THE ARTIFIOIAL PROPAOATION OF MARINE FISH
2U
natural mortality curves. The work subsequently led to a consideration
of mortality factors both in tanks and in the sea, and to basic morphological studies (Shelbourne, 1956, 1956a, b, 1967).
Difficulty in rearing plaice from the egg through and beyond
metamorphosis had left considerable gaps in our understanding of its
early life history. The classical work of McIntosh and Prince (1890)
and Petersen (1894), together with embryological studies by Fullarton
(1891) and Meek (1913), formed the basis of our morphological knowledge, while the researches of Fulton (1898), Milroy (1898), Dakin
(1912) and Rurfield (1928), had thrown light on the early physiology
of the species. Most of the new data, contained in this section of the
present paper have been published recently (Shelbourne et al., 1963;
Shelbourne, 1963a, b, c ; Riley and Thacker, 1963), or is in the press.
A. Y'he development of the plaice
The plaice is sexually mature in its third or fourth year of life,
under natural conditions. At first spawning, a female may produce
something less than 60 000 eggs, and fecundity generally increases with
size. The fertilized egg is buoyant in sea water, transparent and about
2 mm in diameter. Its essential constituents are a thin but tough shell
(the chorion) surrounding a yolky mass covered peripherally by a thin
layer of protoplasm containing the nucleus. After egg fertilization this
cortical layer slowly differentiates into the embryo, which takes 3 weeks
to hatch at 6"C, the incubation time being cloaely related to temperature (Apstein, 1909).
The emergent larva has symmetrical eyes and sufficient yolk to
laat a further 8-10 days at normal sea temperatures. Subsequent larval
feeding stages remain pelagic until the onset of metamorphosis, mme
10 weeks after hatching. The duration of the pelagic phase depends
on food abundance and hereditary factors, as well as temperature.
A t metamorphosis, the left eye migrates to the right side, the body
becomes flattened, and the right side, now the dorsal eurface, develops
heavy pigment in preparation for the demersal habit.
Aurich (1941) produced a series of developmental stages for the
plaice larva, based on the investigations of Dannevig (1897), Reibisch
(1902) and Apstein (1909), which take into account the principal
morphological changes between hatching and metamorphosis. Mr.
A. C. Simpson, formerly of the Lowestoft laboratory, has condensed
Aurich's staging to the following categories :
Stage 1. Yolk etill present,
Stage 2. Yolk resorbed but notochord still straight.
2U
natural mortality curves. The work subsequently led to a consideration
of mortality factors both in tanks and in the sea, and to basic morphological studies (Shelbourne, 1956, 1956a, b, 1967).
Difficulty in rearing plaice from the egg through and beyond
metamorphosis had left considerable gaps in our understanding of its
early life history. The classical work of McIntosh and Prince (1890)
and Petersen (1894), together with embryological studies by Fullarton
(1891) and Meek (1913), formed the basis of our morphological knowledge, while the researches of Fulton (1898), Milroy (1898), Dakin
(1912) and Rurfield (1928), had thrown light on the early physiology
of the species. Most of the new data, contained in this section of the
present paper have been published recently (Shelbourne et al., 1963;
Shelbourne, 1963a, b, c ; Riley and Thacker, 1963), or is in the press.
A. Y'he development of the plaice
The plaice is sexually mature in its third or fourth year of life,
under natural conditions. At first spawning, a female may produce
something less than 60 000 eggs, and fecundity generally increases with
size. The fertilized egg is buoyant in sea water, transparent and about
2 mm in diameter. Its essential constituents are a thin but tough shell
(the chorion) surrounding a yolky mass covered peripherally by a thin
layer of protoplasm containing the nucleus. After egg fertilization this
cortical layer slowly differentiates into the embryo, which takes 3 weeks
to hatch at 6"C, the incubation time being cloaely related to temperature (Apstein, 1909).
The emergent larva has symmetrical eyes and sufficient yolk to
laat a further 8-10 days at normal sea temperatures. Subsequent larval
feeding stages remain pelagic until the onset of metamorphosis, mme
10 weeks after hatching. The duration of the pelagic phase depends
on food abundance and hereditary factors, as well as temperature.
A t metamorphosis, the left eye migrates to the right side, the body
becomes flattened, and the right side, now the dorsal eurface, develops
heavy pigment in preparation for the demersal habit.
Aurich (1941) produced a series of developmental stages for the
plaice larva, based on the investigations of Dannevig (1897), Reibisch
(1902) and Apstein (1909), which take into account the principal
morphological changes between hatching and metamorphosis. Mr.
A. C. Simpson, formerly of the Lowestoft laboratory, has condensed
Aurich's staging to the following categories :
Stage 1. Yolk etill present,
Stage 2. Yolk resorbed but notochord still straight.