Life-history and population dynamics of the range extending Octopus tetricus (Gould, 1852) in south-eastern Australia

Ocean warming may impact the physiology, morphology, or behaviour of marine organisms that are already living close to their limits of thermal tolerance. In response to warming, many marine species are contracting, relocating or extending their geographic range in order to keep pace with their preferred thermal environments. However, variation in the magnitude of range shift responses to ocean warming is great and the mechanisms behind those range shift responses are poorly understood. Most of our understanding on how life history characteristics and population dynamics relate to range shifts is based on studies on terrestrial or marine invasive species but these may differ from climate-driven marine range shifting species. Moreover, examining climate-driven marine range shifts of long-lived species is challenging. The use of short-lived marine range-shifting species may thus facilitate examining how life history characteristics and population dynamics relate to climate-driven marine range shifts, which may allow us to increase our capacity to anticipate changes in species distributions, assess their potential impacts, and develop appropriate management strategies. In the southern hemisphere, Octopus tetricus (Gould, 1852) has undergone a recent polewards range extension from southern Queensland and New South Wales into Victorian and Tasmanian waters, likely associated with the strengthening of the East Australian Current. Therefore, this study investigated how life history characteristics, population dynamics and genetics may relate to the rapid range extension of O. tetricus. In Tasmanian waters, Octopus tetricus had a maximum total weight of 2.3 kg, demonstrated fast growth rates and a short lifespan of approximately 11 months. Temperature, food availability, and gender appeared to influence growth rate. Individuals that hatched during cooler and more productive conditions, but grew during warming conditions, exhibited faster growth rates and reached smaller body sizes than individuals that hatched into warmer waters but grew during cooling conditions. Fast growth and the associated rapid population turnover may provide an advantage over longer-lived native species, facilitating the range extension of O. tetricus. Mature females with developing eggs were found off north-eastern Tasmania. Fecundity was high and more mature females were observed during the austral spring and summer compared to other seasons of the year; the reproductive cycle of females thus seemed to be timed to favourable seasonal environmental conditions, whereas mature males were observed all year round. Females matured later and had larger body size at maturity compared to males. Octopus tetricus has the capacity to reproduce in the new parts of its range and the population has the potential to be self-sustainable, which may enable the size of the emerging population to increase at the extended zone. The population at the range extension zone was sub-structured with a distinct group predominately comprised of individuals from Tasmania. Genetic diversity within the range extension zone was maintained compared to the historical distribution area. The range extension of O. tetricus is likely to be facilitated by high and constant dispersal of paralarvae, high gene flow from a diversity of source areas, and high genetic diversity. These features would favour adaptation to new environments and facilitate establishment beyond the historical geographic limits, potentially allowing O. tetricus to prevail at the range extension areas. These results suggest that fast growth rate, rapid population turnover, high reproductive capacity, synchronization with favourable environmental conditions, high gene flow from a diversity of source areas, and high genetic diversity are some of the enabling factors that may allow the establishment and prevalence of the population of O. tetricus in the range extension zone.