HABITAT SELEOTION B Y AQUA'I'ICJ INVERTEBRATES
339
food if one's preferred food is not available (the first group of muricids)
against the disadvantages of being likely to eat members of one's own
species (the second group of muricids). The concept could however, be
extended to other aspects of habitat selection with certain modifications.
It would be naive to expect animals to differentiate between every
variable they encounter in their environment, and yet few animals are
known to show lack of preferences. The larvae of three polychaetes
(Platynereis, Nereis and Polydora) settle equally well on a range of sediments (Kiseleva, 1967a, b), the larvae of the bivalve Chione cancellata L.
metamorphose with or without contact with sediment although slightly
earlier when sediment is present (D'Asaro, 1967), and the gregarious
larvae of Sabellaria alveolata do not distinguish between tubes of their
own species and those of 8. spinulosa (Wilson, 1968). The oncomiracidia larvae of the trematode Discocotyle sagittata do not appear to
recognize their normal host, the brown trout (Paling, 1969), while
adults of the gastropods Murex firifer and Urosalpinx perragatus
Conrad fed equally readily on three species of bivalve they were
offered as food, without apparent preference (Radwin and Wells, 1968).
Pabricia sabella (Ehrenberg) (Polychaeta) do not distinguish between
different particle sizes (Lewis, 1908), Heterocypris salinus cannot distinguish between waters of different oxygen content (Gaming, 1967)
and Pontoporeia affinis Lindstr6m (Amphipoda) show no humidity
preferences (Lagerspetz, 1963). Pontoporeia is a sublittoral amphipod
and so may have no need of humidity preferences ; on the other hand it
may be limited to the sublittoral region because it cannot detect dry
air-animals stranded in the intertidal zone would in this case be killed
by dehydration. As a final example, the beach amphipod Orchestoidea
corniculata Stout does not detect the shore line of the beach by lunar
orientation (Craig, 1970) in contrast to Talitrus maltator (Papi and
Pardi, 1963), in fact, the animals are very quiescent at full moon (loc.
cit. Table 2) and Craig argues that lunar orientation in this instance
would have little biological value. The reasons for the other examples
are obscure, but must have some ecological significance.
It can be seen, therefore, that a range of unexpected mechanisms
may play a part in habitat selection. We have discussed the possible
importance of coarse and fine selection, of indirect clues, and of the
slope of a preference, and have considered what might happen if a
suitable habitat were not available and also the result of an animal
showing a lack of preference for a particular variable. All these offer
new ways of investigating the mechanisms of habitat selection. However the description of the three dimensional behaviour shown by many
aquatic animals is likely to be a more difficult problem. The only
topic which has received any detailed attention is that of the
339
food if one's preferred food is not available (the first group of muricids)
against the disadvantages of being likely to eat members of one's own
species (the second group of muricids). The concept could however, be
extended to other aspects of habitat selection with certain modifications.
It would be naive to expect animals to differentiate between every
variable they encounter in their environment, and yet few animals are
known to show lack of preferences. The larvae of three polychaetes
(Platynereis, Nereis and Polydora) settle equally well on a range of sediments (Kiseleva, 1967a, b), the larvae of the bivalve Chione cancellata L.
metamorphose with or without contact with sediment although slightly
earlier when sediment is present (D'Asaro, 1967), and the gregarious
larvae of Sabellaria alveolata do not distinguish between tubes of their
own species and those of 8. spinulosa (Wilson, 1968). The oncomiracidia larvae of the trematode Discocotyle sagittata do not appear to
recognize their normal host, the brown trout (Paling, 1969), while
adults of the gastropods Murex firifer and Urosalpinx perragatus
Conrad fed equally readily on three species of bivalve they were
offered as food, without apparent preference (Radwin and Wells, 1968).
Pabricia sabella (Ehrenberg) (Polychaeta) do not distinguish between
different particle sizes (Lewis, 1908), Heterocypris salinus cannot distinguish between waters of different oxygen content (Gaming, 1967)
and Pontoporeia affinis Lindstr6m (Amphipoda) show no humidity
preferences (Lagerspetz, 1963). Pontoporeia is a sublittoral amphipod
and so may have no need of humidity preferences ; on the other hand it
may be limited to the sublittoral region because it cannot detect dry
air-animals stranded in the intertidal zone would in this case be killed
by dehydration. As a final example, the beach amphipod Orchestoidea
corniculata Stout does not detect the shore line of the beach by lunar
orientation (Craig, 1970) in contrast to Talitrus maltator (Papi and
Pardi, 1963), in fact, the animals are very quiescent at full moon (loc.
cit. Table 2) and Craig argues that lunar orientation in this instance
would have little biological value. The reasons for the other examples
are obscure, but must have some ecological significance.
It can be seen, therefore, that a range of unexpected mechanisms
may play a part in habitat selection. We have discussed the possible
importance of coarse and fine selection, of indirect clues, and of the
slope of a preference, and have considered what might happen if a
suitable habitat were not available and also the result of an animal
showing a lack of preference for a particular variable. All these offer
new ways of investigating the mechanisms of habitat selection. However the description of the three dimensional behaviour shown by many
aquatic animals is likely to be a more difficult problem. The only
topic which has received any detailed attention is that of the