Abstract:
(O. nigricans ) is a small euthyneurous pulmonate of the sub-order Stylommatophora, class Gastropoda. It is an inhabitant of the littoral zone and is found clinging to the rocks between tide marks. The animals are usually found in damp shaded places such as in deep dark cracks and under projecting ledges. They are found in large numbers on the surface of the plant Coduim a member if the Chlorophyceae which grows in dense masses on the rocks in some parts.
I have collected this species of Onchidella from
various bays in Cook St., including Island Bay, Lyall Bay and
Titahi Bay. Onchidella however never comes within the limits of
Wellington Harbour. Plentiful along the south coast of the
North Island, as far as Wellington Heads its distribution then
ceases quite suddenly despite the fact that the line of rocks is
continued through the Heads into the harbour itself. Whether
this is the result of the nature of the water, - temperature,
salinity or aeration, or whether it is controlled by the
distribution of food, I have not discovered, but I think it is due
to one of the factors of the former set.
This distribution is rendered more unusual by the fact
that the animal lives quite happily for long periods in the
laboratory provided that it has a small rock covered with lime-
encrusted seaweed, on which to live, and fresh, well aerated
sea-water renewed every second day. Stale water, four or five
days old, always results in a high percentage of mortality due
to suffocation.
The largest specimens I have seen were those collected
from Fishermen's Island which lies about 200 yards out from the
mainland, a deep channel running between the two. Here O. nigricans
was found intermingled with O. flavescens both species occurring
in large numbers. I found them to vary in size up to 2.5 cms
long and 1.75 cms wide whereas those on the mainland seldom
exceeded 1.25 - 1.5 cms in length.
Mr. A.W.B. Powell, Conchologist and Palaeontologist
of the Auckland Institute and Museum reports having found
O. nigricans together with O. patelloides on the West coast in
the neighbourhood of Auckland. He states that O. nigricans
is usually associated with rocks in clean-water situations
while O. patelloides seems to prefer the mud-flats.
