IV.
BIONOMICS.
Most Nudibranchs lead tranquil and sedentary lives ocoupied in growth, nutrition,
pairing, and egg-laying, after which they are believed to die. Vigorous movement, such
as fighting or'the pursuit of active prey, is comparatively rare, and no forms are known to
build homes or nests, though their eggs are protected by various envelopes and arranged
in coils or ribbons on suitable spots.
The duration of life is probably about a year. In England the animals are generally
hatched in the spring or summer. Growth is rapid, and' a microscopic veliger can attain
its full size as an animal two or three inches long in less than twelve months. After a
brief period of spawning it is probable that death supervenes. The evidence on this point
is not entirely conclusive, but there is no record of animals living in captivity for more
than a year: they certainly often die soon after spawning, and dead specimens are often
found on the shore in the summer months. On the other hand captivity is likely to
shorten life; some species which are known to attain a considerable size begin to spawn
when they are relatively small; and some species (e. g. Doris verrucosa, according to
Mazza.relli’s observations at Naples) spawn throughout the year. But it is not likely
that the duration of life is considerable in any case.__
All known Nudibranchiata are marine, with the one exception of Ancyloderis, found in
Lake Baikal. This lake was no doubt once in connection with the ocean, and contains
other indications of an originally marine fauna, but Ancylodoris appears to have departed
in some respects from the usual Nudibranchiate type. A few forms, especially in the
Limapontiidae, frequent brackish estuaries and possess to a limited extent the power and
the habit of quitting the water.1 Doriopsilla areolata, is also said to live in tidal pools and
to sun itself. I t is remarkable that the Nudibranchiata (like the Echinodermata, Polychaets,
and Brachiopods) should be unable to support life except in salt water, but it seems to be
a fact that fresh-water molluscs offer no instances of the shell disappearing or becoming
internal. The main difficulty which life in fresh water offers to Nudibranchs as well as to
Echinoderms, etc., is that the delicate free-swimming larva does not find the food and
other conditions necessary to its existence. In this connection it is interesting to observe
that Germ, which frequents brackish water, has an abnormal larval development, which
takes place within the egg. But even for adults migration from the sea to rivers would
present many dangers and hardships. Soft, slow-moving marine animals, conspicuous
if taken out of their usual surroundings, and mostly carnivorous, would be more exposed
to their enemies in streams and lakes, and less able to find their prey. But these considerations
do not altogether explain why it is that molluscs which have successfully adapted
themselves to life in fresh water do not show the tendency to reject or reduce the shell
visible in both marine and terrestrial gastropods.
1 E. g. Alderia and Cenia.
The principal movements of Nudibranchs consist of creeping and floating. The
former action is performed as in other Gastropods by secreting a layer of mucus along
which the animal slowly slides by expanding and contracting different portions of its
pedal disc. As might be expected from their build, the Cladohepatica and Polyceridas
are much more agile than the Dorididse. This is partly due to the fact that the Dorididae
feed chiefly on sponges and live literally on or partly embedded in their prey. The
majority of JEolids on the other hand eat small Hydroids which require a relatively active
search. Some Dorids, especially the stiff tropical forms with wide mantle-margins and
small feet (e. g. Platydoris scabra), remain in one place until their shape comes to fit into
their surroundings. Among the Cladohepatica the elongate forms such as Facelina,
Goryphelta and Bomella are specially active. When kept in confinement they often eat
one another’s cerata and appear to fight.
Like many other Gastropods (e. g. Skenea and Bissoa), Nudibranchs can float with
the ventral surface uppermost, and in confinement many of them show a great love of
this position. It is doubtless facilitated by the lightness of their weight compared
to their size.1 As far as is known, they are obliged to make their first excursion to the
surface with the .aid of a piece of seaweed or other material support, and possess no power
of levitation of their own. On reaching the surface they secrete from the foot a layer of
mucus and remain in an inverted position with the dorsal region downwards and the foot
and mucus-layer upwards. By bringing the margins of the foot together they can drop
from this position at will, and they frequently remain suspended some inches below the
surface by a thread of mucus connected with the superficial layer.2 Having once established
communication with the surface by means of this thread they are able to climb up
it again at will. A few Nudibranchs have become definitely pelagic (.Phylliroe, Glaucus)
and many others partially so, frequenting seaweed or floating objects (so Scyll&a, Fiona,
and several forms from the Sargasso Sea).
Some Nudibranchs which are provided with wing-like expansions at the side or
with flat tails have some power of swimming, such as Flysia, Hexabranclvus, and especially
Plocamopherus. Even clumsy-looking Dorids may exhibit unexpected powers of movement
in special circumstances, and when dropped into a few feet of water will find their way
to the bottom with movements like those of a flat fish.8 As mentioned above both adult
and young Nudibranchs appear and disappear suddenly in considerable quantities, especially
before spawning and after hatching. Alder and Hancock were disposed to disbelieve in
these migrations, but there is a good deal of evidence for their occurrence. It is not
known how they are performed, but as creeping is an extremely slow process, it is possible
that the animals make use of currents in which they suspend themselves as described.
But though creeping Nudibranchs are quite at home on or under the surface of the water
when they once reach it, it must be remembered that we have no evidence of their power
to rise by their own efforts.
The Dorididse subsist chiefly on sponges, Bryozoa, and compound Ascidians; 1'ritonia
1 According to Heclit an JSolidia 6 centimetres long weighs only 12 grammes.
3 See N. Colgan in Ann. and Mag. N. H., ser. 8, vol. iii, April, 1909, pp. 354—362, “ On Locomotion
and Use of Slime-threads in Marine Mollusca.”
s I have observed this specially in the large East African Dorid Thordisa. crosslo/ndi.