8. THE PHYLUM MOLLUSUA
281
Although these snails may not be very successful in killing the oysters after
the latter reach the size of about 1.0 cm, they, no doubt, interfere with the
oysters’ normal development and growth. This is often shown by the
characteristically deformed shells of the young oysters that came from areas
heavily infested with M . bisutularis. The shells, instead of being flat, which
is normal for shells of young oysters that are not too crowded, are deeply
cupped and may have thickened edges. This abnormality is apparently due
to injuries caused by the activities of M . bisutularis, to the edges of the
oyster mantle, the organ that secretes the shell.
3. Odostomia seminuda Adams, 1839. (Fig. 184)
(Class Gastropoda; subclass Opisthobranchia; order Tectibranchia; family
Odostomia seminuda, considered by some to be a member of the
subgenus Chrysallida, appears to be another North American species.
Originally described by Adams (1839) from four specimens collected
on single valves of Aequipecten irradians, it was rediscovered by Hackney
(1944) from the upper valves of the same scallop and presumed to be a
parasite. Subsequently, it has been found as a parasite on Crepidula
fornicata by Robertson (1957) and on Aequipecten gibbus by Wells and
Wells (1961).
Description. About 4-7 mm long ; shell white, often glossy, stoutly
conical, with six or seven whorls and distinct sutures ; sculptured with
several revolving ridges and cut by vertical striations so that the
surface appears beaded in most instances but consistently with four
rows of tubercles (beads) on each whorl ; with a strong oblique fold or
plait a t base of columella and within aperture.
Ecology. 0. seminuda occurs from Prince Edward Island, Canada,
to the Gulf of Mexico in North America.
Although direct evidence is wanting, the observation by Robertson
(1957) that 0. seminuda is consistently found on or near Crepidula
fornicata although numerous Aequipecten irradians occur in the immediate vicinity suggests that it demonstrates a preference for the
gastropod. This parasite is active and when not feeding may move
around, sometimes away from the host for a short distance.
Robertson has also reported that during the first half of August a t
Woods Hole, Massachusetts, adults measuring up to 4 mm in length
occur concurrently with juveniles, some less than 1 mm long, thus
suggesting that spawning had recently taken place.
Robertson’s hypothesis that 0. seminuda prefers C . fornicata over
A. irradians, and perhaps other hosts, was tested in a series of behavioral
experiments by Boss and Merrill (1965). They collected specimens of
Pyramidellidae )
281
Although these snails may not be very successful in killing the oysters after
the latter reach the size of about 1.0 cm, they, no doubt, interfere with the
oysters’ normal development and growth. This is often shown by the
characteristically deformed shells of the young oysters that came from areas
heavily infested with M . bisutularis. The shells, instead of being flat, which
is normal for shells of young oysters that are not too crowded, are deeply
cupped and may have thickened edges. This abnormality is apparently due
to injuries caused by the activities of M . bisutularis, to the edges of the
oyster mantle, the organ that secretes the shell.
3. Odostomia seminuda Adams, 1839. (Fig. 184)
(Class Gastropoda; subclass Opisthobranchia; order Tectibranchia; family
Odostomia seminuda, considered by some to be a member of the
subgenus Chrysallida, appears to be another North American species.
Originally described by Adams (1839) from four specimens collected
on single valves of Aequipecten irradians, it was rediscovered by Hackney
(1944) from the upper valves of the same scallop and presumed to be a
parasite. Subsequently, it has been found as a parasite on Crepidula
fornicata by Robertson (1957) and on Aequipecten gibbus by Wells and
Wells (1961).
Description. About 4-7 mm long ; shell white, often glossy, stoutly
conical, with six or seven whorls and distinct sutures ; sculptured with
several revolving ridges and cut by vertical striations so that the
surface appears beaded in most instances but consistently with four
rows of tubercles (beads) on each whorl ; with a strong oblique fold or
plait a t base of columella and within aperture.
Ecology. 0. seminuda occurs from Prince Edward Island, Canada,
to the Gulf of Mexico in North America.
Although direct evidence is wanting, the observation by Robertson
(1957) that 0. seminuda is consistently found on or near Crepidula
fornicata although numerous Aequipecten irradians occur in the immediate vicinity suggests that it demonstrates a preference for the
gastropod. This parasite is active and when not feeding may move
around, sometimes away from the host for a short distance.
Robertson has also reported that during the first half of August a t
Woods Hole, Massachusetts, adults measuring up to 4 mm in length
occur concurrently with juveniles, some less than 1 mm long, thus
suggesting that spawning had recently taken place.
Robertson’s hypothesis that 0. seminuda prefers C . fornicata over
A. irradians, and perhaps other hosts, was tested in a series of behavioral
experiments by Boss and Merrill (1965). They collected specimens of
Pyramidellidae )