4. ANALYSIS O F FACTORS INVOLVED IN SYMBIOSIS
49
It still remains unknown just how widely spread such host-factor
stimulated reversals of taxes are in nature although Welsh (1930)
has stated : (‘ Doubtless many parasitic animals become conditioned to
stimuli from the host and show adaptive reactions quite different from
their primitive responses.”
E. Xelectivity of symbiont
Although host-selection by marine symbionts remains t o be studied,
the principle of selectivity of hosts by symbionts has been established
in studies concerned with other types of symbiotic relationships. As
Davenport (1955) has pointed out, true predators among arthropods and
vertebrates with specialized food habits may locate their prey by highly
precise behavior involving, a t least in part, olfactory recognition of the
prey. Although this type of precise recognition during predation has
been demonstrated among certain molluscs (Stehouwer, 1952 ; Kohn,
1961 ; Prings and Frings, 1965) and nudibranchs (Braams and Geelen,
1953), its importance in symbiotic relationships has been little studied
except in the case of a few parasitic insects. Such insects, often termed
parasitoids, are in fact parasites that more often than not gradually kill
their hosts. For this reason, some have chosen to refer t o them as
‘( internal predators ”. However, in following my earlier interpretation
(Cheng, 1964b), I have chosen to consider them as true parasites since
they are obligatorily dependent upon their hosts. That they do cause
the death of their hosts should be considered as a secondary factor which
they share with other pathogenic and/or lethal parasites.
That host-selection does occur implies that the symbiont is capable
of recognizing its natural host prior to establishment. As discussed
above (Section I, C) doubt still remains as to whether the selection
process occurs with all symbionts. Certainly the results of Sudds (1960)
with trematode miracidia indicate that selection prior to penetration
does not necessarily occur. Nevertheless, there is evidence to support
the belief that among certain symbionts of more complex organization,
particularly among the arthropods, host-selection does occur. In
addition, there appears to be host-selection, a t least host-preference,
among certain marine polynoid polychaetes (Davenport, 1950, 1953a,b,
1954; Davenport and Hickok, 1951), and sea anemones (Davenport and
Norris, 1958 ; Ross, 1965).
Among parasitic insects two classical examples may be cited. Laing
(1937), by employing a choice-apparatus, has demonstrated that the
chalcid Alysia manducator may be attracted by olfactory means t o the
environment of its host which is the blowfly larva. The parasite is
attracted, as the result of a powerful chemotaxis, to some factor(s)
A.M.B.-5
5
49
It still remains unknown just how widely spread such host-factor
stimulated reversals of taxes are in nature although Welsh (1930)
has stated : (‘ Doubtless many parasitic animals become conditioned to
stimuli from the host and show adaptive reactions quite different from
their primitive responses.”
E. Xelectivity of symbiont
Although host-selection by marine symbionts remains t o be studied,
the principle of selectivity of hosts by symbionts has been established
in studies concerned with other types of symbiotic relationships. As
Davenport (1955) has pointed out, true predators among arthropods and
vertebrates with specialized food habits may locate their prey by highly
precise behavior involving, a t least in part, olfactory recognition of the
prey. Although this type of precise recognition during predation has
been demonstrated among certain molluscs (Stehouwer, 1952 ; Kohn,
1961 ; Prings and Frings, 1965) and nudibranchs (Braams and Geelen,
1953), its importance in symbiotic relationships has been little studied
except in the case of a few parasitic insects. Such insects, often termed
parasitoids, are in fact parasites that more often than not gradually kill
their hosts. For this reason, some have chosen to refer t o them as
‘( internal predators ”. However, in following my earlier interpretation
(Cheng, 1964b), I have chosen to consider them as true parasites since
they are obligatorily dependent upon their hosts. That they do cause
the death of their hosts should be considered as a secondary factor which
they share with other pathogenic and/or lethal parasites.
That host-selection does occur implies that the symbiont is capable
of recognizing its natural host prior to establishment. As discussed
above (Section I, C) doubt still remains as to whether the selection
process occurs with all symbionts. Certainly the results of Sudds (1960)
with trematode miracidia indicate that selection prior to penetration
does not necessarily occur. Nevertheless, there is evidence to support
the belief that among certain symbionts of more complex organization,
particularly among the arthropods, host-selection does occur. In
addition, there appears to be host-selection, a t least host-preference,
among certain marine polynoid polychaetes (Davenport, 1950, 1953a,b,
1954; Davenport and Hickok, 1951), and sea anemones (Davenport and
Norris, 1958 ; Ross, 1965).
Among parasitic insects two classical examples may be cited. Laing
(1937), by employing a choice-apparatus, has demonstrated that the
chalcid Alysia manducator may be attracted by olfactory means t o the
environment of its host which is the blowfly larva. The parasite is
attracted, as the result of a powerful chemotaxis, to some factor(s)
A.M.B.-5
5