TllE BIOLOGY OF PELAGIC SHRIMPS IN THE OCEAN
283
Swgestea nimilis is connidcwd to have ii life-span similar to that of
Sergia Z~ioens ( I%wham, 1967 ; 1’ctirc.y anti Porss, 1909). Off the Oregon
coast it niiitrircx scxiicilly aftw aboiit, one ycsr when it has attained a
c~waptw lengt,h of 11 mm. Avtmge growth rate was hence about
0.0 mtn cw;qxtet* length p c ~ month, but that of early post-larval
stagw W:LX almost twice the overdl average (l’earcy and Forss, 1969).
~Seelyestes a r d i c m from off Norwtiy also bccomes mature when a year
old tind cull livv nearly t w o yews (Matthews and Pinnoi, 1973). Genthe
(1969) states, however, that S. similis does not maturc in one year in
the Santct Utirhttra Channel, but, relewes a brood in the second year in
the eiimmer untl autumn, anti then dies.
According to Ikematsu (1953) the summer generation of Acetes
jnponicus that l \ i ~ t d ~ e s
in MtL.y and June grows faRt in the warmer
season and spawns the winter generation between August and early
October ; the winter gcneratiori grows for a few months but obviously
growth ceases in winter. They grow again in spring and spawning
takes place aftor May. Sizes at maturity differ markedly between the
two generations. The average body length of adults of the winter
generation is more than 1.5 times that of the summer one. Both
generathiis die iifter spawning and hence the life-span is nine to ten
months for the winter gcneration, whereas it is two and half to three
months in thc aiinimer generation (Fig. 17). Yaeuda P t aZ. (1953) even
estimate t,hat the life-spin of the latter generation to be only twentylive to fifty dnys.
Sivortscn and Holthuis (1 Ofit;) supposed that Acalzthephyra pclagica
iii the North Atlaiitic attains wxiial maturity in the third year of its
lift>. A siniilar result was obtiLiued for dcanthephyra qwdrispinosa in
the northwestern Pacific; it attains inatiirity at two years of age and
is supposed to live a$ least thrtse years (Aizawa. 1968). Table V shows
estimates of tlic growth rates of thc six above-mentioned pelagic species
from egg to rnittllrity. Under environmental conditions characterized
by low temperatures and a poor supply of food. the growth rate of the
lowcr meso- arid bathypelagic Shrimps must be much slower than that
of thc q)ipebgic atid upper mcsopelagic species. Since there will be no
seasonal variations in the chemical and physical environment of the
tlcty SCU. their growth would be influenced mostly by the amount of
food supplied from the upper layers, but no study has been made of
this matt,c~r yet.
The cficets of lower temperature regimes in the bathypclagic
environment ran be defined as decreasing the growth rate. increasing
longevity, and so producing organisms of larger size. and, in Inany
cases, preventing attainment of sexual maturity (Mauchline. 1972).
283
Swgestea nimilis is connidcwd to have ii life-span similar to that of
Sergia Z~ioens ( I%wham, 1967 ; 1’ctirc.y anti Porss, 1909). Off the Oregon
coast it niiitrircx scxiicilly aftw aboiit, one ycsr when it has attained a
c~waptw lengt,h of 11 mm. Avtmge growth rate was hence about
0.0 mtn cw;qxtet* length p c ~ month, but that of early post-larval
stagw W:LX almost twice the overdl average (l’earcy and Forss, 1969).
~Seelyestes a r d i c m from off Norwtiy also bccomes mature when a year
old tind cull livv nearly t w o yews (Matthews and Pinnoi, 1973). Genthe
(1969) states, however, that S. similis does not maturc in one year in
the Santct Utirhttra Channel, but, relewes a brood in the second year in
the eiimmer untl autumn, anti then dies.
According to Ikematsu (1953) the summer generation of Acetes
jnponicus that l \ i ~ t d ~ e s
in MtL.y and June grows faRt in the warmer
season and spawns the winter generation between August and early
October ; the winter gcneratiori grows for a few months but obviously
growth ceases in winter. They grow again in spring and spawning
takes place aftor May. Sizes at maturity differ markedly between the
two generations. The average body length of adults of the winter
generation is more than 1.5 times that of the summer one. Both
generathiis die iifter spawning and hence the life-span is nine to ten
months for the winter gcneration, whereas it is two and half to three
months in thc aiinimer generation (Fig. 17). Yaeuda P t aZ. (1953) even
estimate t,hat the life-spin of the latter generation to be only twentylive to fifty dnys.
Sivortscn and Holthuis (1 Ofit;) supposed that Acalzthephyra pclagica
iii the North Atlaiitic attains wxiial maturity in the third year of its
lift>. A siniilar result was obtiLiued for dcanthephyra qwdrispinosa in
the northwestern Pacific; it attains inatiirity at two years of age and
is supposed to live a$ least thrtse years (Aizawa. 1968). Table V shows
estimates of tlic growth rates of thc six above-mentioned pelagic species
from egg to rnittllrity. Under environmental conditions characterized
by low temperatures and a poor supply of food. the growth rate of the
lowcr meso- arid bathypelagic Shrimps must be much slower than that
of thc q)ipebgic atid upper mcsopelagic species. Since there will be no
seasonal variations in the chemical and physical environment of the
tlcty SCU. their growth would be influenced mostly by the amount of
food supplied from the upper layers, but no study has been made of
this matt,c~r yet.
The cficets of lower temperature regimes in the bathypclagic
environment ran be defined as decreasing the growth rate. increasing
longevity, and so producing organisms of larger size. and, in Inany
cases, preventing attainment of sexual maturity (Mauchline. 1972).