THE SHRIMPING INDUSTRY IN ESSEX
243
individual recorded during the period 1950-56 measured 23/4 inches
long, from the eye socket to the telson. Age, however, cannot be
judged by size alone, as some individuals grow faster than others.
Pink shrimps live three to four years, the females living longer
than the males. Females mature in the first year, but the
majority in the second year. The males, after reaching the
active male phase, usually 7 to 8 months after liberation,
gradually undergo a change of sex, and after approximately 15
months or more they become females. The causes that produce
these changes are not well understood, but there is a possibility
that changes in the hormonal balance may be responsible.
The phenomenon of sex change is not uncommon in the animal
kingdom. It is more common in the lower organisms, limpets
(Patella vulgata), slipper limpets (Crepidula fornicata) and cer-
tain crustacea (Lysmata seticaudata, Calocoris macandreae).
Species belonging to the genus Pandalus show a high percentage
of sex reversal.
The pink shrimp is parasitized by a small bopyrid isopod
(Hemiarthrus (=Phryxus) abdominalis), which is found beneath
the anterior part of the metasome. The percentage of parasitiza-
tion in the Thames Estuary is small, between 0.2 per cent. and
2.5 per cent., but it is higher in the Wash and in northern waters.
This parasite, which can be easily detected in boiled shrimps,
is harmless to human beings and does not affect the animals
either, except that it suppresses the secondary male charac-
teristics at an early stage.
Although pink shrimps had been fished in England for so
many years its life history has not been studied in great detail.
The information given above is part of a study on the biology of
the pink shrimp, which was undertaken by the Ministry of
Agriculture, Fisheries and Food, at Burnham-on-Crouch. Further
research referred to an investigation whether the introduction of
a larger mesh in the shrimp nets would limit the destruction of
non-commercial shrimps.
References
Fishing News, The (1952), No. 2046, 1. London.
Havinga, B. (1929). Krebse und Weichtiere. Handb. Seefisch
Nordeuropas, Vol. 3, Heft 2.
Havinga, B. (1930). Der Granat (Crangon vulgaris Fabr.) in den
hollandischen Gewassern. J. Cons. Int. Explor. Mer., Vol. 5,
p. 57.
Herdman, W. A. (1893). Lancas. Sea-Fish Labor. Report for 1892.
Holdsworth, E. W. H. (1874). Deep Sea-Fishing and Fishing Boats.
Edward Stanford. London.
Lloyd, A. J., and Yonge, C. M. (1947). The biology of Crangon vulgaris
L. in the Bristol Channel and Severn Estuary. J. Mar. Biol.
Asso. U.K., Vol. 26, p. 626.
Kemp, S. (1910). The Decapoda Natantia of the coasts of Ireland.
Fisheries, Ireland Sci. Invest., 1908, No. 1.