610 THE BIOLOGY OF MARINE ANIMALS 



important neurosecretory organs (p. 446) [59a). It is not possible nor de- 

 sirable to consider all the multifarious observations and interpretations 

 recorded on this difficult subject, which awaits further experimental 

 treatment for clarification. 



An additional complication lies in the possibility of host resistance. 

 Among trematodes it has been observed that heavy infections may set up 

 immunity reactions in the final hosts, limiting the number of parasites 

 (Maritrema obcysta in the black-headed gull). The vertebrate host may 

 show increased resistance to infection with age (herring gull infected with 

 Parorchis acanthus) (13, 89). A racial difference exists in the resistance of 

 populations of mussels Mytilus edulis to parasitic infection by the 

 copepod Mytilicola intestinalis in that infected mussels in the Mediter- 

 ranean show no adverse effects, whereas mussels in the North Sea are 

 heavily infested and suffer from gross inanition. It is only recently that 

 Mytilicola has appeared on the coasts of north Europe, but it has long 

 been known in the Mediterranean, where the mussels have become more 

 adjusted to the parasite (27). Dicyemid parasites, as we have observed, 

 live in a balanced relationship with their cephalopod hosts, and a pre- 

 liminary stage of parasitism of this kind may lead to a state of mutual 

 adaptation, perhaps with benefit to both partners, that can be termed 

 symbiosis. 



SYMBIOSIS 



Associations between marine animals and unicellular algae, termed sym- 

 biosis, are of great intrinsic interest and a considerable body of descriptive 

 and experimental evidence is available about them (12, 109, 110). The 

 algae involved are usually classified arbitrarily as green zoochlorellae and 

 brown zooxanthellae, but these terms are without systematic significance. 

 The associations, although usually labelled symbiotic, are seldom true 

 balanced relationships, with equal benefits to both partners, and some 

 symbiotic associations, indeed, are one-sided parasitism. 



Green and brown unicellular organisms are found in the tissues of a 

 host of marine invertebrates, namely in Protozoa, Porifera, Coelenterata, 

 Turbellaria and Mollusca, and less frequently in compound ascidians, 

 bryozoans, polychaetes and echinoderms. Some examples of animal species 

 containing such symbionts are presented in Table 14.1. 



Algal symbionts are small unicellular bodies ranging up to 10// in dia- 

 meter. The zooxanthellae from foraminifers are generally spherical in 

 shape and possess a cellulose covering, chromatophore, pyrenoid body, 

 oil bodies, starch grains and nucleus (22). In true symbiotic associations 

 the algae are located within the tissue cells of the host, but there are some 

 border-line cases in which the algal cells lie in body cavities, or between 

 the cells of the host, as in the compound ascidian Diplosoma virens. Most 

 of these algal symbionts carry on photosynthesis by means of chlorophyll ; 

 as an exception may be cited the flagellates occurring in Beroe which, 

 although pigmented, lack chlorophyll and are incapable of photosynthetic 



