288
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
abdominal segmentation incomplete ; median eye present ; first antenna
of four segments, first segment with fourteen bristles, second with four,
third with two, and fourth with five bristles ; second antenna of three
segments, hook-shaped ; mandible short, strobiliform, always with two
sharp bristles ; maxilla in form of triangular plate with lateral comb of
teeth and medially located feeler ; f i s t maxilliped of female reduced
to small chitinous thickening, that of male hook-shaped at same
position; second maxilliped of male is only noticeable as a weak
chitinous thickening ; two-jointed endopodite and exopodite of first
four pairs of legs with double chitinous ring ; first four pairs of legs
with short setae and spines and fine hairs along exterior margin ; fifth
pair of legs reduced to short bristled pegs; genital openings paired;
egg sac of female long and slender.
Life cycle. Studies on aspects of the life cycle of M . intestinalis
have been contributed by Pesta (1907), Caspers (1939) and Grainger
(1951). The nauplii escape into the host’s alimentary canal from within
egg sacs when the latter rupture at their posterior ends. These nauplii
are enclosed within a membrane. It is still not known how these nauplii
escape from the enclosing membrane but in sea water cultures the
nauplii escape from the egg sacs leaving this membrane behind. The
nauplius and metanauplius are free-swimming. Although in laboratory
cultures the first, second and third copepodid stages can be found freeswimming, Grainger (1951) is of the opinion that M . intestinalis can
become parasitic during the first copepodid stage. Hepper (1953), in
reporting his successful experimental infections of various molluscs,
did not indicate whether i t was the first, second or third copepodid
that invades the molluscs ; however, since Steuer (1902) has reported
that the youngest forms found in naturally infected Mytilus galbprovincialis are 0.83 mm long and with bodies similar to that of the
adult, it may be assumed that these represent first stage copepodids.
Thus M . intestinalis does undergo further development, through the
second and third copepodid stages to the adult, within the molluscan
host. It has not yet been demonstrated how first stage copepodids
enter molluscs but, since they are found within the gut, it may be
assumed that they are ingested.
Ecology. Various investigators have reported that the nauplius of
M . intestinalis is very active and is positively phototactic. The metanauplius is less active and less positively phototactic. The first copepodid,
like the nauplius, is an active swimmer although its tactic response to
sunlight is varied. Some are positively phototactic, others are negatively
phototactic, and still others do not respond to sunlight. The varied
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
abdominal segmentation incomplete ; median eye present ; first antenna
of four segments, first segment with fourteen bristles, second with four,
third with two, and fourth with five bristles ; second antenna of three
segments, hook-shaped ; mandible short, strobiliform, always with two
sharp bristles ; maxilla in form of triangular plate with lateral comb of
teeth and medially located feeler ; f i s t maxilliped of female reduced
to small chitinous thickening, that of male hook-shaped at same
position; second maxilliped of male is only noticeable as a weak
chitinous thickening ; two-jointed endopodite and exopodite of first
four pairs of legs with double chitinous ring ; first four pairs of legs
with short setae and spines and fine hairs along exterior margin ; fifth
pair of legs reduced to short bristled pegs; genital openings paired;
egg sac of female long and slender.
Life cycle. Studies on aspects of the life cycle of M . intestinalis
have been contributed by Pesta (1907), Caspers (1939) and Grainger
(1951). The nauplii escape into the host’s alimentary canal from within
egg sacs when the latter rupture at their posterior ends. These nauplii
are enclosed within a membrane. It is still not known how these nauplii
escape from the enclosing membrane but in sea water cultures the
nauplii escape from the egg sacs leaving this membrane behind. The
nauplius and metanauplius are free-swimming. Although in laboratory
cultures the first, second and third copepodid stages can be found freeswimming, Grainger (1951) is of the opinion that M . intestinalis can
become parasitic during the first copepodid stage. Hepper (1953), in
reporting his successful experimental infections of various molluscs,
did not indicate whether i t was the first, second or third copepodid
that invades the molluscs ; however, since Steuer (1902) has reported
that the youngest forms found in naturally infected Mytilus galbprovincialis are 0.83 mm long and with bodies similar to that of the
adult, it may be assumed that these represent first stage copepodids.
Thus M . intestinalis does undergo further development, through the
second and third copepodid stages to the adult, within the molluscan
host. It has not yet been demonstrated how first stage copepodids
enter molluscs but, since they are found within the gut, it may be
assumed that they are ingested.
Ecology. Various investigators have reported that the nauplius of
M . intestinalis is very active and is positively phototactic. The metanauplius is less active and less positively phototactic. The first copepodid,
like the nauplius, is an active swimmer although its tactic response to
sunlight is varied. Some are positively phototactic, others are negatively
phototactic, and still others do not respond to sunlight. The varied