9
Moltschaniwskyj (1999) on the growth of wild-caught Idiosepius held in aquaria
clearly show that under these conditions the processes of somatic growth can be
modified at the cellular level, the mantle muscle growing thicker, with a greater proportion of mitochondria-rich tissue, muscle fibres with smaller mitochondrial cores
and fewer small muscle fibres. For details on nutrition and growth, see Chap. 5.
1.3 Life Cycle
The whole life cycle of coleoid cephalopods varies, depending on the species, from
about 3–4 months (Idiosepiidae) to several years (Boyle and Rodhouse 2005). The
embryonic phase may vary from about 1 week in pelagic squids producing very
small eggs (Sakurai et al. 1996) to more than 1 year in deep-water species producing very large eggs (Boletzky 1994; Laptikhovsky 1999). In the Pearly Nautilus
(Saunders and Ward 1987), the whole life cycle is much longer, incorporating several years of uninterrupted reproductive activity (an extreme form of the protracted
terminal spawning typical for cephalopods, as indicated in Sect. 1.2.1). The very
large eggs need about 1 year to develop to hatching. This is not unique, since certain deep-water octopods have similar egg sizes and similar durations of embryonic
development (Wood et al. 1998).
1.3.1 Embryos
Depending on the species considered, ovum sizes vary from less than 1 mm to about
40 mm. In spite of these enormous size differences, the basic pattern of embryogenesis is the same in all cephalopods. The ovum is fertilized by a spermatozoon
having passed through the micropyle into the chorion (which, except in the incirrate
Octopoda, is embedded in oviducal jelly and nidamental envelopes). The zygote
undergoes partial cleavage: The cytoplasm concentrated at the animal pole is subdivided by the formation of shallow cleavage furrows, which do not cut into the
underlying yolk mass. While in the centre of this cleavage zone, small blastomeres
are subsequently formed, the peripheral segments (the so-called blastocones) remain in continuity with the yolk mass; they are the founders of the prospective yolk
syncytium, which will break down the yolk and export the nutritive elements to the
blood (Boletzky 2003). From the unilayered blastula, epibolic gastrulation starts
with a progressive expansion of the central cell plate (prospective outer germ layer,
or ectoderm) over the peripheral blastomeres adjacent to the ring of blastocones.
This peripheral complex forms the inner germ layer (mesendoderm), which soon
becomes completely covered by the centrifugally advancing edge of the ectoderm.
This edge forms the incipient blastopore lip of the gastrula.
When looking at subsequent developmental stages of cephalopod embryos and
hatchlings, one has to remember that the marginal blastopore lip of the early gastrula
forms the entire outer yolk sac envelope with its blood lacuna and the muscular
1 Cephalopod Biology
Moltschaniwskyj (1999) on the growth of wild-caught Idiosepius held in aquaria
clearly show that under these conditions the processes of somatic growth can be
modified at the cellular level, the mantle muscle growing thicker, with a greater proportion of mitochondria-rich tissue, muscle fibres with smaller mitochondrial cores
and fewer small muscle fibres. For details on nutrition and growth, see Chap. 5.
1.3 Life Cycle
The whole life cycle of coleoid cephalopods varies, depending on the species, from
about 3–4 months (Idiosepiidae) to several years (Boyle and Rodhouse 2005). The
embryonic phase may vary from about 1 week in pelagic squids producing very
small eggs (Sakurai et al. 1996) to more than 1 year in deep-water species producing very large eggs (Boletzky 1994; Laptikhovsky 1999). In the Pearly Nautilus
(Saunders and Ward 1987), the whole life cycle is much longer, incorporating several years of uninterrupted reproductive activity (an extreme form of the protracted
terminal spawning typical for cephalopods, as indicated in Sect. 1.2.1). The very
large eggs need about 1 year to develop to hatching. This is not unique, since certain deep-water octopods have similar egg sizes and similar durations of embryonic
development (Wood et al. 1998).
1.3.1 Embryos
Depending on the species considered, ovum sizes vary from less than 1 mm to about
40 mm. In spite of these enormous size differences, the basic pattern of embryogenesis is the same in all cephalopods. The ovum is fertilized by a spermatozoon
having passed through the micropyle into the chorion (which, except in the incirrate
Octopoda, is embedded in oviducal jelly and nidamental envelopes). The zygote
undergoes partial cleavage: The cytoplasm concentrated at the animal pole is subdivided by the formation of shallow cleavage furrows, which do not cut into the
underlying yolk mass. While in the centre of this cleavage zone, small blastomeres
are subsequently formed, the peripheral segments (the so-called blastocones) remain in continuity with the yolk mass; they are the founders of the prospective yolk
syncytium, which will break down the yolk and export the nutritive elements to the
blood (Boletzky 2003). From the unilayered blastula, epibolic gastrulation starts
with a progressive expansion of the central cell plate (prospective outer germ layer,
or ectoderm) over the peripheral blastomeres adjacent to the ring of blastocones.
This peripheral complex forms the inner germ layer (mesendoderm), which soon
becomes completely covered by the centrifugally advancing edge of the ectoderm.
This edge forms the incipient blastopore lip of the gastrula.
When looking at subsequent developmental stages of cephalopod embryos and
hatchlings, one has to remember that the marginal blastopore lip of the early gastrula
forms the entire outer yolk sac envelope with its blood lacuna and the muscular
1 Cephalopod Biology