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From the fish keeper's point of view, however, the method of development of the larval
mussel is the greatest drawback to this mollusc.
The female lays a vast number of eggs, many
of these are eaten by the fishes but a certain
number settle on the fishes themselves, principally on the fins, and remain there as
parasites until their development is complete.
Then the tiny mussels drop off and lead the
life of an ordinary mussel. A large, healthy
fish is not greatly inconvenienced by a few
such parasites, but the smaller or weakly
fishes may feel the strain. A look-out should
be kept and ailing fishes examined for the
young mussels ; they can be scraped off with
a knife and the fish returned to the water.
For the aquarium there are several small
species of mussel which feed in a similar manner
to the larger kinds but do not injure the plants
to any extent. They belong to the genera
Pisidium and Sphaerium, otherwise the Fresh-water Cockles.
As a rule dealers do not stock either of these
molluscs and so they must be sought for in the
sand or mud of slow-moving streams or ponds.
Species from fast-moving streams obviously
will not exist in the still water of the aquarium.
The shells are smooth and cockle-shaped,
usually, with dark bands running parallel
with the edge of the shell. The background,
according to the species, may be pale fawn,
pink or bluish-grey. The sizes vary from an
eighth of an inch to half an inch in length.
The young are most interesting ; they move
rapidly about among the stems of water plants,
just like a tribe of monkeys in a forest.
Crustaceans
There is only one large crustacean found
in the fresh waters of this country-the
Crayfish. They are exceedingly difficult to
keep in captivity and are very partial to small
fishes. Mention of them is made here in case
one may be tried, just to see what would
happen.
The other crustaceans are very small, some
are minute, even, yet they are of the greatest
value to the fish keeper ; they form the
finest food of all for the fishes and breed
rapidly in captivity.
Daphnia, the most commonly known of the
small crustaceans, may be purchased alive for
a shilling a can or they may be collected from
country ponds. In the latter case care should
be taken that no undesirables are mixed up
with the daphnae.
Their size varies from a sixteenth of an
inch to an eighth of an inch in length. The
body is rotund and the legs and antennae
are congregated around the head region.
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