Opinion: You couldn’t make it up

Jane Pottas
Jane Pottas

It’s true that the more you look, the more you see and in the past couple of weeks I’ve seen something I’ve heard about but never witnessed before.

Twice - and both times at Boggle Hole. A species of parasitic barnacle called Sacculina invades the bodies of crabs in order to complete its life cycle and it does it by taking over the crab physiology.

The story of this host-parasite relationship is quite incredible – you couldn’t make it up. In the larval stage, female Sacculina have a hard outer shell, like other barnacle larvae, but when she finds a suitable crab host she attaches to the host’s antenna, sheds her hard outer layer and injects her soft inner body into the crab.

At this point the female adult form resembles a tiny slug, but as it parasitizes the crab, it grows tendrils which spread through the crab’s body to obtain nutrients from the crab’s tissues. As it grows bigger it pushes outside the body of the crab and occupies the place where a female crab would carry her eggs protected by the abdominal flap.

At the same time other tendrils surround the crab’s nerve centre and grow along the crab’s nervous system down through its legs and up to and around the cerebral ganglion (the crab equivalent of a brain).

The parasite uses the energy that the crab would invest in reproduction for its own growth and development. In addition, infected crabs are prevented from moulting their shells and re-growing lost limbs and instead all this energy is redirected to the female parasite.

The male Sacculina is extremely small, its only purpose being to fertilize the female’s eggs. The Sacculina parasite produces hormones which modify the behaviour of both male and female infected crabs. Male crabs are effectively chemically castrated.

Their body changes to resemble that of a female crab and they treat the parasite as a female would her own eggs. Infected crabs (both male and female) protect the larval parasites and then disperse them like they would their own eggs.

At this point, the free-swimming Sacculina larvae are entirely independent until they find a new host and so the cycle of infection continues. Although Sacculina generally infects shore crabs (Carcinus maenas) the parasitized crabs I saw were both edible crabs (Cancer pagurus). In any event a quite amazing adaptation.

Male Sacculina find an infected crab and fertilize the eggs in the female’s sac which extends outside the body of the crab protected by the abdomen which in females opens to enclose and protect her eggs following fertilisation.

During reproduction in healthy crabs, the female finds a high rock and releases fertilized eggs from its brooding sac. Parasitized crabs perform the same behaviour, but inadvertently release a cloud of Sacculina eggs. The crab nurtures the Sacculina eggs as if they were its own offspring, and once the

larvae hatch from the eggs and are released into the sea the process begins once again.