A male Smooth-handed Crab, Pilumnopeus serratifrons, can be parasitised by a species of shelless Barnacle Saccula sp. which lives part of its life cycle under the male's abdominal plate. The parasitic barnacle feeds on the living crab's tissue.
This barnacle is a rhizocephalan, which is parasitic on decapod crustaceans. Its body is sac-shaped, and the mantle never develops calcareous plates. It doesn't have any appendages or segments. Therefore, it never looks like a "normal" barnacle.
The barnacle cyprid then excavates a hole through the carapace of the crab allowing the mass of cells to enter into the body of the crab.
Illustrations based on diagrams in
The barnacle feeds by having many outgrowths, called rhizoids, which grow and extend throughout the body of the crab, and take in food for the barnacle.
When the barnacle is sexually mature, it creates a brood chamber which grows out like a mushroom, through another opening near the base of the abdomen (see photo at right)..
A male cyprid of the same species of barnacle attaches itself to the opening of this brood chamber, and then grows inside a special chamber within the female barnacle which itself is inside the body of the parasitised crab. This male growth forms into a testis. The female then effectively becomes a hermaphrodyte, which means that it can fertilise itself.
The effects of this parasitism on the adult crab is quite remarkable. The crab's ability to moult is inhibited. It also reduces or looses its ability to reproduce, and it gains female characteristics and appearance.
I have found a number of these parasitised male crabs near the heated water outflow of coal-fired power stations. The area near Erraring Power Station at Lake Macquarie, NSW is one such area.
Barnes, R. D. (1968) Invertebrate Zoology. p. 467, W.B. Saunders Company, Philadelphia.
Bennett, I. (1987) W.J. Dakin's classic study: Australian Seashores. p.391, Angus & Robertson, Sydney.
Davey, K. (1998) A Photographic Guide to Seashore Life of Australia. p.52, New Holland, Sydney.
Jones, D. & Morgan, G. (1994) A Field Guide to Crustaceans of Australian Waters. p.182, Reed, Chatswood.
W.D. (1970) A Biology of Higher Invertebrates.