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).
Pea crab in Anadara
Pea crab in Atrina
Barnacles Order Rhizocephala
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!
Thompsonia on Portunus
Sacculina on Leptodius
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.
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.