13. EWHAUSIIDS IN THE MARINE ECONOMY
385
organic nitrogenous material and (2) those that require only chitin. A
notable proportion of the population of chitinoclastic bacteria is
attached to the integuments of planktonic crustaceans during ecdysis
and this may be one of the main reasons why very little chitinous
material accumulates in marine sediments, decomposition having taken
place in the water column. Seki (1956a), however, calculates on the
basis of laboratory experiments and observed environmental densities
of chitinoclastic bacteria, that the decomposition of planktonic
organisms must take between 40 and 70 days a t an optimal temperature
of 25°C and that at lower environmental temperatures longer periods
are required-up to 370 days a t 15°C and up to 500 days a t 5°C. The
sinking rates of casts in the sea will probably be slower than Lasker’s
(1966) estimate of 300 m per 24 hr, presumably measured in still water
and, as Seki suggests, the casts probably sink so far and tend to
accumulate a t the boundary surfaces between different water masses.
A certain amount, however, will sink to the surface layers of the sediments where, according to Seki (1965b), the same species of chitinoclastic bacteria are responsible for their degradation. Whichever
happens, the casts become part of the pelagic detritus and may be
consumed by pelagic organisms or they are deposited on the sea bed to
contribute to the bottom detritus. What of nutritive value will be
present in the casts? If they are being degraded by chitinoclastic and
other bacteria then the bodies of the bacteria themselves are of value
to feeding organisms. Further, a large extremely irregularly-shaped
cast with setae present on the limb parts will tend to accumulate fine
detrital material on it as it sinks through the water, so adding to its
bacterial flora, organic content and nutritious quality. If each euphausiid moults once every 5 days and a cast represents 10% of the dry
weight of the euphausiid then a weight of casts equal to seven times the
dry weight biomass of euphausiids will be produced per annum.
Brinton (quoted by Lasker, 1966) suggests that the wet biomass of
Euphausia pacijca throughout its geographical range of 1.3 x
m2
is about 1 g per square metre, a wet biomass which would produce
about 1.5 g dry wt of casts per square metre per year. This weight of
casts contains about 0.25 g carbon and about 0.04 g nitrogen (Lasker,
1966) and about 2 pg of vitamin A and about 2 mg of astaxanthin
(Table XIX). The amount of carbon incorporated in living E . pacijca
throughout its area of occurrence in the Pacific Ocean is equivalent to
about 0.11 g carbon per square metre. According to Lasker’s experiments, about 5% per day of the carbon in the population is required
to maintain growth, respiration and moulting and so about 0.0055 g
carbon per square metre per day must be consumed by the population,
385
organic nitrogenous material and (2) those that require only chitin. A
notable proportion of the population of chitinoclastic bacteria is
attached to the integuments of planktonic crustaceans during ecdysis
and this may be one of the main reasons why very little chitinous
material accumulates in marine sediments, decomposition having taken
place in the water column. Seki (1956a), however, calculates on the
basis of laboratory experiments and observed environmental densities
of chitinoclastic bacteria, that the decomposition of planktonic
organisms must take between 40 and 70 days a t an optimal temperature
of 25°C and that at lower environmental temperatures longer periods
are required-up to 370 days a t 15°C and up to 500 days a t 5°C. The
sinking rates of casts in the sea will probably be slower than Lasker’s
(1966) estimate of 300 m per 24 hr, presumably measured in still water
and, as Seki suggests, the casts probably sink so far and tend to
accumulate a t the boundary surfaces between different water masses.
A certain amount, however, will sink to the surface layers of the sediments where, according to Seki (1965b), the same species of chitinoclastic bacteria are responsible for their degradation. Whichever
happens, the casts become part of the pelagic detritus and may be
consumed by pelagic organisms or they are deposited on the sea bed to
contribute to the bottom detritus. What of nutritive value will be
present in the casts? If they are being degraded by chitinoclastic and
other bacteria then the bodies of the bacteria themselves are of value
to feeding organisms. Further, a large extremely irregularly-shaped
cast with setae present on the limb parts will tend to accumulate fine
detrital material on it as it sinks through the water, so adding to its
bacterial flora, organic content and nutritious quality. If each euphausiid moults once every 5 days and a cast represents 10% of the dry
weight of the euphausiid then a weight of casts equal to seven times the
dry weight biomass of euphausiids will be produced per annum.
Brinton (quoted by Lasker, 1966) suggests that the wet biomass of
Euphausia pacijca throughout its geographical range of 1.3 x
m2
is about 1 g per square metre, a wet biomass which would produce
about 1.5 g dry wt of casts per square metre per year. This weight of
casts contains about 0.25 g carbon and about 0.04 g nitrogen (Lasker,
1966) and about 2 pg of vitamin A and about 2 mg of astaxanthin
(Table XIX). The amount of carbon incorporated in living E . pacijca
throughout its area of occurrence in the Pacific Ocean is equivalent to
about 0.11 g carbon per square metre. According to Lasker’s experiments, about 5% per day of the carbon in the population is required
to maintain growth, respiration and moulting and so about 0.0055 g
carbon per square metre per day must be consumed by the population,