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Index of flora
Index of fauna
SYMBIOSIS:Living Together—Parasitism
Pea Crab Family Pinnotheridae
and Bivalves

People who have eaten bivalves (e.g., the blood cockle, Anadara) (Family Arcidae) may have encountered a small pea-sized crab inside the shell. This is the pea crab (Pinnotheres). The crab not only derives shelter and protection from the shellfish, it also shares the food gathered by the host. Owing to the size of the crab, the host however, does not suffer significantly.

Two are shown, a hairy species found in Atrina (Family Pinnidae) and the common white species in Anadara. (Crab shell width up to l cm).
Photo by Tan Bee Hong
Pea crab in Anadara

Pea crab in Atrina
Parasitic Barnacles Order Rhizocephala
and Crab

Parasitic barnacles have become so modified that they frequently resemble fungi. The larvae of these parasites resemble those of the typical barnacles, and is the only clue to their origin. In Thompsonia, the adult ramifies through a host (in this case the flower crab, Portunus pelagicus) like a root system, and later breaks through the joints of the host shell to produce many brownish-yellow egg-sacs. On bursting, the free-swimming larvae are released.

Sacculina also branches through the tissues of its host (here the common shore crab Leptodius exaratus) but produces only one large brood-sac externally under the host's abdomen, containing its many eggs. The interesting feature is that the parasite makes the host crab, if it's a male, appear more like a female, in effect, "unsexing" it!
Photo by Tan Bee Hong
Thompsonia on Portunus

Photo by Tan Bee Hong
Sacculina on Leptodius

Photo by Kang NeeCymothoid Isopod
and Fish

These isopods resemble free-living ones but have specialised piercing mouthparts for sucking blood and terminal hooks on their limbs for clamping onto their fish host. Shown here is a cymothoid isopod on a whiting (Sillago). The isopod has attached to the lower jaw of the fish where it has direct access to the soft vascular (blood) tissue.

Photo by Tan Bee HongBopyrid Isopod
and Snapping Shrimp

The most specialised of the parasitic isopods are the bopyrids, which attach themselves to the gills of crustaceans, feeding on the blood. They cause characteristic swellings on the shell of prawns, shrimps and crabs, but are not poisonous to man if eaten.
Shore environment
From A Guide to Seashore Life by Dr Leo W H Tan and Peter K L Ng
Published by the Singapore Science Centre and sponsored by BP

@Raffles Museum of Biodiversity Research and Singapore Science Centre