ZLEARINU OF BIVALVE MOLLU8KY
119
Success of fertilizing eggu stripped from conditioned females depended upon the degree of their ripeness. Early in the season, when
the largest of the eggs were only 6611. nolie were fertilizable. Later,
when the clams were riper, a large perccntage of stripped eggs underwent cleavage but, as is often the cast in using eggs obtained in this
manner, most of the larvae were abnormal (Fig. 40). Costello et al.
(1957) also reported that eggs of E . directus can be fertilized artificially
and they remain fertilizable for about 5 hr.
The egg of a razor clam is surrounded by a narrow perivitelline
space enclosed in n thin vitelline menibrane. Measurements of eggs
discharged in a normal spawning showod that they varied in diameter
from about 64 to 73 p, the majority being 71 p, thus indicating that
this is the modal size of iiormal eggs. Lebour (1!)38) gave the diameter
of eggs of Ensis Riliqua as about 7 0 p, which closdy resembles measurements of eggs of our species. Jcirgensen (1946) gnve references to other
workers who studied eggs and larvae of En.& and related forms.
Eggs and larvae were usually cultured at about 22°C. The smallest
straight-hinge larvae were 80 p in length, but only two individuds
this small were observed. Even larvae as small as 85 p in length were
scarce, while those measuring approximately 92 x 78 p were common.
This size is considerably smaller than the 140 x 120 p indicated by
Sullivan (1948) as the smallest larvae of this species.
The larvae are light in color and more transparent than those of
M . mercenaria. The umbo appears in litrvne only 116 p in length and
becomes well developed in larvae 135 p long. Larvae of this size have
definitely passed the straight-hinge stage. The umbo is so pronounced
in larger larvae that it is impossible for them to lie flat. As a result,
considerable variation in the length-width ratio is obtained when
large larvae are measured under a microscope.
Setting of E . directus began soon after some larvae reached a length
of about 210 p. A t 24°C this often occiirred only about 10 days after
fertilization. However, some larvae rtbmained swimming until they
reached a much larger size before undergoing metamorphosis. For
example, in one culture fed Chlorella sp. larvae measuring 250 to 260 p
in length were still swimming 27 days after fertilization and possessed
both a powerful velum and a well-developed Soot. The largest swimming
larva measured 270p, while in the same culture many fully metamorphosing individuals showing a well-developed foot, gills and
siphons were only 220 p long.
The appearance of the " eye " was irregular and it was not located
where this structure is usually found in most bivalve larvae. The
smallest individual possessing thig struct ure was only 126 p long, but
119
Success of fertilizing eggu stripped from conditioned females depended upon the degree of their ripeness. Early in the season, when
the largest of the eggs were only 6611. nolie were fertilizable. Later,
when the clams were riper, a large perccntage of stripped eggs underwent cleavage but, as is often the cast in using eggs obtained in this
manner, most of the larvae were abnormal (Fig. 40). Costello et al.
(1957) also reported that eggs of E . directus can be fertilized artificially
and they remain fertilizable for about 5 hr.
The egg of a razor clam is surrounded by a narrow perivitelline
space enclosed in n thin vitelline menibrane. Measurements of eggs
discharged in a normal spawning showod that they varied in diameter
from about 64 to 73 p, the majority being 71 p, thus indicating that
this is the modal size of iiormal eggs. Lebour (1!)38) gave the diameter
of eggs of Ensis Riliqua as about 7 0 p, which closdy resembles measurements of eggs of our species. Jcirgensen (1946) gnve references to other
workers who studied eggs and larvae of En.& and related forms.
Eggs and larvae were usually cultured at about 22°C. The smallest
straight-hinge larvae were 80 p in length, but only two individuds
this small were observed. Even larvae as small as 85 p in length were
scarce, while those measuring approximately 92 x 78 p were common.
This size is considerably smaller than the 140 x 120 p indicated by
Sullivan (1948) as the smallest larvae of this species.
The larvae are light in color and more transparent than those of
M . mercenaria. The umbo appears in litrvne only 116 p in length and
becomes well developed in larvae 135 p long. Larvae of this size have
definitely passed the straight-hinge stage. The umbo is so pronounced
in larger larvae that it is impossible for them to lie flat. As a result,
considerable variation in the length-width ratio is obtained when
large larvae are measured under a microscope.
Setting of E . directus began soon after some larvae reached a length
of about 210 p. A t 24°C this often occiirred only about 10 days after
fertilization. However, some larvae rtbmained swimming until they
reached a much larger size before undergoing metamorphosis. For
example, in one culture fed Chlorella sp. larvae measuring 250 to 260 p
in length were still swimming 27 days after fertilization and possessed
both a powerful velum and a well-developed Soot. The largest swimming
larva measured 270p, while in the same culture many fully metamorphosing individuals showing a well-developed foot, gills and
siphons were only 220 p long.
The appearance of the " eye " was irregular and it was not located
where this structure is usually found in most bivalve larvae. The
smallest individual possessing thig struct ure was only 126 p long, but