FISHERY BULLETIN: VOL. 71, NO. 4 



terrestrial arthropods microanatomical studies 

 on copepods (Lowe, 1935; Fahrenbach. 1962; 

 Park, 1966) reveal a highly organized central 

 nervous system, one that supports inner- 

 vated integumental sensilla and glands distri- 

 buted symmetrically over the body. The 

 receptors may monitor the environment, and 

 the effectors may release information in response 

 to appropriate stimulators. 



Among semispecies, i.e., somewhat differen- 

 tiated populations of a superspecies (Mayr, 1970), 

 the primary reproductive barrier is extrinsic 

 inasmuch as potential hybridizers lack access 

 to one another. Assuming no significant contact 

 since the inception of allopatry, intrinsic repro- 

 ductive barriers, i.e., prezygotic mechanisms, if 

 any, are untested and probably imperfect or 

 nonexistent. Similar populations of recent 

 common ancestry newly brought together 

 through the expansion of ranges are likely to 

 interact unfavorably across a spectrum of fun- 

 damental organismic processes and thereby ex- 

 perience pronounced selection pressures. When 

 sympatry replaces allopatry among sets of 

 semispecies hybridization is often intense in the 

 area of geographical overlap (Remington, 

 1968; Rising, 1970). 



The only stabilizing alternatives for such 

 interacting ])opulations are: 1) elimination of 

 one or both; 2) genetic displacement; or 3) 

 coalescence. Alternatives 1 and 2 will prevail 

 if, in general, hybrid offspring are less fit and at 

 a selective disadvantage and only postzygotic 

 barriers are available. Development of increased 

 efficiency of prezygotic barriers will be at a 

 premium and the rate of their development and 

 distribution will be a function of the rate of 

 interactions and the mobility of the populations. 

 According to MacArthur and Wilson (1967) a 

 computer model by Bossert has indicated that 

 full displacement is achieved rapidly and, under 

 ideal conditions, equilibrium may be achieved 

 in as few as 10 generations. 



As seen in each of the species groups of Euca- 

 lanus, differences in the integumental organs 

 between species groups include features com- 

 mon to all members and those common to mem- 

 bers comprising the group. Within a group 

 differences tend to occur on or near the genital 

 segment. Moreover, there is a general trend for 



differences to be more pronounced in the popula- 

 tions whose distributions are most extensive 

 and bring them into contact with the largest 

 number of other members within the species 

 group. Thus, the pattern of integumental organs 

 in Eucalayius show configurations that appear 

 to have been shaped by character displacement 

 (Brown and Wilson, 1956) in the broadest sense 

 or what some (Blair, 1963; Littlejohn, 1965) 

 have referred to as reinforcement. If so, inte- 

 gumental organs in copepods may play a 

 significant role in the mating process. 



CONCLUSIONS 



1. This study provides unique and compelling 

 evidence of the extensive information content 

 applicable to systematics, phylogenetics, evolu- 

 tion, and population biology present in the in- 

 tegumental sensory and effector organs of 

 l^lanktonic copepods. 



2. Integumental organs appearing on the 

 body segments in Eucalayius are distributed in 

 bilaterally symmetrical, serially homologous 

 patterns that appear in both dorsal (tergal) and 

 lateral (pleural) sets corresponding to the so- 

 mites of the body. The organs fall into two basic 

 groups: a) receptors that appear as hairs, pegs, 

 or pits and b) integumental glands that commu- 

 nicate with pores on the surface of the exoskele- 

 ton. Within the set of a somite each designated 

 site is constant in position and in morphological 

 type of organ relative to the others comprising 

 the set. 



3. Analysis and survey of the gross morphol- 

 ogy, distribution, and variation of these integu- 

 mental organs was facilitated by a combination 

 of techniques including a) digestion of internal 

 tissues and intensive staining for light micros- 

 copy, b) clearing and integumental staining of 

 intact specimens for light microscopy, and c) 

 scanning electron microscopy. Processing of 

 relatively large numbers of specimens was 

 accomplished most efficiently using method a. 



4. Comparative survey of integumental organs 

 in 17 species recognized in this study as com- 

 prising the genus Eucalanus indicated that the 

 numbers and arrangement of these structures 

 reflect generic similarities, species group simi- 

 larities, and individual species patterns, all 



1006 



