Metamorphosis of the Shell in the 
California Sea Hare, Aplysia calif ornica Cooper 
Lindsay R. Winkler 1 
Despite attempts by Carazzi (1905), Maz- 
zarelli (1893), Saunders and Poole (1910), 
and the present writer, the metamorphosis of 
Aplysia has not yet been observed, nor has it 
been possible to follow through the changes 
in the shell which take place as the animal 
metamorphoses from a planktonic veliger to 
a sluglike sea hare. This is because the veliger 
shell is lost sometime during metamorphosis 
so that the evidence of metamorphosis in the 
animal becomes obscured. In one case only 
has the writer seen a small outmoded veliger 
shell hanging in place by a very thin mem- 
brane in the apexial concavity of a very young 
specimen from a new species obtained from 
the Gulf of California. 
However, the neaplysiids which are known 
only from the west coast of North America 
are distinct in that they possess a special 
attachment plate extending from the inner 
apexial area. This plate contains the built-in 
veliger and the metamorphic shell. This is 
best seen in young shells, as older animals 
tend to strengthen their shells by overlaying 
with added shell material. 
MATERIALS AND METHODS 
The shells of Aplysia californica Cooper, two 
to five inches in length, were procured from 
newly sacrificed young animals. The shells 
were preserved in 70 per cent alcohol. Study 
was done under a dissecting microscope. 
EXPERIMENTAL DATA 
The position of the metamorphic shell is 
indicated in Figure 1. The portion of this 
shell outline which normally comprises the 
veliger shell at hatching may be approxi- 
1 Present address: School of Tropical and Preventive 
Medicine, Loma Linda, California. This work was done 
at the Allan Hancock Foundation, University of South- 
ern California. Allan Hancock Contribution No. 229. 
mated by comparing the shell of laboratory 
incubated egg strings at the time of hatching 
with the inner areas of the shell nucleus. Size 
measurements are helpful in determining the 
areas involved in the prehatching shell. The 
size of the veliger from top of the hood is 
approximately 150 /x. 
The angle of the hood in relation to the 
base of the shell at the time of hatching (Fig. 
2), compared to the corresponding dimension 
in the shell nucleus, indicates the approxi- 
mate starting point of free-living existence. 
The end of the pelagic period probably oc- 
curred at the time of the first major change in 
shell character in the area built after the initi- 
ation of free-living existence. 
In many specimens the shell built between 
the time of hatching and this supposed end 
of the pelagic period is of slightly different 
color. It is a less transparent yellow and has 
fine growth lines, perhaps reflecting the 
change from the prehatching yolk food to 
the food that may have been available to it in 
the free-swimming state. 
An examination of the veliger shell (Fig. 2) 
shows that the tendency to build a spiral 
shell is already indicated at time of hatching 
by the asymmetrically thickened edge of the 
initial spire. The tendency to spiral rapidly in- 
creases during the early postveliger existence. 
The writer believes that this is a crawling 
stage in which the snail’s foot has taken over 
the locomotion for which it was not yet pre- 
pared at the time of hatching. This early 
stage, though very different from the adult, 
is probably very gastropodlike, with the ani- 
mal crawling on its foot and feeding upon 
microscopic algal forms but retracting within 
its spiral shell which it closes with its opercu- 
lum. This constitutes the first stage of 
metamorphosis. 
348 
