planktotrophic larvae – larvae that must feed in the water column to later
metamorphose into deep benthos. The lecithotrophic types, those with yolky eggs that
develop directly to benthic young (in some animals metamorphosing to the benthic
phase before hatching), show no ovarian cycling. Thus, isopods are not particularly
cyclic in reproduction. This contingency of cycling on reproductive mode suggests
that the adaptive aspect is anticipation by spawners of good feeding conditions for
their young, possibly on the pulsed fallout of the spring bloom. However, there are
few seasonal cues, apart from the pulsing of food, by which to regulate reproductive
timing. Perhaps they simply reproduce when resources allow. Along with
reproduction, there is strong evidence that bulk metabolism of the deep-sea benthos
responds with little delay to the variation of food supply – evidence considered in the
next chapter.
Fig. 13.29 Two aspects of reproductive cycling in two species of clam sampled three
times per year in Rockall Trough, northeast Atlantic. In both species, gonad size
increases from June to about March, and then decreases again. (a) Each “kite
diagram” shows the proportion of individuals in progressively later stages of ovarian
development. (b) Oocyte diameter follows the same timing.
(After Lightfoot et al. 1979.)