IOO A BIOLOGY OF CRUSTACEA 



snail no longer in existence. One wonders how long this can go 

 on. particularly as there does not seem to he any other suitable 

 shell on the island. 



The mollusc shells carried hv the hermits provide the hase for 

 the attachment of sea anemones. The position occupied hy the 

 anemone may he variable, as in the association between Eupagnrus 

 bernhardus and the anemone Calliactis parasitica, or there may be 

 a constant relation between the tentacular crown of the anemone 

 and the mouth of the crab. Eupagurus prideauxi occupies shells 

 too small completely to cover its abdomen, which is protected by 

 the cloak-like base of the anemone Adamsia palliata. The mouth 

 of the anemone is always positioned just behind that of the crab, 

 so that any food caught by the crab is shared by the anemone. The 

 association between the two species is obviously very close, and it 

 has been found that the anemone produces toxins which are fatal 

 when injected into other crabs, but which are neutralised by anti- 

 bodies when injected into Eupagurus prideauxi. 



Hermit crabs often have colonies of small hydroids, particularly 

 of the genus Hydractinia, attached to the mollusc shells which they 

 carry. These hydroids are also found attached to rocks and sea- 

 weeds, so that they are not obligatory commensals. A peculiar 

 feature of the colonies carried by the hermits is that they have an 

 additional type of zooid, a spiral one, the function of which is 

 not really known, but it is thought to be protective. One species of 

 Hydractinia, H. epiconcha from Japan, forms dense colonies which 

 overlap the edges of the mollusc shell, and so provides protection 

 for the hermit as it grows and relieves it of the task of finding a 

 larger shell. 



Hydroids are also sometimes found attached to parasitic cope- 

 pods. Usually the hydroid concerned is a well-known, unmodified, 

 free-living species, such as Obelia geniculata, which has been 

 found on Lernaeocera branchialis (p. 113) attached to the gills of a 

 cod (Gadus). A peculiar modified hydroid, Ichthyocodium 

 sarcotretis, which lacks tentacles, has been found on a copepod, 

 Sarcotretes scopeli, which attaches itself to the bodv of the small 

 pelagic fish, Scopelus glacialis. It is not known how Ichthyocodium 

 obtains its food. 



Other creatures may join the hermit crab association. Nereis 

 fucata is a polvchaete worm which inhabits the whelk shells carried 

 by Eupagurus bernhardus, and when the hermit captures some food 

 the worm pokes its head quickly between the mouthparts of the crab 



