298
THE BIOLOGY OF EUPHAUSIIDS
We described the development of the larvae in Chapter 4 and used
lengthlfrequency histograms to show that similar larvae of the same
species in different sea areas often differ in size (Figs 45,49). Mauchline
(1965a) analysed, on lengthlfrequency histograms, the population of
larvae of Thysanoessa raschii in the Clyde and found that a furcilia,
extra to those described by Einarsson (1945) for this species a t Iceland,
was present in the development. This stage, the third, would have
remained undetected if these histograms had not been constructed
because the larvae of the second and third stages are closely similar
morphologically, the main difference being in size (Fig. 113). Few
investigators have examined the rates of growth in body length of
larvae although reasonable approximations can be made. Ruud (1932)
calculated that furciliae of Euphausia superba grow in total length from
about 1.5 to 3.5 mm in about 30 days. Using the data on body length
and durations of each larval stage given in Mauchline (1959, 1965a) the
rates of increase in length of the furciliae of Meganyctiphanes norvegica
and Thysanoessa raschii a t comparable stages of development are
1.5-6.0 and 1.5-4.3 mm respectively. These rates of growth are faster
than those of larvae of Euphausia superba developing in considerably
higher latitudes. No experimental observations on the growth rates of
larval stages under different conditions of food and temperature have
been made and this is a line of investigation which could explain some
of the differences found in the sizes of specific larval stages in different
sea areas (see Chapter 4).
Growth in body length of euphausiids, like that of all crustaceans,
is not continuous because of the more or less rigid exoskeleton.
Consequently, increase in body length takes place a t moulting, a period
of constant body length then prevailing until a further increase in
length takes place a t the next moult. It is, therefore, instructive to
examine the results of Lasker’s (1966) observations on growth of
individual E. pacijka in the laboratory before discussing the various
rates of growth determined for other species by statistical analyses of
natural populations. Lasker maintained seven experimental animals
in 1-litre containers of sea water kept a t 10°C and observed the sizes of
the animals each time they moulted (Fig. 114). The growth of the three
juveniles was steady and linear with regular moults, maintaining an
incremental addition to length of 0.034-0.048 mm per day. The growth
of the adults (Fig. 114) was not as regular as that of the juveniles nor
was the rate so great, being about 0.01-0.02 mm per day. These experimental animals were of course kept under excellent conditions of food
and temperature and so growth rates of 0.048mm per day may be
approaching optimal rates in the natural environment rather than
THE BIOLOGY OF EUPHAUSIIDS
We described the development of the larvae in Chapter 4 and used
lengthlfrequency histograms to show that similar larvae of the same
species in different sea areas often differ in size (Figs 45,49). Mauchline
(1965a) analysed, on lengthlfrequency histograms, the population of
larvae of Thysanoessa raschii in the Clyde and found that a furcilia,
extra to those described by Einarsson (1945) for this species a t Iceland,
was present in the development. This stage, the third, would have
remained undetected if these histograms had not been constructed
because the larvae of the second and third stages are closely similar
morphologically, the main difference being in size (Fig. 113). Few
investigators have examined the rates of growth in body length of
larvae although reasonable approximations can be made. Ruud (1932)
calculated that furciliae of Euphausia superba grow in total length from
about 1.5 to 3.5 mm in about 30 days. Using the data on body length
and durations of each larval stage given in Mauchline (1959, 1965a) the
rates of increase in length of the furciliae of Meganyctiphanes norvegica
and Thysanoessa raschii a t comparable stages of development are
1.5-6.0 and 1.5-4.3 mm respectively. These rates of growth are faster
than those of larvae of Euphausia superba developing in considerably
higher latitudes. No experimental observations on the growth rates of
larval stages under different conditions of food and temperature have
been made and this is a line of investigation which could explain some
of the differences found in the sizes of specific larval stages in different
sea areas (see Chapter 4).
Growth in body length of euphausiids, like that of all crustaceans,
is not continuous because of the more or less rigid exoskeleton.
Consequently, increase in body length takes place a t moulting, a period
of constant body length then prevailing until a further increase in
length takes place a t the next moult. It is, therefore, instructive to
examine the results of Lasker’s (1966) observations on growth of
individual E. pacijka in the laboratory before discussing the various
rates of growth determined for other species by statistical analyses of
natural populations. Lasker maintained seven experimental animals
in 1-litre containers of sea water kept a t 10°C and observed the sizes of
the animals each time they moulted (Fig. 114). The growth of the three
juveniles was steady and linear with regular moults, maintaining an
incremental addition to length of 0.034-0.048 mm per day. The growth
of the adults (Fig. 114) was not as regular as that of the juveniles nor
was the rate so great, being about 0.01-0.02 mm per day. These experimental animals were of course kept under excellent conditions of food
and temperature and so growth rates of 0.048mm per day may be
approaching optimal rates in the natural environment rather than