48 



The Can^vdian Field-Naturalist. 



[Vol. XXV. 



ASSOCIATION, COMMENSALISM AND PARASITISM AMONG MARINE 

 ANIMALS IN THE STRAIT OF GEORGIA. 



By C. McLean Fraser, Ph.D., F.R.S.C. 

 Professor of Zoology, University of British Columbia, Vancouver, B.C. 



The conditions affecting marine life are 

 sufficiently diversified in the waters of 

 the strait of Georgia to supply material 

 for investigation in every branch of zoo- 

 logy but no branch is better served than 

 ecology. 



In making ecological observations one 

 is necessarily struck with the numerous 

 instances of association, commensalism and 

 parasitism, states which differ only in 

 degree and between which complete inter- 

 grading is evident. The association may 

 be quite casual as in cases where two or 

 more species are found in proximity on 

 account of the fact that they live on the 

 same kind of food. The relationship is 

 much closer in other cases where the spe- 

 cies remain permanently in proximity, in 

 some instances as much for mutual pro- 

 tection as for common feeding. It is but 

 a short' step from this to commensalism 

 where two species (seldom more) bear 

 such an intimate relation to each other 

 that one is seldom found or never found 

 without the other. They may be mutually 

 helpful, and, if not evidently so, at least 

 one must not be harmful to the other. 

 The condition of one species feeding with 

 another differs comparatively little from 

 that of one species feeding on another, and 

 so parasitism appears, going from the one 

 extreme Avhere the parasite is free to at- 

 tack its host, to move about from one host 

 to another or even to live apart from the 

 host, to the other extreme where the par- 

 asite in the adult state has lost all power 

 of locomotion and is otherwise so degener- 

 ate that it lives only by absorbing highly 

 complex nourishment from the host. 



The term "parasitism" is often loosely 

 used in connection with such sessile forms 

 as hydroids, bryozoa, etc., where there is 

 no evidence of such. The larvae of these 

 forms, when they reach the end of the 

 free-swimming period settle down, in most 

 cases, on anything available for support 

 throughout the rest of their life. In very 



few instances is there any evidence that 

 they receive any nourishment from the 

 supporting object which consequently can- 

 not be a host. It evidently cannot be a 

 host when the support is a piece of rock 

 or dead shell, and it is usually no more so 

 if this is an animate object. 



As an instance of more or less casual as- 

 sociation the young herring and the stick- 

 leback will serve. The herring hatches out 

 in March or April, and by July or August 

 has taken on the real herring appearance. 

 It gathers in schools to feed around whar- 

 ves and floats and near the shore at var- 

 ious places, where it remains until it is 

 about a year old. During this period the 

 individuals mingle freely with the stick- 

 lebacks that are in that vicinity for the 

 same reason, so that one seldom sees the 

 one species without the other. Later the 

 herring goes out to deeper water and the 

 association is at an end. 



Without going into detail other instan- 

 ces may be noted. The little blenny, Ano- 

 plarchus, is found with the cling fish, Cau- 

 larchus, under stones that are left exposed 

 at low tide, and with these the singing fish, 

 Porichthys, often appears. Schools of 

 sand launces are often associated with 

 schools of herring. Viviparous perch, Pa- 

 nerodon, and sometimes Tceniotoca, occur 

 in the same locality as the rock cod, Sehas- 

 todes. This is true of the two species of 

 clams, Saxidomus and Paphia, and of the 

 heart-urchin, Schizaster, the brittle-star, 

 Amphiura, and the holothurian, Molpadia. 



A somewhat closer association appears 

 between two Chaetopterid annelids, a large 

 one and a small one, and both of them with 

 a Phoronopsis. They are all tubiculous 

 species, living in the fine sand a short 

 distance above lowest low-water mark. 

 Where one is found the others are sure to 

 be present. In certain localities, only ex- 

 posed at the lowest tides, where coarse 

 sand or fine gravel is mixed with mud, a 

 Synapta is plentiful, and with it a brittle- 



