584 



THE BIOLOGY OF MARINE ANIMALS 



stratum, but at least one species is parasitic and sends feeding stolons 

 into the host's tissues (38). 



Commensalism Proper 



Loose associations verging on commensalism are seen in the behaviour 

 of certain fish. The sea bass Serranus is described as keeping watch out- 

 side the retreats occupied by devil fish {Octopus), in order to seize the 

 remnants of crustaceans captured by the latter. Beebe (5) gives an example 

 of a mutually beneficial relationship in the behaviour of the rainbow parrot 

 fish Pseudoscarus guacamaia of Bermuda. This fish browses on living coral, 



and at intervals it assumes a vertical stance 

 in the water and allows small wrasse to 

 clean its teeth and scales of adhering debris. 

 Fixed and constant associations between 

 fishes are shown by the pilot fish Naucrates 

 ductor and the shark-sucker Echeneis nau- 

 crates, which accompany pelagic sharks 

 and other large fish. In the remora or shark- 

 sucker the dorsal fin is transformed into a 

 large sucking disc which is provided with 

 transverse pleats and occupies the dorsal 

 surface of the head (Figs. 14.1 and 14.2). It 

 uses this structure to attach itself to the 

 body of the shark, and occasionally releases 

 its hold to snatch some of the shark's 

 food. 



Commensalism permits the different 

 adaptations and structural specializations 

 of two species to be shared, and this in- 

 creases the total response range and en- 

 vironmental experience of the group. For 

 the benefits accruing, however, there are 

 frequently restrictions in other directions, 

 resulting from a lower level or range of activity in one partner, which acts as 

 a brake on the total activity of the ensemble. Many examples of this are seen 

 in associations with anemones, in which more active animals are restricted 

 to the environs of their host actinian, etc. Some degree of morphological 

 differentiation is often evident in commensal associations, and one example 

 involving the sucking disc of the remora has already been cited above, 

 while others will be noted in subsequent pages. Physiological adaptations 

 are more subtle and still await exploration. One interesting case involving 

 the chromatophore responses of the amphipod Hyperia galba, which is 

 commensal with such medusae as Rhizostoma and Chrysaora, has been 

 treated in Chapter 12. It is obvious that such fixed associations must also 

 involve considerable specialization of sensori-neural processes and be- 

 havioural activities, either of an inherent or of an acquired character, and 



Fig. 14.1. Sucker on the Top 



of the Head of the Remora 



or Shark Sucker 



