136
MALCOLM R. CLARKE
ova buried in a slightly viscous, buoyant, oviducal secretion which is
enveloped in a thick capsule of a viscous, albuminous, nidamental
gland secretion. The egg mass is probably normally deposited on the
sea floor in cavities, to judge from the habits of the captive animals.
Hamabe found that spawning took place a t 15"-20"C and at a chlorinity
of 19~0P19~08°/00. The spawning process is thought to start by a
discharge of a mass of nidamental gland secretion alongside the arms
(Fig. 16). The eggs are passed to the buccal membrane where they are
fertilized and then transferred to the tips of the first and fourth arms.
An oviducal secretion is then discharged and the fertilized eggs are
pushed into it. Further nidamental secretion covers the egg mass and
as it is denser than sea water it settles on the substratum. The whole
process probably takes about two hours.
Hamabe (1963) has shown that, after copulation and spawning,
degeneration of body organs takes place. This led him to conclude
that females probably die after spawning and sink to the sea floor,
because similar changes occur in Sepia esculenta whose buoyant condition after death makes the mortality at spawnhg evident at the sea
surface.
The elliptical or semi-spherical eggs measure 0.7-0.8 mm in greatest
diameter and are telolecithal. A partial cleavage takes place, and 12 h
after spawning the blastoderm extends beyond the equator, and by
24 h the germ layer extends almost over the entire surface of the egg;
by this time the shell and mantle rudiments develop at the animal
pole. Development proceeds and at 40-60 h a number of granules are
discharged from the embryo into the perivitelline space and the
embryo then rotates. The rotation slows down, the embryo adheres to
the egg membrane, Hoyle's organ secretes a membrane-softening agent
and hatching takes place about 102-113 h after spawning.
The newly hatched larvae measure 0.7P1.02 mm long (Fig. 17).
In the sea hatching took place during May and, in the aquarium, the
larvae lived up to 54 h at 15O-18"C and a chlorinity of 19~07-19~330/00.
The very young larvae were unable to swim clear of the bottom. At a
length of 3.0-15.0 mm the larva has the typical " rhynchoteuthis "
form with fused tentacles forming a " rostrum " (Fig. 17) and after
at least 2 weeks the tentacles separate. No prerhynchoteuthis stage
described by Hayashi and Iszuka (1953) was found by Hamabe (1962a)
or Okiyama (196513).
Okiyama (1965b) has found eggs in plankton hauls and suggests
that, whilst they are normally spawned on the bottom, they can become
small floating masses secondarily. Both eggs and larvae increased in
abundance with depth down to 50 m in the daytime suggesting their
MALCOLM R. CLARKE
ova buried in a slightly viscous, buoyant, oviducal secretion which is
enveloped in a thick capsule of a viscous, albuminous, nidamental
gland secretion. The egg mass is probably normally deposited on the
sea floor in cavities, to judge from the habits of the captive animals.
Hamabe found that spawning took place a t 15"-20"C and at a chlorinity
of 19~0P19~08°/00. The spawning process is thought to start by a
discharge of a mass of nidamental gland secretion alongside the arms
(Fig. 16). The eggs are passed to the buccal membrane where they are
fertilized and then transferred to the tips of the first and fourth arms.
An oviducal secretion is then discharged and the fertilized eggs are
pushed into it. Further nidamental secretion covers the egg mass and
as it is denser than sea water it settles on the substratum. The whole
process probably takes about two hours.
Hamabe (1963) has shown that, after copulation and spawning,
degeneration of body organs takes place. This led him to conclude
that females probably die after spawning and sink to the sea floor,
because similar changes occur in Sepia esculenta whose buoyant condition after death makes the mortality at spawnhg evident at the sea
surface.
The elliptical or semi-spherical eggs measure 0.7-0.8 mm in greatest
diameter and are telolecithal. A partial cleavage takes place, and 12 h
after spawning the blastoderm extends beyond the equator, and by
24 h the germ layer extends almost over the entire surface of the egg;
by this time the shell and mantle rudiments develop at the animal
pole. Development proceeds and at 40-60 h a number of granules are
discharged from the embryo into the perivitelline space and the
embryo then rotates. The rotation slows down, the embryo adheres to
the egg membrane, Hoyle's organ secretes a membrane-softening agent
and hatching takes place about 102-113 h after spawning.
The newly hatched larvae measure 0.7P1.02 mm long (Fig. 17).
In the sea hatching took place during May and, in the aquarium, the
larvae lived up to 54 h at 15O-18"C and a chlorinity of 19~07-19~330/00.
The very young larvae were unable to swim clear of the bottom. At a
length of 3.0-15.0 mm the larva has the typical " rhynchoteuthis "
form with fused tentacles forming a " rostrum " (Fig. 17) and after
at least 2 weeks the tentacles separate. No prerhynchoteuthis stage
described by Hayashi and Iszuka (1953) was found by Hamabe (1962a)
or Okiyama (196513).
Okiyama (1965b) has found eggs in plankton hauls and suggests
that, whilst they are normally spawned on the bottom, they can become
small floating masses secondarily. Both eggs and larvae increased in
abundance with depth down to 50 m in the daytime suggesting their