no. 3602 CRAYFISHES — HOBBS, HOLT, AND WALTON 75 



There are instances of branchiobdellids known from only one host 

 species, but these are associations in which the worm is known from 

 only one or a few collections. The one such association of this type 

 from our collections (A. legaeus on Cambarus sciotensis) clearly does 

 not indicate host-species specificity since A. legaeus is known from 

 other species of crayfishes outside of the Mountain Lake area (Holt, 

 1965, p. 12) Similarly, the associations (table 1) in which it might 

 appear that there is a "preference" by a branchiobdellid species for 

 one or another host species are surely to be interpreted as the con- 

 sequence of the ecological relationships of hosts and worms or as the 

 product of the migration (distribution) patterns of the two. Host 

 specificity in the sense of host-species specificity is not an important 

 feature of the relationship between the annelids and the crustaceans; 

 yet, the relationship is an obligate one for the branchiobdellids. 



In occupying the exoskeleton of the crayfish host, the ostracods 

 and branchiobdellids have found niches that remind one of the 

 occupation of a stream by two or more species of crayfishes. Although 

 detailed observations on the distribution of the ostracods on their 

 hosts are limited to those that are reported above, it is reasonable to 

 assume that most multiple infestations result in a partial segregation 

 of the species on different areas of the host, similar to that exemplified 

 by As. asceta and Dt. chalaza on C. carolinus, in which the former is 

 concentrated among the setae of the gnathal appendages and the 

 latter on the setiferous portions of the abdomen. In terms of obvious 

 morphological adaptations to microhabitats on the crayfishes, the 

 ostracods have not approached the refinements exhibited by some of 

 the branchiobdellids (see p. 8). The contrast in body form of the 

 vermiform, gill-inhabiting B. illuminatus, the plumose, venter- 

 dwelling P. alcicornus, and the depressed, tennis-racquet-shaped, 

 chela-dwelling Xg. instabilius reflect the extreme conspicuous adaptive 

 modifications accomplished by the members of this order in becoming 

 specialized for a particular microhabitat on the crayfish host. Even 

 in the absence of such adaptive features, there must be equally 

 strong as yet unrecognized ones that are in effect in the ostracods, 

 for in the associations just mentioned the gnathal areas of all specimens 

 of C. carolinus examined were dominated by As. asceta and the 

 abdominal region by Dt. chalaza. 



While the adaptations of the crayfishes to riffles and pools are not 

 so conspicuous as those among the branchiobdellids in their modifi- 

 cations for living on different body regions of the crayfish, the riffle- 

 dwelling C. I. longulus is smaller, less spiny, and has smoother chelae 

 than the pool-dwelling C. acuminatus. The burrowing C. carolinus, 

 in contrast to C. I. longulus, has a compressed rather than depressed 



