Geoscience Reference
In-Depth Information
other gains such as the ability to adopt a very streamlined form which enables them
to penetrate narrow channels and crevices in the soil. Some species are very largely
subterranean notably the three species of
Testacella
(
Plate 11
). These can sometimes
be found crawling freely on the surface at night, and can easily be recognized by the
small external shell, resembling a tiny mussel shell, perched on the rear end. In all the
other slugs the shell is reduced to a small plate or mass of granules embedded within
the body mantle at the front end of the body.
Colour variations in slugs are common among adults as well as between juven-
iles and adults. Too much reliance on external characters, coupled with incomplete or
inaccurate descriptions and on dissections of specimens pickled in alcohol, has led to
much confusion in the past. Kerney and Cameron's field guide in 1979 included four
species not recognized in H.E.Quick's monograph published in 1961, and four more
species have been added since then. It was amateur collectors rather than professional
biologists who first noticed behavioural differences between apparently similar slugs,
especially during breeding. Morphological differences were then found in the repro-
ductive organs of freshly killed specimens. This increase from 23 to 30 species in a
group of quite large soil animals is an interesting example of the significant additions
that may still be made through careful observation without sophisticated equipment.
One of these 'new' species, the Durham
Arion
in Eversham and Jackson's 1982
key, has, at the time of writing, only just received its Latin epithet
flagellus.
This spe-
cies is rather similar externally to the true but apparently less common
Arion lusit-
anicus
so many of the earlier records of
lusitanicus
are incorrect. Similarly, the once
'easily recognized' and commonly recorded garden slug
A. hortensis
, with its bright
orange or yellow sole, is actually composed of three species
A. hortensis, A. owenii
and
A. distinctus
, of which the last is by far the most common. Confusion over two
or three species of slugs may be thought a somewhat academic problem, but it could
lead to erroneous conclusions in experiments on slug control.
Most snails and slugs are hermaphrodite, like earthworms, so mating usually res-
ults in both partners laying eggs, often in batches under logs or in pockets excav-
ated in the soil. Some species are protandrous, that is they behave first as males and
later in life as females. A few slugs are self fertilizing and can therefore reproduce
without having to find a mate. Some populations of the marsh slug
Deroceras laeve
are parthenogenetic, the eggs being produced without even the normal exchange of
chromosomes that occurs in self fertilization. Parthenogenesis is quite common in
earthworms and eelworms and widespread among arthropods including mites, milli-
pedes and aphids, but this was the first record in molluscs.
Most snails and slugs feed on dead and decaying plant and animal matter in-
cluding dung. Fungi are much favoured by some species while others rasp algae and
