REARING OF BIVALVE MOLLUSKS
117
Since studies on larvae of this species were carried on when good
food organisms, such as the naked flagellates, were not available, the
larvae were fed mixed cultures of microorganisms in which ChloreUa
was usually the predominating form. Larvae grew well on this alga,
although it is quite certain that they would have grown faster if given
better foods.
Larvae of P. pholadifomnis were grown at temperatures ranging
from 18" to 30°C. The best cultures were observed at room temperatures
ranging from 20" to 25°C. A t somewhat lower temperatures, between
17' and 18"C, eggs developed and larvae grew, but growth was slow.
At 30°C egg development was rapid, but larvae were feeble and the
majority died within 2 to 4 days. Perhaps mortality was not due
exclusively to temperature alone, but to the heavy bacterial flora that
quickly established itself under these conditions.
The shortest period from fertilization of the egg to metamorphosis
at room temperature was 13 days. In several instances, however, the
larvae of these cultures did not begin to set until the 20th or 21st day.
The differences were probably due to variations in quality and quantity
of food.
Two characteristics were noticeable in young clams soon after
metamorphosis. First, the color of their shells becomes muoh denser
and, secondly, the edges of the shells opposite the umbo acquire a
serrated, roughened appearance.
P. Ensis directus (Conrad)
The general characteristics of larvae of E. directus were given
recently by Sullivan (1948). Costello et al. ( I 957) gave a brief description of early HtageH of (:gg dsvnlopmctnt, whilc l,ohoiir ( 1 IW) tirul nLiidiod
larvae of fiaxin diquu. I n gt:nc:ral, howt:vc:r, lrirvao of 1C. djrectw
have bcon l c s ~ iritcnsively studied than thoso of other common bivalves
of our coast.
In our laboratory, observations on E'. directua and its larvae were
conducted for three winters. Each year these clams were brought
from the field and placed in conditioning trays in late December. It
was found, however, that when these clams arc removed from the oil
they soon begin to die because the powerful hinge pulls the clam
shells apart. This can be prevented by placing tight rubber bandu
mound each clam to counteract the action of the hinge and kccp itH
shells closed. Using this simple method thew clam8 havo hcen kept
alive without any soil for periods aH long
Attempts to induce spawning of E. directm were madc during tho
middle of January after 2 or 3 wecks of conditioning. Although at
A month.
117
Since studies on larvae of this species were carried on when good
food organisms, such as the naked flagellates, were not available, the
larvae were fed mixed cultures of microorganisms in which ChloreUa
was usually the predominating form. Larvae grew well on this alga,
although it is quite certain that they would have grown faster if given
better foods.
Larvae of P. pholadifomnis were grown at temperatures ranging
from 18" to 30°C. The best cultures were observed at room temperatures
ranging from 20" to 25°C. A t somewhat lower temperatures, between
17' and 18"C, eggs developed and larvae grew, but growth was slow.
At 30°C egg development was rapid, but larvae were feeble and the
majority died within 2 to 4 days. Perhaps mortality was not due
exclusively to temperature alone, but to the heavy bacterial flora that
quickly established itself under these conditions.
The shortest period from fertilization of the egg to metamorphosis
at room temperature was 13 days. In several instances, however, the
larvae of these cultures did not begin to set until the 20th or 21st day.
The differences were probably due to variations in quality and quantity
of food.
Two characteristics were noticeable in young clams soon after
metamorphosis. First, the color of their shells becomes muoh denser
and, secondly, the edges of the shells opposite the umbo acquire a
serrated, roughened appearance.
P. Ensis directus (Conrad)
The general characteristics of larvae of E. directus were given
recently by Sullivan (1948). Costello et al. ( I 957) gave a brief description of early HtageH of (:gg dsvnlopmctnt, whilc l,ohoiir ( 1 IW) tirul nLiidiod
larvae of fiaxin diquu. I n gt:nc:ral, howt:vc:r, lrirvao of 1C. djrectw
have bcon l c s ~ iritcnsively studied than thoso of other common bivalves
of our coast.
In our laboratory, observations on E'. directua and its larvae were
conducted for three winters. Each year these clams were brought
from the field and placed in conditioning trays in late December. It
was found, however, that when these clams arc removed from the oil
they soon begin to die because the powerful hinge pulls the clam
shells apart. This can be prevented by placing tight rubber bandu
mound each clam to counteract the action of the hinge and kccp itH
shells closed. Using this simple method thew clam8 havo hcen kept
alive without any soil for periods aH long
Attempts to induce spawning of E. directm were madc during tho
middle of January after 2 or 3 wecks of conditioning. Although at
A month.