The role of hatchling size in generating the intrinsic size-at-age variability of cephalopods: extending the Forsythe Hypothesis

Abstract

Cephalopods are characterised by extreme variability in size-at-age, with much of this variation attributed to effects of temperature and food. However, even siblings reared under identical conditions display a wide range of sizes after a period of growth. Hatchling size may represent a source of variation encompassed within adult size-at-age data (i) within a given cohort (variation in hatchling size suggests that a cohort's growth trajectory will have a 'staggered start') and (ii) as hatchling size also varies as a function of incubation temperature this will vary across broader scales (i.e. between cohorts). Field-hatchling size data for Sepioteuthis australis were used in simple deterministic simulations, extending Forsythe's (1993) temperature hypothesis, to investigate the influence of hatchling size on adult size-at-age variability. Within a cohort, our growth projections suggest that after 90 days, a large hatchling growing at a specific constant percentage daily growth rate (% BW day −1 ), would be approximately double the size of the small hatchling growing at exactly the same rate, irrespective of the growth rate used. When considering growth of different cohorts, decreases in hatchling size, as temperatures increase during a spring/summer spawning season, may be partially counteracting the 'Forsythe-effect' of increased growth rate at higher temperatures.

Modelling size-at-age in wild immature female octopus: a bioenergetics approach

“…Hatchling size varies both within a cohort (intracohort variation) and between seasons (inter-cohort variation) (Pecl et al 2004a). The model was parameterised with 3 seasonal distributions, namely summer, autumn and winter distributions for hatchling size, from which individuals were randomly selected.…”

“…0.258 g) and the proportional change in mean hatchling size to be 0.5% d -1 of additional incubation. This was based on an observed change in hatchling size of 25% for a 48 d difference in incubation duration between the summer and autumn cohorts in Sepiotheuthis australis (Pecl et al 2004a), a species with a similar thermal range and hatchling size.…”

“…Other generally overlooked factors, such as hatchling size (Pecl et al 2004a, Leporati et al 2007, can also affect size-at-age. Within a species, hatchling size is known to vary by at least 147% in squid (e.g.…”

“…Within a species, hatchling size is known to vary by at least 147% in squid (e.g. Sepioteuthis australis; Pecl et al 2004a) and 239% in cuttlefish (e.g. Sepia officinalis; Domingues et al 2001).…”

“…Most cephalopods are believed to follow a 2-phase growth pattern, starting with a rapid exponential phase followed by a slower second growth phase, often represented by a power function (Semmens et al 2004, Boyle & Rodhouse 2005. Given this growth pattern, differences between individuals at hatching can amplify throughout the lifespan (Pecl et al 2004a) and have an impact on size-at-age. Hatchling size variation has multiple origins that can be divided into environmental effects, maternal effects and genetic effects.…”