74
VICTOR L. LOOSANOFF AND HARRY C. DAVIS
maximum lengths of larvae at time of setting and water temperature.
Finally, no relationship was found between shape of larvae (ratio of
mean lengths to mean widths) at the time of setting and water temperature.
Incidentally, these studies gave us an opportunity to verify the
opinion expressed by some authors that forms Ijving in warmer waters
have just as long a pelagic life as related species living in northern
waters, and that eggs and larvae of southern species develop more
slowly than those of northern species of the same genus if both species
are kept under identical temperature conditions. We conducted these
studies with M. mercenaria of Long Island Sound and M. eampecfiienais
imported from the Gulf of Mexico. Both groups were ripened in our
laboratory under identical conditions and induced to spawn by the
usual methods. Eggs and larvae of both species were gro& at a
temperature of 21°C in triplicate cultures, and random samples of
100 larvae from each culture were taken for length and mortality
measurements every two days from fertilization to metamorphosis.
Results clearly indicated that rates of growth and mortality of
larvae of the two species were practically identical and that setting of
larvae of both groups began simultaneously. On the basis of these
observations we may conclude that the belief that eggs and larvae
of southern species, when grown at the same temperatures, develop
more slowly than those of the northern species of the same genus is
not true in all cases.
As has already been mentioned, in some bivalves the beginning of
metamorphosis may be delayed because of unfavorable conditions.
I n addition, in at least one case, namely, that of A m i a simplez, we
observed what appeared to be partial or interrupted metamorphosis
(Loosanoff, 1961). While in larvae of this species metamorphosis
commonly occurs at a length of approximately 200 p , hundreds of
individuals considerably larger than 215 p were seen in some cultures
crawling on the bottom by means of a powerful ciliated foot. One of
these individuals megsured 577 x 514 p.
Partial metamorphosis was characterized by disappearance of the
velum but retention of a functional foot. Moreover, examination of
these crawlers showed another common characteristic, namely, a
definite narrow band on their shells, which probably indicated the
edge of prodissoconch, or larval shell, and the beginning of dissoconch,
or post-larval shell. We assume that the band dividing the two portions
of the shell was formed during an important event in the life of the
young mollusk, possibly when the velum had entirely disappeared
or some other equally important anatomical or physiological change
VICTOR L. LOOSANOFF AND HARRY C. DAVIS
maximum lengths of larvae at time of setting and water temperature.
Finally, no relationship was found between shape of larvae (ratio of
mean lengths to mean widths) at the time of setting and water temperature.
Incidentally, these studies gave us an opportunity to verify the
opinion expressed by some authors that forms Ijving in warmer waters
have just as long a pelagic life as related species living in northern
waters, and that eggs and larvae of southern species develop more
slowly than those of northern species of the same genus if both species
are kept under identical temperature conditions. We conducted these
studies with M. mercenaria of Long Island Sound and M. eampecfiienais
imported from the Gulf of Mexico. Both groups were ripened in our
laboratory under identical conditions and induced to spawn by the
usual methods. Eggs and larvae of both species were gro& at a
temperature of 21°C in triplicate cultures, and random samples of
100 larvae from each culture were taken for length and mortality
measurements every two days from fertilization to metamorphosis.
Results clearly indicated that rates of growth and mortality of
larvae of the two species were practically identical and that setting of
larvae of both groups began simultaneously. On the basis of these
observations we may conclude that the belief that eggs and larvae
of southern species, when grown at the same temperatures, develop
more slowly than those of the northern species of the same genus is
not true in all cases.
As has already been mentioned, in some bivalves the beginning of
metamorphosis may be delayed because of unfavorable conditions.
I n addition, in at least one case, namely, that of A m i a simplez, we
observed what appeared to be partial or interrupted metamorphosis
(Loosanoff, 1961). While in larvae of this species metamorphosis
commonly occurs at a length of approximately 200 p , hundreds of
individuals considerably larger than 215 p were seen in some cultures
crawling on the bottom by means of a powerful ciliated foot. One of
these individuals megsured 577 x 514 p.
Partial metamorphosis was characterized by disappearance of the
velum but retention of a functional foot. Moreover, examination of
these crawlers showed another common characteristic, namely, a
definite narrow band on their shells, which probably indicated the
edge of prodissoconch, or larval shell, and the beginning of dissoconch,
or post-larval shell. We assume that the band dividing the two portions
of the shell was formed during an important event in the life of the
young mollusk, possibly when the velum had entirely disappeared
or some other equally important anatomical or physiological change