class of polyunsaturated aldehydes with molecular weight around 152 daltons. Not all
diatoms produce these toxins, but when those that do, for example Thalassiosira
pacifica, are abundant and eaten they can cause sharply reduced egg viability in
Calanus and, particularly, Pseudocalanus (Halsband-Lenk et al. 2005). The toxic
mechanism involves disruption of mitotic spindle formation at cell division. This
suggests that in addition to poisoning of embryos by a maternal diatom diet,
malformations of segmentation of the body and legs can be induced in copepodite
stages that eat toxic diatoms (C. Miller, unpublished). Newly forming articulations are
the principal sites of mitosis in later stages. This may produce more immediate
protection of diatoms from herbivores than poisoning of embryos. Leising et al.
(2005) showed that species-specific grazing rates of female Calanus pacificus were
significantly lower on toxic diatoms than on others, even when the toxic species were
dominant. Possibly, toxic aldehydes or their chemical precursors can be detected and
appropriately evaluated by grazers, providing both the copepod and the diatom with a
direct selective benefit.
(^) From egg production experiments, we have some idea of the input of new young to
plankton populations. Egg mortality rates for copepods that spawn freely into the
water (as opposed to those carrying their eggs to hatching) are high, which is likely
true for all freely spawning plankton. Evaluation of the relative abundance of Calanus
eggs at different embryonic stages shows an excess of early division stages, with
implied mortality before hatching on the order of 60% (Miller et al. in prep.).
Probably the bulk of that mortality is predation. Whatever the loss, healthy young
must often hatch in good numbers (40% may be very good), because freely spawning
forms are consistent and abundant components of the plankton community.
(^) There are also many kinds of copepod (Pseudocalanus, Euchaeta, Euchirella,
Oithona, and others) in which the females carry eggs in pouches until they hatch.
Pouches are formed by pushing eggs at spawning into secretions from glands adjacent
to the genital opening. The female then tows these pouches until the eggs hatch.
Survival to hatching is dramatically increased because the eggs benefit from the
mother’s capability to escape from predators (Kiørboe & Sabatini 1994). Therefore,
development can be slower and more complete. In such species, first nauplii are
relatively larger than those of free-spawners, more completely developed, and can
begin to feed at hatching, instead of at N3 as is typical of free-spawners. Greater
survival to hatching allows much smaller numbers of eggs and longer inter-clutch
intervals. Trade-offs between many young each at high risk given low parental care
and fewer young each given greater parental investment are seen all through the
animal kingdom.