Energetics of successful settlement in spiny lobsters

Abstract

Spiny lobsters have an oceanic larval phase (i.e. phyllosoma) that can persist for over a year in some species. These larvae are active and specialised predators of macrozooplankton. The phyllosomal phase in spiny lobsters ends when they metamorphose into a transitional post-larval puerulus, a nektonic, non-feeding phase that relies on energetic reserves accumulated during the previous phyllosomal phase, a phenomenon known as secondary lecithotrophy. Pueruli actively swim towards the coast, and upon arrival in shallow coastal waters they seek out suitable benthic habitat to settle and moult into benthic juveniles, and recommence feeding. Secondary lecithotrophy limits the energy available to make this onshore migration, and it has been postulated that increasing seawater temperatures increase the bioenergetic requirements for the directed movement of pueruli. To test this hypothesis, the energetics of the nektonic phase and early juvenile stages of two species of spiny lobsters were studied: Jasus edwardsii and Sagmariasus verreauxi. Wild caught pueruli of J. edwardsii were experimentally swum for periods of up to 5 days whilst measuring changes in their total lipid and protein content. Using this method pueruli of this species were estimated to use 96 J per day while in transit. A similar experiment was done for wild pueruli of S. verreauxi at three seawater temperatures (i.e., 17, 20 and 23 °C) with the pueruli using 23.1 J, 42.4 J and 48.6 J per day respectively. The time that pueruli of S. verreauxi spent swimming decreased markedly with increasing temperature with a 27% reduction in swimming when temperature increased from 17 to 23 °C. Swimming experiments of pueruli of both species showed them matching the speed of the currents inside the tanks (0.15-0.17 m s-1) and exhibiting rheotaxis. Pueruli would swim both during the night and the day. Starvation experiments were performed in both species to investigate post-pueruli energetics in first instar juveniles. Once pueruli had moulted into first instar juveniles they were not allowed to feed until they perished, with their lipid and protein content compared to levels in control pueruli pre metamorphosis. It was estimated that first instar juveniles of J. edwardsii use 25.0 J per day and S. verreauxi use 11.7 J per day during starvation. There was no difference in the energy use of first instar juveniles of S. verreauxi starved at 17, 20 or 23 °C. For S.verreauxi the recovery time post-starvation was also examined, this was calculated to be 30.4 days, if refed within this time frame 50% can recover and continue developing. The results of this study indicate the lowest levels of lipid that are viable for survival and continued development in pueruli and first instar juveniles of S. verreauxi is around 6% (dryweight), and when they approach this threshold lipid is spared and protein catabolism is used to meet metabolic demands. Collectively the results of this research indicate that the transport success of pueruli into inshore and coastal waters is likely to be negatively affected by increasing seawater temperatures. However, once pueruli moult to the first instar juvenile stage they become much more tolerant of elevated temperatures. This can result in reduced recruitment of lobsters into the population.