102 A BIOLOGY OF CRUSTACEA 



pincers and holds them on its back until they became fixed. The 

 sponge grows and covers the back of the crab, giving it protection 

 by virtue of its distastefulness to fish. 



The echinoderms, which include the starfish, sea urchins and sea 

 cucumbers, are another group with which the Crustacea frequently 

 enter into special relationships. Often we do not know enough about 

 the details of the relationship to be sure whether it is commensalism 

 or parasitism. This is so in the case of Micropontius ovoides, a 

 copepod which lives on the irregular urchin, Spatangus purpureus. 

 The female of Micropontius is remarkable for producing only one 

 large cg^ in each of her two ovisacs. 



Frequently one finds that the associated crustacean and echino- 

 derm are similarly coloured. Copepods of the genus Ascomyzon 

 often match exactly the colour of the starfish on whose surface 

 they live. 



Sometimes the crustacean lives on a particular part of the echino- 

 derm's surface. A crab, Lissocarcinus orbicularis, from Madagascar, 

 lives near the mouth of a sea cucumber, and is sometimes actually 

 enclosed within the tentacles of its host, but it comes to no harm. 

 Another crab, Eumedon convictor, from the Gambier Islands, lives 

 on the top of a sea urchin, Echinothrix turca, and causes the host's 

 shell to fold back to form a sort of gall. 



One of the simplest relationships with another animal is to live 

 on its surface and to enjoy the benefits of free transport. Many 

 barnacles attach themselves by chance to other animals, often on 

 other large Crustacea, but some species regularly attach themselves 

 to a particular type of animal. Coronula attaches to whales, Stoma- 

 tolepas is found on turtles, and perhaps the most surprising, 

 Platylcpas ophiophilus has been found on the sea snake, Enhydris 

 curta, in the Malay Archipelago. 



A number of small creatures reverse the roles, and attach them- 

 selves to various Crustacea. 



An association between certain species of Daphnia and the rotifer 

 Brachionus rubens has been analysed in a delightful manner by 

 Viaud (1947). The rotifer (fig. 45) attaches itself to Daphnia, using 

 its foot, and so gets transported by the water flea. Feeding currents 

 are produced by the corona of the rotifier, and it obtains its food by 

 selecting from the particles brought in by these currents. In this 

 way the rotifer is independent of Daphnia for its food supply and 

 so cannot be considered as a parasite. Moderate numbers of the 

 rotifer do not appear to harm Daphnia, but large numbers, such as 



