Photoperiodic modulation of
metamorphosis in Atlantic halibut (Hippoglossus hippoglossus L.)
J.S.
Solbakken, K. Pittman-2004
Aquaculture, 232: 613-625
Abstract:
The effect of abrupt changes in photoperiod on
induction of metamorphosis in Atlantic halibut was investigated. Larvae were
reared in a 3.7-m3 tank on enriched brine shrimp (Artemia)
from first feeding at 265 day degrees (d°C) (44 days post hatch, DPH) until
66 days (DPH) under continuous illumination or to a standard length of 17 mm
corresponding to the beginning of metamorphosis. Thereafter, the larvae were
divided into two groups; one further exposed to continuous light (24L:0D),
the other given a 12L:12D regime, with four replicates within each
treatment. The experiment was terminated at 116 DPH.
An abrupt decrease in photoperiod at commencement of
metamorphosis briefly stimulated eye migration and slowed growth. Growth was
not significantly different at the end of the experiment. The eye migration
to standard length ratio was significantly higher under 12L:12D than in
larvae under continuous light at 116 DPH. Continuous light accelerated the
appearance of haemoglobin in the circulation, gave a higher frequency of
normal pigmentation, and higher survival than did 12L:12D in metamorphosing
halibut larvae. The sequence of measurable metamorphic events was generally
neural change, growth and skeletal change, circulatory change and finally
establishment of the pigmentation pattern. Heterochrony of metamorphic
events was observed for eye migration and for the appearance of haemoglobin
with respect to age but not size. The results indicate that a short period
of decreased photoperiod may act as an initiator for some metamorphic events
The underlying mechanism may be the action of photoperiod on melatonin and
stage specific thyroid hormone interaction. The interaction between
ontogeny, photoperiod and endocrinology should be further investigated.
(Department of Fisheries and Marine Biology,
University of Bergen, Box 7800, 5020, Bergen, Norway, e-mail of K. Pittman: karin.pittman@ifm.uib.no)