10
elements (derived from the mesodermal part of the inner germ layer), which spread
over the yolk mass. These muscular elements form a network organized to create
peristaltic waves of surface contractions generating the early embryonic blood circulation. From the outer yolk sac the blood space extends around the remaining yolk
mass inside the ‘embryo proper’ (i.e. the embryonic body derived from the gastrula).
In embryos developed from small eggs, the outer yolk sac is smaller than, or
about the same size as, the embryo proper, whereas in embryos developing from
large eggs, the outer yolk sac is huge compared to the embryo proper. Whatever
the size, however, the remaining yolk mass (i.e. the yolk not yet ‘used up’ during
embryogenesis) is finally taken up by the body of the embryo before it hatches. This
so-called inner yolk sac (belonging to the digestive gland) is the persistent part of
the embryonic ‘yolk organ’ (Boletzky 2010).
To leave the egg case (i.e. the chorion and complementary envelopes), cephalopod hatchlings use a special hatching gland situated on the mantle tip. The local
digestion of the envelopes by the hatching enzyme produces an opening, through
which the hatchling extracts itself, generally using auxiliary hatching equipments
(e.g. integumental ciliatures). The different hatching mechanisms are closely related
to the structure and consistency of the egg cases, which may have undergone strong
modifications during the development of the embryos (Boletzky 1998).
1.3.2 Hatchlings
All cephalopods hatch as complete little animals, which are devoid of true larval
features (hence the term paralarva proposed by Young and Harman 1988).
Remaining yolk reserves are absorbed to depletion while the young animal
begins to ingest and digest food, generally live prey (e.g. crustacean larvae),
which are captured using visual hunting techniques. This parallel embryonic and
postembryonic nutrition may last several days or weeks. Early growth of hatchlings shows a phase with no net growth in dry weight due to the yolk consumption
(Vidal et al. 2002). Once the yolk reserves are used up, hatchlings depend on
active foraging and—to a limited extent—on energy sources stored in the cells
of certain tissues. Depending on the actual hatching time, the young animal starts
out under more or less favourable conditions of individual fitness. Premature
hatching, which is frequently triggered by artificial stimuli (Villanueva and
Norman 2008), may substantially reduce survival chances, since the remaining
outer yolk sac is lost, and a large inner yolk sac represents a heavy ballast for
an actively swimming animal, limiting the jet propulsion power by reducing the
available water volume of the mantle cavity.
In most cephalopods, the hatchlings have a lifestyle that is rather similar to the
adult lifestyle. Thus, the hatchlings of nektonic and macroplanktonic squids are
planktonic in that they are transported by currents, but within a given water mass
in the cubic metre range, they move about actively by jet propulsion (Bartol et al.
2009), approaching potential prey by forward swimming and escaping from wouldbe predators by backward jetting (often combined with inking). This is essentially
S. von Boletzky and R. Villanueva
elements (derived from the mesodermal part of the inner germ layer), which spread
over the yolk mass. These muscular elements form a network organized to create
peristaltic waves of surface contractions generating the early embryonic blood circulation. From the outer yolk sac the blood space extends around the remaining yolk
mass inside the ‘embryo proper’ (i.e. the embryonic body derived from the gastrula).
In embryos developed from small eggs, the outer yolk sac is smaller than, or
about the same size as, the embryo proper, whereas in embryos developing from
large eggs, the outer yolk sac is huge compared to the embryo proper. Whatever
the size, however, the remaining yolk mass (i.e. the yolk not yet ‘used up’ during
embryogenesis) is finally taken up by the body of the embryo before it hatches. This
so-called inner yolk sac (belonging to the digestive gland) is the persistent part of
the embryonic ‘yolk organ’ (Boletzky 2010).
To leave the egg case (i.e. the chorion and complementary envelopes), cephalopod hatchlings use a special hatching gland situated on the mantle tip. The local
digestion of the envelopes by the hatching enzyme produces an opening, through
which the hatchling extracts itself, generally using auxiliary hatching equipments
(e.g. integumental ciliatures). The different hatching mechanisms are closely related
to the structure and consistency of the egg cases, which may have undergone strong
modifications during the development of the embryos (Boletzky 1998).
1.3.2 Hatchlings
All cephalopods hatch as complete little animals, which are devoid of true larval
features (hence the term paralarva proposed by Young and Harman 1988).
Remaining yolk reserves are absorbed to depletion while the young animal
begins to ingest and digest food, generally live prey (e.g. crustacean larvae),
which are captured using visual hunting techniques. This parallel embryonic and
postembryonic nutrition may last several days or weeks. Early growth of hatchlings shows a phase with no net growth in dry weight due to the yolk consumption
(Vidal et al. 2002). Once the yolk reserves are used up, hatchlings depend on
active foraging and—to a limited extent—on energy sources stored in the cells
of certain tissues. Depending on the actual hatching time, the young animal starts
out under more or less favourable conditions of individual fitness. Premature
hatching, which is frequently triggered by artificial stimuli (Villanueva and
Norman 2008), may substantially reduce survival chances, since the remaining
outer yolk sac is lost, and a large inner yolk sac represents a heavy ballast for
an actively swimming animal, limiting the jet propulsion power by reducing the
available water volume of the mantle cavity.
In most cephalopods, the hatchlings have a lifestyle that is rather similar to the
adult lifestyle. Thus, the hatchlings of nektonic and macroplanktonic squids are
planktonic in that they are transported by currents, but within a given water mass
in the cubic metre range, they move about actively by jet propulsion (Bartol et al.
2009), approaching potential prey by forward swimming and escaping from wouldbe predators by backward jetting (often combined with inking). This is essentially
S. von Boletzky and R. Villanueva