ASSOCIATIONS 585 



recent studies, such as those by Davenport (18), draw attention to the 

 wealth of fertile problems in this particular field. 



There are numerous instances of commensalism involving a coelenterate 

 and some other animal, and it is probably such associations that are best 

 known. Permanent associations occur between certain spider-crabs and 

 sea-anemones, in which the anemone is used for protection. The long- 

 legged spider-crab Macropodia rostrata (=Stenorhynchus phalangium) is 

 always found in the neighbourhood of the opelet anemone Anemonia 

 sulcata. Thomson (101) observed that the crab usually stations itself close 

 to the anemone, and when disturbed it works its way back upon the crown 

 of the anemone, which makes no attempt to seize its guest. Both members 

 derive benefit from this relationship, the spider-crab obtaining protection 



Fig. 14.2. Sand Shark {Carcharias taurus) with two Remoras {Echeneis 

 naucrates) Attached. (Drawn from a photograph in Nat. Hist. N.Y., 1950.) 



from the batteries of nematocysts and the anemone seizing food from the 

 claws of the crab. 



Much more intimate associations are established by other brachyurans. 

 A species of Chilean crab Hepatus chilensis nearly always carries an 

 actiniarian Actinoloba reticulata upon its back. It is the anemone, appar- 

 ently, which takes the initiative in this association. When separated from a 

 crab, it first of all rests attached to the substratum. When it makes contact 

 with a crab it attaches itself to the latter's foot, and then works its way up 

 to the back of the crab. Somewhat similar is the behaviour of the crab 

 Dromia which carries a piece of living sponge on its back, held in position 

 by means of its last two pairs of legs. There are two other crabs, Melia 

 tessellata and Polydectus cupulifera, which carry anemones in their pincers 

 (Fig. 14.3). The chelae of the crab are slender and mobile, and bear a row 

 of sharp teeth for holding the anemone. The anemone is held with the 

 mouth upwards and the tentacles projecting away from the crab, and when 

 the latter is disturbed, it waves its anemone-armament from side to side. 

 The crab also utilizes its commensal associate in feeding, and removes 

 particles of food from the tentacles of the anemone with the aid of its 

 second pair of limbs. When the anemones are taken away from the crab, 



M.A.— 19* 



