Order Pyramidellacea

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Family: Pyramidellidae

There are a wide variety of snails that feed on clams and oysters. The majority of these belong to the family Pyramidellidae, of which there are at least 1000 species in the Pacific alone (Cumming, 1988). These snails are generally quite small reaching a maximum length of 2-4 mm (0.08-0.16 in.), and look like small grains of rice. The majority of what is known about these snails comes from the species that are commonly found feeding on oysters and clams in commercial operations. Very little is known about how many species may affect tridacnid clams. Tathrella iredalei and Pyrgiscus sp. are two species that have been isolated in Australian tridacnid clam farms (Cumming, 1988). J. Sprung observed a small snail which fed on stony corals, using a long stylet to suck up fluids as in other pyramidellidae. It apparently causes no harm to the coral as a single parasite, but if allowed to multiply within the aquarium it could become a pest.

The typical feeding location of Tathrella snails, along the upper shell edge of a Tridacna crocea. J.C. Delbeek.


Studies of Pyrgiscus have shown them to be veiy serious pests of tridacnids in aquaculture systems. They can exhibit an extremely rapid population growth when in land-based seawater tanks or in trays raised above the substrate in the wild (Cumming, 1988).

The protrusible proboscis of one of these Tathrella sp. snails is evident as it pierces the mantle of a

7". derasa clam. Note the egg mass on the shell of the clam (far right). The upside-down vase like object above and to the right of the snails is a harmless common syconoid sponge that often multiplies in reef aquariums. S.W. Michael.


Reproduction of these simultaneous hermaphrodites is quite rapid, with a 2.5 mm (O.l in.) snail capable of producing 2-3 egg masses/day, each with up to 120 eggs (Cumming, 1988). The eggs are held in a jelly-like mass, often with several egg masses close together on the clam shell. The eggs hatch after 10 days and the larvae are capable of feeding within 3 days of hatching (Cumming, 1988). The young tend to remain on the clam they hatch out on. As a result, all it takes is two of these snails to cause a rapid population growth on a single clam.

Pyramidellid snails feed mainly at night. During the day they stay near the base of the clam or, in species with large scutes such as Tridacna squamosa, between the scutes, out of direct sunlight. At night they migrate up towards the lip of the shell where they extend their proboscis and, using their needle-like stylus, poke a hole into the mantle of the clam. They then suck out fluid contents from the mantle. When left unchecked, large numbers of these snails can easily kill a clam within days or months, depending on the size of the clam.

Removal: Interestingly, these snails are relatively rare in the wild, which indicates that some form of namral biological control must be in place. One such natural predator is the portunid crab Thalamita sima. Small specimens of these crabs (1-1.5 cm; 0.4-0.6 in.) are often found in clam beds and have been used to control pyramidellid snails in aquaculture projects with some success, however, they have also been found to feed on small (4 mm; 1.6 in.) tridacnid clams (Cumming, 1988). Certain wrasse species are more promising predators. Members of the genus Halichoeres, specifically

Halichoeres melanurus, will feed on parasitic snails of tridacnid clams. S.W. Michael.

Halichoeres MelanurusPseudocheilinus Tetrataenia

The Six-Line wrasse, Pseudocheilinus hexataenia, will feed on parasitic snails {Tathrella sp.) of tridacnid clams. J.C. Delbeek.

H. melanurus and H. chloropterus, have been observed to feed on these snails in the wild (B. Carlson, pers. comm.). Also the Six-lined Wrasse, Pseudocheilinus hexataenia, has been found to eat pyramidellid snails in the aquarium as has the Four-lined Wrasse P. tetrataenia. It is likely that many more wrasse species will be found to be predators of these snails in the aquarium. However, unless the snails are exposed on the shell during the day, the fish will not be able to find them. Therefore, nocturnal extraction by the aquarist may still be required.

Phylum: Annelida

Class: Polychaeta

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The COMPLETE guide to Aquariums

The COMPLETE guide to Aquariums

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  • yonas kifle
    What fish tank parasite looks like rice?
    8 years ago

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