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apparently as the result of migration of the juveniles. According to
Kearn ( 1988 ), the monogenean Entobdella soleae, after transfer to another
host individual, the sole Solea solea,migratesfromtheuppertothelower
surface via the head; some transferred individuals, particularly adults,
spend longer than others on the head, thereby improving the chances of
cross-fertilization between worms arriving on the host at different
times. Another example is the parasitic prosobranch snail Thyca crystal-
lina. Juveniles settle largely on the upper surface of the distal parts of the
arms of the sea star Linckia laevigata,wheretheyarerandomlyorientated,
whereas large females face the starfish mouth in a small area on the right
side of the ambulacral groove on the surface of the oral arm (Elder
1979 ).
The mating hypothesis of niche restriction is not limited to parasites,
as indicated by the observations of Zw¨ lfer ( 1974a , b ) on trypetid flies,
most of which use certain host plants as a meeting place for mating. And
this phenomenon is indeed common in insects. According to Zw¨ lfer
(personal communication, and examples in Zw¨ lfer and Bush 1984 ), the
''Treffpunkt'' or ''rendezvous principle'' applies to almost all host-specific
phytophagous insect species (Diptera, Coleoptera, Hymenoptera, and
many Lepidoptera). Males of the species use and defend the meeting
places as temporary, resource-based territories. In species with endophytic
larvae such meeting places are the rule.
Niche segregation to ensure reinforcement of reproductive
barriers
Interspecific competition has not only been proposed as the principal
cause of niche restriction, but also of niche segregation. However, as in
the case of niche restriction, there are other alternatives to explaining
niche segregation by interspecific competition. An important alternative
explanation (beside random selection of niches in largely empty niche
space, which was discussed on pp. 39-48) is reinforcement of reproduc-
tive barriers. Miller ( 1967 ) has already stressed that two explanations for
character displacement are possible, i.e., competition (''ecological charac-
ter displacement''), or reinforcement of reproductive barriers (''Wallace
effect''). Several authors (e.g., Grant 1972 ; Wilson 1975 ; Connell 1980 ;
Arthur 1982 ) have pointed out that the ecological aspect of character
displacement is weak in many situations. Concerning ectoparasites of fish,
Rohde ( 1989 ) concluded that there is little or no evidence for the view
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