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224 PROCEEDINGS OF THE CALIFORNIA ACADEMY OF SCIENCES
Series 4, Volume 60, No. 10
cephalaspideans in Cephalaspidea sensu strictu.
Next, there are several families that (evidently) retain the herbivorous diet: Bullidae,
Haminoeidae, and Runcinidae, and perhaps Diaphanidae and Notodiaphanidae (which are not
closely related to each other) as well. Gizzard plates are present in the Bullidae, Haminoeidae and
Runcinidae, and the latter two families can be united by the structure and manner of functioning of
the gizzard plates, which rock against each other from anterior to posterior and shred the algae or,
in the case of a small group within Haminoea, cyanobacteria, upon which the animals feed. The
resemblance is highly detailed, not, as Mikkelsen (2002:98) says, superficial, although there are
four plates, not three. Such characters as the presence of a seminal bulb that forms spermatophores
are possible synapomorphies connecting them with Cephalaspidea sensu strictu However, molecular
studies seem to indicate that the family Runcinidae, although monophyletic, is not closely
related to other cephalaspideans (Malaquias, Mackenzie-Dobbs, Gosliner & Reid, 2009). The trouble
is that these studies do not indicate the relationship of Runcinidae to any other lineage. In the
Bullidae the plates are less elaborate and crush rather than shred the food, as do those of other
groups to be discussed later. This herbivorous assemblage is probably a paraphyletic grade rather
than a single lineage. However, it does contain some well-supported monophyletic groups, most
notably one consisting of Atys, Haminoea, Phaneropthalmus, and Smaragdinella (see Malaquias,
Mackenzie-Dobbs, Gosliner & Reid, 2009), for which we have a considerable amount of data on
chemical defense.
And finally, there are several families of carnivorous opisthobranchs in which there are gizzard
plates, or in which there is good reason to believe that such plates have been lost: Retusidae,
Scaphandridae, Philinidae, Aglajidae, Gastropteridae and Philinoglossidae. These plates are usually
three in number, not counting any accessory ones that may be present. They are used in crushing
shelled prey, such as mollusks and even foraminiferans. The plates squeeze together due to the
action of muscles, and in the most extremely developed forms they act rather like a nutcracker.
Shells are well developed in some members of the group but these tend to become much reduced.
When there has been a shift towards eating somewhat more motile prey items that are not well
defended by shells, such as opisthobranchs, the gizzard and shell both become reduced. It seems
likely that this is a polyphyletic assemblage, reflecting a shift to carnivory. But there is good evidence
for a monophyletic group that consists of Philine, Navanax, Aglaja, and related genera.
Again, there are data on chemical defense for this assemblage.
The following tree corresponds to the best estimate we can give at this time of the relationships
among the cephalaspideans with chemical defense:
┌ Acteonidae
─┤
│ ┌ Cylichnidae
│┌┤ Philinoidea
││└ Aglajidae
└┤┌ Bullidae
└┤┌ Haminoeidae
└┤
└ Smaragdinellidae
The Acteonidae and allies are treated as the first branch to have diverged from the cephalaspidean
stock. There is evidence that these are the closest relatives of nudibranchs and notaspideans.
A second lineage of cephalaspideans, herbivorous and without gizzard plates, gave rise to the
remaining opisthobranchs, including Cephalaspidea sensu strictu, the first branch of which,
according to Malaquias, Mackenzie-Dobbs, Gosliner and Reid (2009) consists of the genus