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E. A. G. Vidal and S. von Boletzky
making the duration of the hatching period longer and hatching asynchronous (Paulij
et al. 1991). In addition, it was also shown that L. vulgaris embryos exposed to constant light conditions produced slower statolith growth when compared to embryos exposed to manipulated summer (16L/8D) or winter (8L/16D) photoperiod (Villanueva
et al. 2007). This is consistent with observations made for D. opalescens intermediate
stage eggs exposed to continuous illumination and to natural photoperiod, but under
the same water quality and temperature conditions. Hatchlings from eggs exposed to
constant light were smaller and had consumed nearly all their internal yolk reserves,
when compared with those under natural photoperiod, suggesting that light might also
affect the efficiency of yolk utilization, and consequently, hatchling size and condition
(E.A.G. Vidal, personal observation). This requires validation, but it seems imprudent
to expose the eggs to constant illumination if the main purpose of the experiment is
rearing of paralarvae. Hatchlings exposed to suboptimal or stressed conditions during
embryogenesis may be weak, have poor functionality or be more susceptible to infection.
Viability and competence of paralarvae will, to a large extent, depend on the
developmental history and environmental influences during egg development. Environmental conditions are considered to play important roles on eggs and larval
quality of invertebrates (Benzie 1998). Nonetheless, little progress has been made in
elucidating the connections between the influence of environmental factors during
embryonic development and the production of high-quality and competent paralarvae for rearing. Studies that focus on evaluating tolerances and conditions during
embryonic development should provide information on a fundamental aspect of egg
quality and development: paralarval survival and competence. For instance, temperature has a dramatic impact on embryonic development of cephalopods (Boletzky
1987) and both the rate and the efficiency of yolk utilization are temperature dependent (Vidal et al. 2002b, 2005). Eggs of both L. vulgaris and D. opalescens incubated at lower temperature yield larger hatchlings than those incubated at higher
temperatures (Villanueva 2000a; Vidal et al. 2002b). This shows that development
at lower temperatures maximizes yolk conversion efficiencies, with the opposite
effect observed at high temperatures (Vidal et al. 2002b). The high metabolic demands of late-stage embryos maintained at the highest temperature tolerance limits
may impose a substantial constraint on the efficiency of conversion of yolk into
tissue. This is coherent with the production of premature hatchlings of L. vulgaris
when incubated at 24.7 °C (Villanueva et al. 2003) and 19 °C (Rosa et al. 2012). Observations under experimental conditions showed that the yolk reserve at hatching,
although variable, is proportional to the body mass, representing around 33–65 %
of the body dry weight (dw) of D. opalescens paralarvae (Vidal et al. 2002b). Most
importantly, the yolk content at hatching is crucial for the survival of hatchlings during the transition from endogenous (yolk) to exogenous (prey) feeding, when prey
capture skills are developing and the highest mortality rates are registered.
An essential fact here is the absence of a morphological or physiological ‘hatching stage’ in cephalopods (D’Aniello et al. 1989; Boletzky 2003). According to
Boletzky (2004), hatchlings can be: (1) premature, without complete absorption of
E. A. G. Vidal and S. von Boletzky
making the duration of the hatching period longer and hatching asynchronous (Paulij
et al. 1991). In addition, it was also shown that L. vulgaris embryos exposed to constant light conditions produced slower statolith growth when compared to embryos exposed to manipulated summer (16L/8D) or winter (8L/16D) photoperiod (Villanueva
et al. 2007). This is consistent with observations made for D. opalescens intermediate
stage eggs exposed to continuous illumination and to natural photoperiod, but under
the same water quality and temperature conditions. Hatchlings from eggs exposed to
constant light were smaller and had consumed nearly all their internal yolk reserves,
when compared with those under natural photoperiod, suggesting that light might also
affect the efficiency of yolk utilization, and consequently, hatchling size and condition
(E.A.G. Vidal, personal observation). This requires validation, but it seems imprudent
to expose the eggs to constant illumination if the main purpose of the experiment is
rearing of paralarvae. Hatchlings exposed to suboptimal or stressed conditions during
embryogenesis may be weak, have poor functionality or be more susceptible to infection.
Viability and competence of paralarvae will, to a large extent, depend on the
developmental history and environmental influences during egg development. Environmental conditions are considered to play important roles on eggs and larval
quality of invertebrates (Benzie 1998). Nonetheless, little progress has been made in
elucidating the connections between the influence of environmental factors during
embryonic development and the production of high-quality and competent paralarvae for rearing. Studies that focus on evaluating tolerances and conditions during
embryonic development should provide information on a fundamental aspect of egg
quality and development: paralarval survival and competence. For instance, temperature has a dramatic impact on embryonic development of cephalopods (Boletzky
1987) and both the rate and the efficiency of yolk utilization are temperature dependent (Vidal et al. 2002b, 2005). Eggs of both L. vulgaris and D. opalescens incubated at lower temperature yield larger hatchlings than those incubated at higher
temperatures (Villanueva 2000a; Vidal et al. 2002b). This shows that development
at lower temperatures maximizes yolk conversion efficiencies, with the opposite
effect observed at high temperatures (Vidal et al. 2002b). The high metabolic demands of late-stage embryos maintained at the highest temperature tolerance limits
may impose a substantial constraint on the efficiency of conversion of yolk into
tissue. This is coherent with the production of premature hatchlings of L. vulgaris
when incubated at 24.7 °C (Villanueva et al. 2003) and 19 °C (Rosa et al. 2012). Observations under experimental conditions showed that the yolk reserve at hatching,
although variable, is proportional to the body mass, representing around 33–65 %
of the body dry weight (dw) of D. opalescens paralarvae (Vidal et al. 2002b). Most
importantly, the yolk content at hatching is crucial for the survival of hatchlings during the transition from endogenous (yolk) to exogenous (prey) feeding, when prey
capture skills are developing and the highest mortality rates are registered.
An essential fact here is the absence of a morphological or physiological ‘hatching stage’ in cephalopods (D’Aniello et al. 1989; Boletzky 2003). According to
Boletzky (2004), hatchlings can be: (1) premature, without complete absorption of