Growth, development, and aging methods of the midget octopus, Octopus huttoni
This is the first study to be completed on the growth, developmental rates, and aging methods in the midget octopus, Octopus huttoni. Understanding life histories and population dynamics are important for fisheries management. Although O. huttoni may not be a commercial species, it is commonly caught as bycatch from oyster dredging suggesting that populations are heavily impacted. The overall aim of this study was to compare life history data from O. huttoni populations in Southern New Zealand. Morphometries and ages were compared between three populations with different habitats and substrate; Foveaux Strait (Bluff), Otago Harbour, and Offshore Otago. Morphometries were measured from 121 individuals, ages were estimated from 109 individuals using beak length, stylet weight, and beak and stylet increment analysis, and lipofuscin volume was calculated for 106 individuals. Paralarval young were also bred in captivity and reared at different temperatures to observe how growth changes with temperature. Aging methods were compared to determine the best method for wild-caught animals. Beaks provided the highest estimate, assuming that one increment is laid down daily, but are still suspected to be an underestimate because of feeding erosion. Stylets were validated to have daily growth rings but provided estimates that were much lower than beak estimates due to poor visualization of the nucleus and are therefore, not recommended as an accurate aging method. Lipofuscin accumulated slightly when compared to beak age, but the uneven distribution of granules may have resulted in increased variation as only 10 photos were taken per sample. Therefore, more photos (20-30) per sample and known-aged individuals should be used to establish a baseline trend. Individuals from the Foveaux Strait were found to be significantly smaller and younger than those found in the Otago Harbour. Those from Munida were smaller and younger than those from the Otago Harbour, but older than those from Bluff. These results support the ontogenetic shift hypothesis that paralarvae of merobethic species such as O. huttoni drift offshore during the planktonic stage, settle to the benthos, and then migrate inshore to spawn when mature. Since individuals from the Foveaux Strait were caught offshore, further studies should also collect octopus from inshore Bluff to compare and further test this hypothesis. The smallest individual was estimated to be 28 days old, but O. huttoni are hypothesised to spend 40-70 days in the plankton before settlement. The lack of growth rings in laboratory reared paralarvae suggest that increments may start depositing only after settlement. Future studies should observe increments in known-aged, recently settled individuals and if this is true, that would indicate that beak estimates can only be representative of age post-settlement. Results from paralarval rearing confirmed that at lower temperatures, embryonic development is slower, and that mother size and age does not influence offspring size. Overall, this comprehensive study provides vital life history information for three different populations of O. huttoni in Southern New Zealand.
Advisor: Lamare, Miles; Damsteegt, Erin; Lokman, Mark
Degree Name: Master of Science
Degree Discipline: Marine Science
Publisher: University of Otago
Keywords: Southern New Zealand; Allometry; Lipofuscin; Octopus huttoni; Growth rates; Ageing
Research Type: Thesis