Decoupling of copepod grazing rates, fecundity and egg-hatching success on mixed and alternating diatom and dinoflagellate diets
J.T.
Turnet, A. Ianora, A. Miralto, M. Laabir, F. Esposito-2001
Marine Ecology Progress Series, 220: 187-199
(from Current Contents)
Abstract :
Experiments were conducted over 10 to 20 d periods to
study the grazing and reproductive success of the copepod Temora sWera fed
on unialgal cultures of the diatom Thalassiosira rotula (THA) or the
dinoflagellate Prorocentrum minimum (PRO), as well as mixtures of THA and
PRO (MIX experiments) and alternating diets of THA and PRO switched daily
(SWITCH experiments). Adult females ate both THA and PRO, and while rates of
feeding on the 2 diets were similar in terms of carbon ingestion, egg
production was generally higher on the diatom diet. In contrast, copepod
egg-hatching success was low on the diatom diet, declining rapidly after 2 d
from > 80 to 0% by Day 17. The diminution in hatching success was slower
when females were fed MIX or SWITCH diets, but nonetheless diminished to 0
and < 25% by the end of the experiment, depending on the incubation
method. Only in the case of the PRO diet was egg viability high and stable
with time (87 to 96%), regardless of whether female and male couples were
incubated as individual couples in crystallizing dishes or as triplicate
couples in rotating bottles. However, in most other cases, the incubation
method (crystallizing dishes vs rotating bottles) had very strong effects on
egg and fecal pellet production, and hatching success. Higher egg production
rates were generally obtained when females were incubated in crystallizing
dishes, whatever the diet, although fecal pellet, production rates were
significantly higher in the rotating bottle experiments in most cases.
Egg-hatching success was also strongly affected by incubation method, with
generally higher hatching rates in the rotating bottles. This was probably
due to the fragility of non-viable eggs, which were more easily destroyed by
mechanical disturbance in rotating bottle experiments. The results support
the recent discovery that reproductive failure in copepods can be due to
deleterious antimitotic compounds present in some diatoms that arrest normal
embryonic division. Reduction in egg viability was not only visible when
females were fed unialgal diatom diets, but also when they were fed mixed
diets. However, on mixed diets there was a 'dilution effect' in that
hatching was reduced by approximately half, and this took about twice as
long to occur. The evolutionary advantages for diatoms in producing
antimitotic compounds are discussed, as well as questions of why copepods
feed on diatoms with impunity, even though some diatoms are detrimental to
copepod reproductive success.
(Univ Massachusetts, Sch Marine Sci & Technol,
706 S Rodney French Blvd, New Bedford, MA 02744, USA, e-mail: jturner@umassd.edu)