Larval development and larval rearing
After fertilisation, cleavage started and the embryo
further developed inside the fertilisation membrane.
The cleavage was complete and holoblastic.
The blastula stage occurred within an hour after
fertilisation and it became a typical gastrula stage
after one day. The larval development of H. scabra
consisted of auricularia larva, which is a feeding
stage, and then the non-feeding doliolaria larval
stage and finally the pentactula larval stage. Auricularia
is a slipper-shaped, transparent larva with
ciliated bands around the body. It has a pre-oral
loop in the anterior part and an anal loop posterior.
Three larval developments stages – early, middle
and late auricularia – can be identified. Early
auricularia larvae were formed after two days and
late auricularia were formed after five or six days
and remained for 10–13 days. The digestive tract,
comprising foregut, mid gut, hindgut and larval
anus, was observed in the late auricularia stage.
Auricularia larvae are pelagic in habit and feed on
microalgae.
Doliolaria larvae are smaller and more compact
than auricularia larvae. They have a barrel-shaped,
dark-brown body with five ciliated bands (Fig. 4).
In this stage, rapid changes can be observed inside
their body and all adult features begin to form. This
stage remained for 2–3 days (day 14 to 16 in their
life cycle) and plastic sheets (PVC) were transferred
to the larval tanks as soon as the first doliolaria
larva was observed, as they need favourable substratum
to settle on and metamorphose into pentactula
larvae.
Pentactula larvae were observed after 16 days. It
is a tubular-shaped larva with five tentacles at the
anterior end and a single posterior foot. The body
colour is dark with a greenish-grey tinge. Tube feet
develop all over the body. The pentactula creeps
over the sides and bottom of the tank and they
actively feed on benthic algae and other detritus.
After one month they become typical sea cucumbers
with the same body shape as adults, but in the
early juvenile stages there are two long tube feet at
the posterior end. In this study, they were tightly
attached to the settlement substrata but could perform
slow activities. The juveniles produced in the
indoor hatchery tanks (~ 3 cm length) were nursed
in outdoor fibreglass tanks until they reached
around 5–10 g (Fig. 4). The nursed juveniles were
grown out in a lagoon pen.
The development stages of P. violaceus and C. quadrangularis
are very similar to those of H. scabra. In
both these species, the non-feeding doliolaria stage
appeared around 13–14 days but juvenile growth
was very low compared to H. scabra juveniles. Juveniles
of these species took more than two months to
attain an average length of 5–8 mm and P. violaceus
juveniles attained an average length of 1 cm only
after four months (Fig. 5).
Grow-out of juveniles
H. scabra juveniles with an average weight of
11 g ± 4 were transferred into a pen which was constructed
in Puttlam lagoon. The size of the pen was