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210 PROCEEDINGS OF THE CALIFORNIA ACADEMY OF SCIENCES
Series 4, Volume 60, No. 10
Part 2. Prokaryotes
Prokaryotes are distinguished from eukaryotes on the basis of the structure of the cell. The
prokaryote cell is simpler than the eukaryote cell and lacks such features as a nucleus and the membrane
that surrounds it. The prokaryotes consist of two major groups of bacteria, Archaebacteria
and Eubacteria. The Eukaryotes are more closely related to the Archaebacteria than to Eubacteria:
in other words the bacteria are a paraphyletic group. The Eubacteria include all of the bacteria that
are of interest in this discussion, and these belong to two closely-related lineages, the
Actinobacteria and Cyanobacteria. The Cyanobacteria were formerly considered plants and were
called blue-green algae, or Cyanophyta. They form small filaments that are readily visible to the
naked eye, and superficially resemble some true algae. A case of poisoning that occurred when people
in Hawaii ate the red alga Gracillaria coronopifolia was later attributed to the cyanobacteria
growing epiphytically on it (Nagai, Yasumoto & Hokama, 1997).
Animals that feed on prokaryotic or eukaryotic “algae” sometimes have switched from one to
the other during the course of evolution. This is but one example of how the texture and metabolite
content of the food are often more important than its taxonomy. Bacteria are noteworthy for
their chemical versatility. Many of them synthesize metabolites that eukaryotes do not. They are
particularly important as symbionts that synthesize alkaloids, macrolides, and polyketides. Bacteria
that live as symbionts within the bodies of marine animals such as sponges and sea-squirts are the
source of alkaloids and some other secondary metabolites. (For review of marine microorganisms
and fungi with secondary metabolites, see Pietra, 1997; Bernan, 2001; Engel, Jensen & Fenical,
2002).
Some unfortunate claims have occasionally made, even in print, about the putative role of
symbionts in producing secondary metabolites. A particularly glaring example is to be found in a
recent publication by Castoe, Stephens, Noonan and Calestani (2006:“2”): “Polyketide compounds,
used as defensive mechanisms to deter predation, have been isolated from some marine
invertebrates (e.g., sponges and mollusks; Garson, 1989), although these polyketides were subsequently
found to be produced by bacterial symbionts (and not encoded in the animal genomes).”
What the authors allege simply is not true. They do not cite a single publication in favor of their
claim, and had anything of the sort been published it would surely have been brought to our attention.
One way or another, what perhaps began as a mere speculation thrown out in casual conversation
has become a rumor, or even what is sometimes called an “urban myth.” Now, given the fact
that some, indeed many, animals make use of metabolites that are produced by their symbionts, it
is perfectly reasonable to ask, in any given case, whether it is the host or some guest that synthesizes
it. There are two possibilities and we need to apply the proper canons of evidence to decide
between them, just as we do when deciding whether metabolites are produced endogenously or are
derived from food. Many opisthobranchs obtain metabolites by eating other organisms that in turn
get them from symbionts. It has not been difficult to recognize those symbionts, which exist in substantial
numbers and are located in particular tissues and specialized organs within the body of the
host. In opisthobranchs there are no tissues or organs that are reasonable candidates for harboring
such a population of symbionts, and no such populations have been identified. In those cases where
biosynthesis has been studied experimentally, and in which the biology of the animals is well
understood, the production of metabolites by symbionts, although perhaps not as rigorously
excluded as a hostile critic might demand, is highly implausible. In the cephalaspidean Haminoea,
for example, the animal biosynthesizes defensive metabolites called fusipyrones. But the animals
also biosynthesize a series of metabolites called haminols, which are not defensive. Rather they
function as alarm pheromones, which alert conspecifics to attack by predators, and probably also