238 THE ESSEX NATURALIST
Life history of the shrimps
(a) The brown shrimp (Crangon vulgaris L.).
The brown shrimp is very generally distributed along the
coasts of the North Sea and the brackish waters connected there-
with. It is also found in the Baltic, White Sea, north of Iceland,
and as far as the Mediterranean. It has been recorded from the
east coast of North America and from several localities in the
Pacific, but these specimens referred to an extremely closely
allied form which differs from the brown shrimp only in the shape
of an anatomical organ.
The brown shrimp prefers a soft bottom, because it buries
itself in it to escape from its many enemies. As a typical coast
dweller it avoids deep water, it is seldom found in large quantities
at a depth exceeding 15 fathoms, although in the Norwegian
fiords it has been caught at depths of 50 to 60 fathoms. In many
river mouths it is taken occasionally in water which is almost
completely fresh, but at other seasons its capture is possible only
in the vicinity of the open sea. Brown shrimps can withstand a
wide range of temperature and salinity. Under aquarium con-
ditions they have survived after ice has formed over the surface
of the undiluted sea-water. If at a low tide they have been left
in pools, they continue to live even when water temperatures
reach the high figure of 86° P.
The brown shrimp is an omnivorous animal, its diet consisting
of various crustaceans (Corophium, Gammarus, Praunus, various
Copepods, cyprid larvae of Balanus), marine worms (Nereis spp.),
various bivalves (Mytilus, Macoma, Cardium) and gastropods. In
addition, algae (Ulva, Enteromorpha), fish eggs and fry are men-
tioned as food. Occasionally the stomach is full of silt and mud.
The brown shrimp has two spawning seasons, although these
are not distinct and often overlap. Brown shrimps carrying
spawn are found, however, almost all the year round, though the
proportion is lowest during the autumn months. The number
of eggs carried are related to the size of the female shrimp.
Murie found that the number of eggs carried varied between 850
and 3,600, and Herdman mentions that 5,000 eggs are deposited
by a fully-mature shrimp. According to Havinga (1930) the
number varied from 1,500 (total length of the shrimp about 11/2
inches) to 15,000 (total length of the shrimp about 3 inches).
When the young shrimps escape from the egg it is only less than
1/10 of an inch long. After certain stages of development, five
in larval stage, and four in post-larval stages, the shrimps measure
between 1/4 and 2/5 of an inch long. During the early part of their
lives growth is similar in both sexes, but subsequently the females
grow more rapidly. Females live longer and attain a maximum
size of over 3 inches, while males are smaller, just over 2 inches
long. The females mature at an age of about 20 months and
at a length of less than 2 inches. Most probably they spawn once
in the second year, twice in the third and fourth years, while a
few may survive to spawn in the fifth.