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208 PROCEEDINGS OF THE CALIFORNIA ACADEMY OF SCIENCES
Series 4, Volume 60, No. 10
2005). This is kin selection, or, if you prefer, familial selection (Harvey, Bull, Pemberton & Paxton,
1982). Basically the idea here is that the unit that gets selected is not an organism, but a family unit
made up of close relatives. This would work in such terrestrial animals as butterflies, in which the
larvae (caterpillars) hatch out of eggs that are laid in the same place and then hatch out and grow
up in close proximity to their siblings. If one of them gets eaten by a predator, and if the predator
is repelled, then the remainder of the family will be spared. With most marine gastropods, including
most opisthobranchs, these conditions are not met, as was first pointed out by Faulkner and
Ghiselin (1988). Tulrot and Sandberg (1991) provided the same argument. It was further developed
by Rosenberg (1989, 1991). The usual pattern in opisthobranchs is for the fertilized eggs to be laid
in a clutch protected by transparent coverings made up largely of mucopolysaccharide secretion
from the genital duct. The embryos remain in the egg mass while they develop. During that period
they are in close proximity to their siblings, and therefore the conditions for kin selection are in fact
met. Indeed, the eggs of opisthobranchs are often brightly colored and loaded with metabolites as
well. By the end of their embryonic development, however, they hatch out as larvae, swim about
in the water until they find a suitable place for metamorphosis, and settle on the bottom. Only then
do they grow up into mature adults. In consequence, the gene pool gets “stirred up” and the siblings
are separated from one another.
It would appear, then, that the kin or familial mechanism of selection will not work. But things
are not all that straight forward after all (Cimino & Ghiselin, 2001). Opisthobranchs, although they
are simultaneous hermaphrodites, generally do not self-fertilize. They fertilize their eggs with
sperm from the prospective father that is stored in a special organ (receptaculum seminis) until the
mother fertilizes the eggs. More sperm than are necessary for fertilization are produced, and some
of the partner’s sperm may be digested in an organ called the gametolytic gland or bursa copulatrix.
This allows for a kind of recycling, allowing the sperm-donors to contribute to the next generation
but bringing about sperm competition among them. The animals tend to congregate in small
groups, and they may copulate repeatedly. Sometimes, as in the case of Notaspidea, they can be
found in the field living as pairs. The body of any such animal that has copulated contains not only
his and her own germ plasm in the form of unfertilized eggs, but also that of other individuals in
the form of viable sperm. Consequently, from a Darwinian point of view a dead opisthobranch is
still very much alive, so long as the sperm are capable of engendering offspring.
Perhaps the best way to close this methodological excursus is to emphasize the point that quite
a variety of evidence can be brought to bear upon the problems of adaptive significance.
Experiments, whether in the field or the laboratory, comparisons whether of behavior or anatomy,
and the historical data of biogeography and phylogenetics all provide useful evidence and lend one
another mutual support.
CHAPTER III
PRODUCERS AND TRANSFORMERS OF SECONDARY METABOLITES
Part 1. Introduction
Although many opisthobranchs biosynthesize defensive metabolites de novo, their doing so is
a later evolutionary development. Originally they obtained the metabolites from food, and this
remains the general rule. Therefore some background material on the organisms from which the
metabolites are obtained will be useful in helping us understand the evolution of chemical defense