Distribution and recruitment patterns of Evechinus chloroticus and other New Zealand echinoderms: The role of pre- and post-settlement events

Abstract

<p>Numerous environmental and biological processes are responsible for shaping community structure in temperate rocky reefs. Replenishment of most marine invertebrate populations is largely determined by recruitment success, but it can be highly variable at different spatial and temporal scales. Recruitment is a complex process that involves larval supply, settlement (attachment to a suitable substrate and metamorphosis) and survival and growth of settled juveniles. However, factors controlling recruitment of mobile invertebrates, such as sea urchins, remain largely unknown. This study examines the major biotic and abiotic processes affecting early life stages of the sea urchin Evechinus chloroticus, and to what extent these events control the size and distribution of adult populations. This thesis contributes to a better understanding of the processes shaping population structure of E. chloroticus and other echinoderm species. In Chapter 2, I investigated the spatial and temporal variation in settlement of E. chloroticus, during the austral summer, at five sites in two locations of the Wellington region (Harbour and South coast) that differ in population structure and environmental conditions. The highest mean seawater temperature and lowest salinity were observed in the Harbour, while the South coast displayed higher chlorophyll and turbidity values. Density of adult sea urchins was significantly higher in the Harbour compared to the South coast and was significantly correlated to the predominant habitat type (shells). The mean size of urchins was larger in the South coast sites and juveniles (<20 mm) were only found at one site in the Harbour. Settlement was very infrequent and spatially variable. Pulses of settlement occurred in February 2018 and January 2019, in the Harbour sites and Breaker Bay (mouth of the Harbour), but not on the South coast. The presence of a young adult cohort (40 – 60 mm) at these sites suggests that there has not been a large episode of recruitment in 2 to 3 years. The low recruitment patterns observed here could be similar in other parts of New Zealand where sea urchin barrens are not very common. In Chapter 3, I described the echinoderm assemblage in the Wellington region and their recruitment patterns. Echinoderms play important ecological roles in the benthic community, but the ecological processes that regulate species assemblages are still poorly understood. The aim of this study was to analyse variation in abundance, settlement, and the correlation with environmental variables. Five echinoderm species were responsible for the assemblage variation within sites and their density was highly correlated with habitat composition. Cushion stars (Patiriella regularis) were highly abundant at all sites, while eleven-armed sea stars (Coscinasterias muricata) and snake brittle stars (Ophiopsammus maculata) were dominant in the Harbour and Moa Point respectively. Settlement of echinoderms was moderately correlated with temperature, and showed significant differences between the Harbour and the South coast. There was an important pulse of eleven-armed sea star settlers in 2019, while ophiuroid settlers were present in lower numbers but constantly during the studied period. These findings demonstrate how interannual variation in echinoderm recruitment can occur. One or more years of poor recruitment can follow a relatively good year for recruitment, and species-specific processes, as well as environmental variables, might be contributing to the observed differences. In Chapter 4, I examined possible factors limiting larval development and settlement success of E. chloroticus in the Wellington region, which frequently experiences coastal runoff. I experimentally assessed the combined stress of low salinity and sediments, in a short-term exposure, similar to runoff after a major rain event, on E. chloroticus larval and juvenile performance. Larval development was reduced in the lowest salinity and suspended sediment treatments, but these differences were no longer visible near the end of larval development. Larval survival was not affected by the treatments. However, settlement success and juvenile development was reduced by both larval exposure to the lowest salinity and in the presence of deposited sediments. At 28 d post-settlement, there were no surviving juveniles that had been exposed either to the lowest salinity, or to the highest level of suspended sediment, early in larval life. These findings suggest that even short exposure to runoff stress during early development can have strong consequences for later larval settlement and juvenile performance, which could compromise recruitment to adult populations. Predation is an important source of mortality of young marine invertebrates. In Chapter 5, I highlighted predation as a potential factor affecting recruitment success of E. chloroticus. I experimentally assessed mortality due to predation of new settlers (<1 mm) and juveniles (10 – 40 mm), as well as the role of sea urchin size and the availability of refuges on their survival. Predation on E. chloroticus new settlers had not been investigated before, and I identified hermit and decorator crab juveniles as the most effective micropredators. The availability of refuges reduced mortality of settlers and juveniles by half. Adult decorator crabs were also effective predators of sea urchin juveniles, and only lobsters were able to prey on individuals >42 mm. Although the role of macroinvertebrate predation in shaping sea urchin populations remain unclear, these findings suggest that micropredation (especially by crabs) should be considered as an important factor driving recruitment success. Together, this research provides insights on the distribution and recruitment patterns of echinoderms, which have been rarely studied in New Zealand (and not at all in the Wellington region), and attempts to elucidate the role of pre- and post-settlement events in determining recruitment to the adult population, using E. chloroticus as a model species. Findings suggest that stressors such as runoff and predation largely contribute to post-settlement mortality, and that carry-over effects of larval experience can have important consequences for juveniles. In addition, the infrequent settlement pulses and low densities of sea urchin juveniles indicate that recruitment might occur every 2 to 3 years in the Harbour, and every 3 to 4 years on the South coast. The differences in population structure observed between the two locations might be largely due to differential recruitment patterns and environmental conditions. For relatively long-lived species, even if recruitment is sporadic populations can be sustained if adult mortality is low. However, E. chloroticus is subject to exploitation in the Wellington region and across the country. This research contributes not only to a better understanding of the processes that shape population structure, but also to a better management of the fishery of E. chloroticus, as well as other invertebrate species subject to exploitation.</p>