Abstract
For benthic marine keystone species, the fitness of new recruits is an important determinant of their long-term population dynamics and their ecosystem impact. Marine invertebrates have complex life cycles, with each stage exposed to stage-specific stressors that influence development and fitness. In particular, the planktonic larval stage is directly influenced by phytoplankton food supply, and often exhibits phenotypic plasticity to adapt to highly variable environmental conditions such as patchy food. Carryover effects describe experiences from an earlier life stage that influence the fitness of a subsequent life stage. Because carryover effects impact individual fitness, they play an important role in shaping recruitment and ultimately influence adult population structures. Global climate change and anthropogenic pressures are exacerbating environmental instability, particularly in near-shore waters. This includes changes in pelagic productivity patterns, which can have direct impacts on larval development and survival, and in turn, recruitment and post-settlement fitness.
This study aimed to evaluate larval responses and carryover effects (post-settlement fitness) of variable phytoplankton diets on two ecologically distinct echinoderm species: the crown- of-thorns sea star Acanthaster cf. solaris and the New Zealand sea urchin Evechinus chloroticus. Across tropical coral reefs of the Indo-Pacific, A. solaris maintains habitat diversity through coral grazing, and E. chloroticus has a similar ecological role in the temperate kelp forests of New Zealand. Echinoderms have boom-bust population dynamics, and high abundances, termed ‘outbreaks’, can be environmentally detrimental. The rise in outbreak frequency of A. solaris is a key contributor to the global decline in coral cover and resilience. The nutrient enrichment hypothesis suggests that outbreaks are driven by increased terrestrial runoff and phytoplankton blooms in coastal waters, increasing larval survival (Birkeland, 1982). In northern New Zealand, E. chloroticus outbreaks transform productive kelp forests into ‘urchin barrens’. Although both species have well-described larval stages and display plastic responses to different phytoplankton concentrations, little is known about the carryover effects of larval nutrition in either species.
In separate laboratory experiments, larvae of both species were reared under either constant high, constant low, or variable phytoplankton regimes (low to high and high to low, with the food switch occurring halfway through development). Larval developmental progression was tracked using stage classification, and larvae were sampled for morphometric analysis before and after the food switch. Both univariate and multivariate morphometric analyses were completed to identify the presence of allometric growth and overall body shape differences, respectively. Larvae were provided with a settlement substrate upon metamorphic competency, and settlement development indices were used to track metamorphic success. Newly metamorphosed juveniles were measured in diameter as a metric of early-juvenile fitness.
For A. solaris, early larval nutrition influenced larval developmental rates and body size, while late-stage larval nutrition had carryover effects on early juvenile size. Larvae with adequate early nutrition displayed few effects of late-stage food limitation, and had the highest settlement of all treatments (25.0 ± 0.85%), including constantly well-fed larvae. However, less than half of the settlers that experienced late-stage food limitation had survived at 6 days post-settlement (dps) (11.73 ± 1.79%), suggesting these larvae did not have sufficient nutritive reserves to complete metamorphosis. In contrast, larvae that experienced early-food limitation and a late-stage food increase had a lower number of settlers and juveniles (<1% of larval volume). A key finding of this study was that variable larval food conditions lead to carryover effects for early pre-metamorphic juvenile size. Juveniles from diet switch treatments were significantly smaller than juveniles of constantly high-fed larvae. Interestingly, juveniles of larvae that were switched from early food limitation to high late-stage food availability were larger than juveniles of late-stage food-limited larvae by 6 days post-settlement (413 ± 13 μm, 389 ± 6 μm, respectively). These findings further highlight the importance of late-stage food availability in early juvenile success, as size is vital in reducing predation of the largely defenceless early juveniles.
For Evechinus, when released from early food limitation later in development, larvae displayed slight compensation in body size. These larvae were larger and more developed by the end of the experiment than larvae in constant low food conditions. Larvae switched to low food later in development did not increase the length of feeding appendages, supporting previous research that highlights plasticity in larval arm length is greatest in early development for Evechinus. Late-stage food limitation caused minor developmental delays, with less advanced larvae than constantly well-fed larvae at 19 days post-fertilisation (dpf). However, PCA analyses showed that these larvae had similar body sizes, suggesting limited morphological plasticity to decreased food availability later in development. Larvae that experienced early food limitation were unable to fully catch up in size or development to early well-fed larvae that experienced late-stage food limitation. Larvae from variable food treatments exhibited the highest post-oral arm length asymmetry following the food switch. Larvae that experienced late food limitation had significantly higher post-oral arm length asymmetry than constantly well-fed larvae at 16 dpf (15.63 ± 1.07%, 10.12 ± 1.06%). These findings suggest that while adequate early nutrition may support larval resilience in response to later food decreases, it does not mitigate abnormal development. Settlement was unsuccessful in the Evechinus experiment, and carryover effects could therefore not be identified, yet the negative carryover effects of poor late-stage larval nutrition in Acanthaster suggest that further research should follow Evechinus across the metamorphic boundary to assess potential carryover effects.
The findings of this thesis suggest that variability in larval nutrition, and the timing of food limitation, can lead to carryover effects in the size of a post-metamorphic juvenile echinoderm. In particular, the negative carryover effects of late-stage food limitation observed in A. solaris highlights the importance of adequate nutrition throughout the development of juvenile structures (i.e. rudiments). Evechinus demonstrated evidence of compensatory growth upon release from food limitation, which may buffer individuals against short-term phytoplankton fluctuations. However, further research is needed to conclude the long-term carryover effects of such plasticity to understand how this may affect population stability. Simulating variable food environments better reflects natural phytoplankton variability, and long-term monitoring of these individuals could lead to meaningful conclusions about wild population dynamics and recruitment under increasing environmental instability.