March, 1921. 



The Canadian Field-Naturalist. 



49 



star, Amphiura, with long, sleuder rays. 

 In appearance tliey have very little in com- 

 mon and yet tiiey alwa^'s seem to occur 

 together. 



The best as well as tlie most common 

 and most extensive association is found 

 almost everywhere in rough, rocky bottom 

 at a depth of 25 or 30 fathoms in the open 

 strait. The most conspicuous part of the 

 association is made up of three sponges, 

 Bathydonis dawsoni, Rhadocalyptus dow- 

 lingii and AphrocalUstes whiteavesianus, 

 the first two in particular being supplied 

 J with numerous siliceous spicules that make 

 them troublesome to handle. With these 

 are associated a spiny ascidian, Tethjjum 

 igaboji (the only very spiny ascidian to be 

 found in the strait), a spiny brittle-star, 

 Ophiophohis sp., and two species of the 

 genu Euphrosyne, spiny annelids. These 

 are all spiny creatures and hence they 

 " form a very w^ell protected group. With 

 these there may be other species of rarer 

 occurrence; other species of annelids and 

 sponges, and a brittle-star. Many species 

 of bryozoa and hydroids are attached to 

 the sponges but none of them seem to be 

 essential to the association. 



Examples of commensalism are jiumer- 

 ous. The two species feeding together 

 usually belong to different phyla and hen- 

 ce they are hard to group. It may suffice 

 to arrange them according to the phyla to 

 which the major members belong. The 

 vertebrates do not supply instances. Ap- 

 parently there is in this region no counter- 

 part of the relation between the pilot-fish 

 and the shark. Possibly the nearest ap- 

 proach to it is the case of the little seulpins 

 that wait around for the crumbs that are 

 dropped when the crab. Cancer productns, 

 is having its meal of fresh barnacles. 



The ascidians serve better, as with Asci- 

 diopsis paratropa in particular, but also 

 with Phallusia ceratodes, Ciona intestinalis 

 and Tethyum aurantium, the hydroid, En- 

 docrypta hufitsmani, finds a close relation- 

 ship, since it is found attached to tlie wall 

 of the brancliial cavity. A Pinnotlierid 

 has been found commensal with Tefhiium 

 igaboji. 



Among the molluscs, the most familiar 

 examples are found in the clams, Mun and 

 Schizofhan-us, wliere various Pinnotherids 

 or pea-crabs are much at liome in the 



mantle cavity. The condition here is ex- 

 treme, for the body of the horse-clam in 

 particular may be 16 or 18 inches down in 

 the sand. The crab must therefore depend 

 on the circulation in the clam, for its oxy- 

 gen as well as for its food supply. An- 

 other bivalve, the scallop or Pecten, may 

 have either one of two species of sponges 

 living commensal with it, the two species 

 being Esperella adhcerens and Myxilla pa- 

 rasitica. With a much smaller bivalve, 

 Axinopsis sericata, occurs the hydroid, Mo- 

 nobrachium , parasihim. Tlie offensive 

 zooids of Monobrachium extend over the 

 ventral border of the valve and probably 

 protect the mollusc as well as the hydroid 

 colony. The key-hole limpet, almost in- 

 variably, has an annelid lying in the man- 

 tle cavity parallel to the foot, of a color 

 that matches that of the foot and gills. 



Among the Crustacea there is what might 

 be called a forced commensalism in the 

 case of the decorating crab. Hydroids, 

 sponges, bn-ozoa, etc., grow from many 

 places on its exoskeleton, but the begin- 

 nings of the colonies are placed there by 

 the crab itself. The hydroid, Hydractinia 

 aggregata, is always found growing on & 

 shell that is inhabited by a hermit crab, 

 Pagurus splendescens (possibly other spe- 

 cies as well). Here the arrangement is 

 .evidently mutually beneficial. The hy- 

 droid colony disguises the home of the crab 

 and on the other hand it obtains food mat- 

 erial let slip by its messmate. This is de- 

 finitely indicated by the fact that the nu- 

 tritive zooids are much more numerous 

 near the margin of the shell, to the ex- 

 clusion of the generative zooids that are 

 always situated farther back. In some 

 eases the hermit-crab, Pagunis ochotensis, 

 (and possibly other species), has a closer 

 association than that with the hydroid, 

 wlien it has an annelid, Xereis, living with 

 it within the shell. 



No instance has appeared where an an- 

 nellid is the major commen.sal exigent 

 where both commensals are annelids. The 

 Polvnoid that lives in the tube of an Am- 

 phifrifr is an example. 



Among the Ecbinoderms one starfish, 

 Evasterias troschelii, is seldom found with- 

 out a commensal Polvnoirl. that matches 

 tlie color nai-tiddarlv well b"t no l)etter 

 tlian the Polvnoid found on the Holothu- 



