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Crab Parasitism

Photo of the Smooth-handed Crab

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.

Graphic of parasitic barnacle naupius Graphic of parasitic barnacle cypris larvae
After hatching, the first Sacculina sp. life stage is a nauplius. After a couple of moults it becomes a cypris

This barnacle's cyprid, which will grow to be a female, attaches itself by its first antennae to the base of the adult crab's setae.

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.

Graphic of cypris barnacle first entering host

Graphic of parasitic barnacle changing into a kentrogen larvae

Graphic of parasitic barnacle tissues entering host

Illustrations based on diagrams in
Russell-Hunter. W.D. (1970) A Biology of Higher Invertebrates
Macmillan, London.

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.

Photo of parasitised barnacleWhen 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.

Russell-Hunter. W.D. (1970) A Biology of Higher Invertebrates.
p. 80, Macmillan, London.


Tubercled Crab
Reef Crab
Smooth-handed Crab
Variegated Shore Crab
Red Bait Crab
Sowrie Crab
Mudflat Sentinal Crab
Smooth Shore Crab
Seaweed-decorator Crab
Crab reproduction
Crab parasitism

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photo of Keith DaveyLife on Australian Seashores
by Keith Davey (C) 2000

Learning Consultant - Media
The University of Newcastle

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Scientific Consultant: Phil Colman
site created 01.01.98 : updated 01.04.2000