1 62 A BIOLOGY OF CRUSTACEA 



through the cuticle and this will usually involve passing a layer 

 containing fats. The exact mode of action of these poisons is not 

 known, but mercury is known to inhibit the action of certain 

 enzymes, so that a general interference with metabolism may be 

 the end result. When compounds of mercury have been tested on 

 prawns large amounts of the poisons have been found in the 

 excretory organs, suggesting that the crustacean body makes an 

 attempt to rid itself of the toxic material. 



Damage by marine boring animals is also of great economic 

 importance, and of all the creatures responsible for such damage 

 two are outstanding : one is a mollusc, Teredo, the ship-worm; the 

 other is an isopod crustacean, Limnoria, the gribble. There are some 

 fourteen wood-boring species of Limnoria known, and a further 

 seven are known which burrow in the holdfasts of large seaweeds. 



Piles supporting wharfs and piers seem to be particularly sus- 

 ceptible to damage by marine borers. It has been estimated that 

 the annual damage around the coasts of the U.S.A. amounts to 

 over fifty million dollars. The gribble seems able to attack creosote- 

 impregnated wood earlier than the ship-worm. It makes shallow 

 tunnels along the grain of the wood and this allows water to 

 penetrate further into the wood and leach out more of the protective 

 creosote. 



Wood which is exposed to the sea is quickly attacked by a 

 variety of fungi, and it is probable that such an attack is necessary 

 before the gribble can successfully establish itself. Adult gribbles 

 die in about four days if they are kept with sterile wood in sterile 

 water, but they live for much longer periods if fed on cultures of a 

 marine fungus. Another indication of the importance of fungi as 

 food for Limnoria is revealed when the amino-acid constitution of 

 the fungus and the isopod are compared. The fungus, Lulworthia 

 floridana, contains the same thirteen amino acids as Limnoria 

 tripunctata, from which the gut has been removed to avoid con- 

 tamination. Sterile pinewood only contains seven of these acids, 

 However the gribble also obtains some nutriment from the wood, 

 and its gut contains enzymes which are capable of digesting 

 cellulose and other substances in wood. 



Infestation of new wood is carried out by sexually mature 

 gribbles, but the females are not carrying eggs when they migrate 

 to the new habitat. Pairing occurs after they have become estab- 

 lished in their new burrows. Each burrow usually contains a 

 female at the point of active extension, and a male a little further 



