22
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
attached to empty shells. Subsequently, Ross and Sutton (1961), as
the result of a series of tests designed to determine the frequency and
speed of the clinging response of C. parasitica under various conditions,
have confirmed that Buccinum shells that had never been occupied by
crabs evoked the clinging response most consistently and rapidly.
Ross and Sutton (1961), while reinvestigating Ross’ (1960) earlier
observation that C. parasitica is not attracted to Buccinum shells
that have been boiled in strong alkali, found that removal of the
periostracum, either by boiling in alkali or by mechanical means, will
reduce, but not abolish, clinging by the anemone. Furthermore, the
anemone does respond to isolated strips of periostracum as to untreated
shells. In addition, C . parasitica does not cling to inactivated shells when
these become reoccupied by crabs, nor does it cling to dummy shells or to
shells coated with a thin plastic layer. From these experiments it was
concluded that the clinging response depends entirely on a general
molluscan “ shell-factor ” associated with the periostracum. An
extension of this work by Davenport et al. (1961) has revealed that
C. parasitica attaches itself to Buccinum shells partially by tentacular
nematocyst activity. Moreover, the threshold of nematocyst discharge
changes markedly in accordance with the attachment behavior of the
animal. The discharge threshold is lower in free animals than in animals
already attached by their pedal disks to a shell. Thus it would appear
that an anemone receives information from the contact of the pedal
disk with the shell which in turn influences the discharge threshold.
It has been suggested that chemoreceptors in the pedal disk may be
responsible for initiating this reaction after responding to some organic
material in the periostracum.
Although the studies cited above strongly suggest chemotaxis of
C. parasitica to the periostracum of Buccinum undatum in the
laboratory, Davenport (personal communication) has related a still
unexplained phenomenon that occurs in nature. According to him, one
can dredge up numerous Buccinum in the English Channel, but one
almost never finds Calliactis parasitica on these. It is only on empty
shells harboring hermit crabs that C. parasitica is found, or on empty
shells alone. Since it is known that the hermit crab, Eupagurus
bernhardus, does not aid the anemone in becoming attached (Ross,
1960), the question arises as to whether the reaction of the anemone to
the periostracum of Buccinum undatum is a conditioned reflex that
only appears after the molluscs vacate their shells. This remains to be
determined.
Another study which demonstrates chemotaxic attraction of a
symbiont to its molluscan host was carried out by Muriel A. Wikswo
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
attached to empty shells. Subsequently, Ross and Sutton (1961), as
the result of a series of tests designed to determine the frequency and
speed of the clinging response of C. parasitica under various conditions,
have confirmed that Buccinum shells that had never been occupied by
crabs evoked the clinging response most consistently and rapidly.
Ross and Sutton (1961), while reinvestigating Ross’ (1960) earlier
observation that C. parasitica is not attracted to Buccinum shells
that have been boiled in strong alkali, found that removal of the
periostracum, either by boiling in alkali or by mechanical means, will
reduce, but not abolish, clinging by the anemone. Furthermore, the
anemone does respond to isolated strips of periostracum as to untreated
shells. In addition, C . parasitica does not cling to inactivated shells when
these become reoccupied by crabs, nor does it cling to dummy shells or to
shells coated with a thin plastic layer. From these experiments it was
concluded that the clinging response depends entirely on a general
molluscan “ shell-factor ” associated with the periostracum. An
extension of this work by Davenport et al. (1961) has revealed that
C. parasitica attaches itself to Buccinum shells partially by tentacular
nematocyst activity. Moreover, the threshold of nematocyst discharge
changes markedly in accordance with the attachment behavior of the
animal. The discharge threshold is lower in free animals than in animals
already attached by their pedal disks to a shell. Thus it would appear
that an anemone receives information from the contact of the pedal
disk with the shell which in turn influences the discharge threshold.
It has been suggested that chemoreceptors in the pedal disk may be
responsible for initiating this reaction after responding to some organic
material in the periostracum.
Although the studies cited above strongly suggest chemotaxis of
C. parasitica to the periostracum of Buccinum undatum in the
laboratory, Davenport (personal communication) has related a still
unexplained phenomenon that occurs in nature. According to him, one
can dredge up numerous Buccinum in the English Channel, but one
almost never finds Calliactis parasitica on these. It is only on empty
shells harboring hermit crabs that C. parasitica is found, or on empty
shells alone. Since it is known that the hermit crab, Eupagurus
bernhardus, does not aid the anemone in becoming attached (Ross,
1960), the question arises as to whether the reaction of the anemone to
the periostracum of Buccinum undatum is a conditioned reflex that
only appears after the molluscs vacate their shells. This remains to be
determined.
Another study which demonstrates chemotaxic attraction of a
symbiont to its molluscan host was carried out by Muriel A. Wikswo