HEARING O F BIVALVE MOLLUSKS
89
reached 275 p, and a few individuals measuring between 295 arid 305 p
in length still retained a functional velum.
While some larvae metamorphosed in about 12 to 14 days after
fertilization, many others in the same cultures were still free-swimming
when the experiment was discontinued after 43 days (Fig. 30). I n
one of these cultures, while there were still free-swimming larvae from
250 to 29.5 p long, there were also several thousand completely metamorphosed juvenile mussels that ranged up to l mm in length. Why
so many of the larvae were still in the free-swimming stage, while others
in thc same cultures had mctamorphoscd and showed considerable
post-larval growth, remaincd uncxplaitied.
According to Sullivan (1948) the smallest straight-hinge larvae of
M . demissus measure approximately 112 x 90 p. This size quite
closely agrees with our measurements. However, the maximum size
given by Sullivan, 205 x 180 p, is considerably smaller than that
observed in our cultures, which was about 303 x 260 p. We also
differ with Sullivan on the color of ribbed mussel larvae because she
states that these larvae are a deep yellow in the early stages, whereas
we found that throughout their development they are brown.
Jarrgensen ( 1946) discussed larvae of closely-related species. In
his references to Modiolus modiolus Jsrgensen stated that larvae of this
species have not been identified with certainty in plankton samples
taken in Danish waters. He thinks that this is probably due to the
very close resemblance of this species to veligers of Mytilus edulis. We
disagree with Jarrgensen since, in our studies of larvae of these two
species, we found them to be distinctly different in their lengthwidth measurements. Furthermore, the difference in their general
appearance, especially during later stages of development, is too great
to confuse these two groups of veligers.
Rees (1950) stated that in the smallest veligers of Modiolus and
Mytilus the shape is similar, although in the former the narrow end
is less pointed. Nevertheless, in later stages the shapes of the two
genera become more differentiated. Rees also thought that the small
specimens of the two mussels are distinguished more easily by their
texture than by their shape. He concluded that Jsrgensen’s drawing
(Fig. 167d) is probably of Modiolus modiolus, instead of Mytilus edulis,
as claimed by Jsrgensen. However, examination of the photomicrographs of, presumably, lclodiolus modiolus offered by Rees clearly
shows that their shape significantly differs from that of Modiolus
demissus grown in our cultures from known parents. This is especially
true of his larvae, approximately 250 p in length, which have very
little resemblance to our larvae of the same length.
89
reached 275 p, and a few individuals measuring between 295 arid 305 p
in length still retained a functional velum.
While some larvae metamorphosed in about 12 to 14 days after
fertilization, many others in the same cultures were still free-swimming
when the experiment was discontinued after 43 days (Fig. 30). I n
one of these cultures, while there were still free-swimming larvae from
250 to 29.5 p long, there were also several thousand completely metamorphosed juvenile mussels that ranged up to l mm in length. Why
so many of the larvae were still in the free-swimming stage, while others
in thc same cultures had mctamorphoscd and showed considerable
post-larval growth, remaincd uncxplaitied.
According to Sullivan (1948) the smallest straight-hinge larvae of
M . demissus measure approximately 112 x 90 p. This size quite
closely agrees with our measurements. However, the maximum size
given by Sullivan, 205 x 180 p, is considerably smaller than that
observed in our cultures, which was about 303 x 260 p. We also
differ with Sullivan on the color of ribbed mussel larvae because she
states that these larvae are a deep yellow in the early stages, whereas
we found that throughout their development they are brown.
Jarrgensen ( 1946) discussed larvae of closely-related species. In
his references to Modiolus modiolus Jsrgensen stated that larvae of this
species have not been identified with certainty in plankton samples
taken in Danish waters. He thinks that this is probably due to the
very close resemblance of this species to veligers of Mytilus edulis. We
disagree with Jarrgensen since, in our studies of larvae of these two
species, we found them to be distinctly different in their lengthwidth measurements. Furthermore, the difference in their general
appearance, especially during later stages of development, is too great
to confuse these two groups of veligers.
Rees (1950) stated that in the smallest veligers of Modiolus and
Mytilus the shape is similar, although in the former the narrow end
is less pointed. Nevertheless, in later stages the shapes of the two
genera become more differentiated. Rees also thought that the small
specimens of the two mussels are distinguished more easily by their
texture than by their shape. He concluded that Jsrgensen’s drawing
(Fig. 167d) is probably of Modiolus modiolus, instead of Mytilus edulis,
as claimed by Jsrgensen. However, examination of the photomicrographs of, presumably, lclodiolus modiolus offered by Rees clearly
shows that their shape significantly differs from that of Modiolus
demissus grown in our cultures from known parents. This is especially
true of his larvae, approximately 250 p in length, which have very
little resemblance to our larvae of the same length.