302
E. A. G. Vidal and S. von Boletzky
A reproducible high rate of survival and growth of paralarvae is necessary to
achieve successful culture. Reliable methods to assess paralarvae quality should be
developed for use soon after hatching and allow comparison of results. A good measure of paralarvae competence seems to be their ability to withstand short periods of
starvation. It has been shown that starvation elicits a strong selection for inherently
faster growing and stronger paralarvae (Vidal et al. 2006). Behaviour may also be
an effective tool for the evaluation of paralarval quality. Weak or abnormal hatchlings will display irregular swimming and behavioural patterns.
16.6.5 Growth
It has been shown experimentally that there are two distinct growth phases during
the early development of loliginid squid: (1) the no net growth phase and (2) the
exponential growth phase. The no net growth phase comprises a negative and a
positive growth sub-phases. In the first, a decrease in body weight of hatchlings is
observed (from 20 to 30 %; Vidal et al. 2002b) because the yolk is being consumed
at exponential rates and there is little, if any, conversion of yolky matter into somatic tissue due to the high metabolic cost of swimming and maintenance (Vidal
et al. 2002b; Rosa et al. 2012). In the positive sub-phase, the weight loss is regained
over the next few days by prey capture, resulting in a very short phase where no net
growth takes place. As a consequence, at the end of this no net growth phase, the
body weight of paralarvae is the same as at hatching. The duration of this phase is
temperature dependent and indicates the time required to recover the weight loss
due to yolk utilization. For example, for D. opalescens the no net growth phase
lasted 7 days at 16 °C and 15 days at 12 °C. This early growth pattern has also been
observed in L. vulgaris (Villanueva 2000a) and L. reynaudii (Vidal et al. 2005)
reared in the laboratory.
Interestingly, this short phase where no net body growth occur has a reflection in
the growth of the statoliths as demonstrated by Villanueva et al. (2007). Statoliths
growth rates of newly hatched L. vulgaris showed a conspicuous decrease of more
than half in comparison with those of pre-hatching embryos, clearly indicating the
negative growth sub-phase as a result of the exponential rate of yolk utilization.
This no net growth phase set the limits of a critical period in the early life history
of squid that correspond to the transition from endogenous to exogenous feeding
and is mainly caused by the constraints imposed by the high metabolic demands
of the active swimming mode of hatchlings (Rosa et al. 2012) and their short-lasting energy reserves to overcome metabolic suppression and starvation (Vidal et al.
2002b, 2006). The high mortalities that occur during this critical period are due to
the inability of hatchlings to withstand food deprivation and to capture prey in the
required amounts for daily maintenance and growth. This reveals a short initial
delay in growth of paralarvae, which are subjected to an intense and effective selection for successful feeders. The survivors shortly attain the exponential growth rates
that are the fundamental hallmark of most cephalopod species (Forsythe and Van
Heukelem 1987).
E. A. G. Vidal and S. von Boletzky
A reproducible high rate of survival and growth of paralarvae is necessary to
achieve successful culture. Reliable methods to assess paralarvae quality should be
developed for use soon after hatching and allow comparison of results. A good measure of paralarvae competence seems to be their ability to withstand short periods of
starvation. It has been shown that starvation elicits a strong selection for inherently
faster growing and stronger paralarvae (Vidal et al. 2006). Behaviour may also be
an effective tool for the evaluation of paralarval quality. Weak or abnormal hatchlings will display irregular swimming and behavioural patterns.
16.6.5 Growth
It has been shown experimentally that there are two distinct growth phases during
the early development of loliginid squid: (1) the no net growth phase and (2) the
exponential growth phase. The no net growth phase comprises a negative and a
positive growth sub-phases. In the first, a decrease in body weight of hatchlings is
observed (from 20 to 30 %; Vidal et al. 2002b) because the yolk is being consumed
at exponential rates and there is little, if any, conversion of yolky matter into somatic tissue due to the high metabolic cost of swimming and maintenance (Vidal
et al. 2002b; Rosa et al. 2012). In the positive sub-phase, the weight loss is regained
over the next few days by prey capture, resulting in a very short phase where no net
growth takes place. As a consequence, at the end of this no net growth phase, the
body weight of paralarvae is the same as at hatching. The duration of this phase is
temperature dependent and indicates the time required to recover the weight loss
due to yolk utilization. For example, for D. opalescens the no net growth phase
lasted 7 days at 16 °C and 15 days at 12 °C. This early growth pattern has also been
observed in L. vulgaris (Villanueva 2000a) and L. reynaudii (Vidal et al. 2005)
reared in the laboratory.
Interestingly, this short phase where no net body growth occur has a reflection in
the growth of the statoliths as demonstrated by Villanueva et al. (2007). Statoliths
growth rates of newly hatched L. vulgaris showed a conspicuous decrease of more
than half in comparison with those of pre-hatching embryos, clearly indicating the
negative growth sub-phase as a result of the exponential rate of yolk utilization.
This no net growth phase set the limits of a critical period in the early life history
of squid that correspond to the transition from endogenous to exogenous feeding
and is mainly caused by the constraints imposed by the high metabolic demands
of the active swimming mode of hatchlings (Rosa et al. 2012) and their short-lasting energy reserves to overcome metabolic suppression and starvation (Vidal et al.
2002b, 2006). The high mortalities that occur during this critical period are due to
the inability of hatchlings to withstand food deprivation and to capture prey in the
required amounts for daily maintenance and growth. This reveals a short initial
delay in growth of paralarvae, which are subjected to an intense and effective selection for successful feeders. The survivors shortly attain the exponential growth rates
that are the fundamental hallmark of most cephalopod species (Forsythe and Van
Heukelem 1987).