hardly separable from the tissue of the host. These extreme cases of endoparasitism are poorly investigated, but they show that some molluscs have found ways to this highest level of parasitism.

Parasitism on and in fishes
The larvae of fresh water mussels face the problem that the current would carry them into the sea. The populations would be swept out of the rivers. The problem was solved by using fish as vector against the current. The glochidian-larvae of almost all freshwater mussels spend the time of their development-circle in the gills of fish moving them upstream. The European freshwater pearl-mussel Margaritifera is dependent on the presence of the river-trout Salmo trutta fario. The introduction of the larger, faster growing and easy to breed rainbow-trout Oncorhynchus mykiss to European rivers and lakes, as well as pollution has depleated the populations of the river-trout and brought the pearl-mussel near extinction.

But there is also another bizarre kind of parasitism that was discovered only a few years ago, involwing parrot fish and the snails of the buccinid genus Colubraria. For a long time it was unknown what these shells ate as they have no radula but a very long proboscis. This is one of the spectacular photos of a sleeping parrot fish being stung into its mouth by Colubraria obscura, now known as the "vampire-snail".

Colubraria obscura stinging a sleeping parrot-fish

The Cystiscid Kogomea sucking blood above the fin of a parrotfish. Photo Courtesy Scott Johnson

Parasitism among relatives- "nurse-eggs"
As mentioned above, many gastropods whose larval development lacks a free swimming stage perform an interesting and apparently effective strategy. A single egg capsule contains many hundred larvae, only one of which develops further, eating the others in the capsule.


An egg-capsule of Cypraeovula algoensis. Only one specimen has developed to the pediveliger stage.

In the Plathelminthes the tendency towards parasitism is interpreted as a strategy to compensate the lack of a protective shell or cuticula. The main function of the host is to find shelter from outside influences. In the Nemathelminthes the protective, acid-resistant cuticula has encouraged parasitism simply because Nematods dont seem to care in what medium they are as long as there is a food supply. In the Arthropod groups with parasitic forms there are two aspects that have supported parasitic lifes. First, the development of most effective tools to suck, sting and bite, and second, the extreme selective pressure by the success of this group, and the resulting urge to find new ecological niches.

In the Mollusca, three main factors have lead to parasitism. First, to overcome the problem of immobility and the consequenting danger of populations being swept out of rivers (freshwater mussels). This immobility might have lead to phoretic stages that later lead to parasitism. It is probable that most parasitic relationships with Echinoderms result from phoresis. Second, the pressure to find ecological niches in the densely populated marine biocoenosis. And third, the potential to be parasitic by having developed organs variable enough to gather all kinds of food and by body-shapes capable of hiding amongst- and clinging to- the potential hosts.

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This was uploaded in 2001. Subsequently I was asked to write a book chapter on Mollusca in: Marine Parasitology (2005) Csiro Publishing, Melbourne, p. 240ff