Extended phenotypes on coral reefs: cryptic phenotypes modulate coral-vermetid interactions.

Phenotypic variation can lead to variation in the strength and outcome of species interactions. Variation in phenotypic traits can arise due to plastic responses to environmental stimuli, underlying genetic variation, or both, and may reflect differences in the focal organism or aspects of the extended phenotype (e.g., associated microbes). We used a reciprocal transplant experiment of Porites corals to evaluate the role of plasticity vs. heritable diversity on phenotypic traits and performance of corals that varied in their prior exposure to vermetid gastropods, an organism known to reduce coral growth and survival. We measured a suite of phenotypic traits associated with coral performance, many of which showed a plastic response to vermetid exposure. Vermetids decreased calcification of corals, increased microbial diversity, and shifted microbial composition. Most traits also showed a signature of previous exposure environment that persisted even when exposure was reversed: i.e., under the same conditions, corals naïve to vermetids had slower calcification rates, thicker tissues, higher Symbiodiniaceae densities, and different microbiomes than corals previously exposed to vermetids. We suggest the phenotypic differences are heritable, as reefs with and without vermetids were comprised of two different mitotypes, that revealed high, consistent genetic variation. Vermetids were only found on the fast-growing coral mitotype that was characterized by thin tissue, and that likely had a history of disturbance. As extended phenotypes can have community impacts, we suggest vermetid, in addition to microbes, are part of the extended community phenotype of these corals. Coral genotypes can establish different reef trajectories, with thin-tissue types more prone to disturbance and subsequent colonization by other species, like vermetids, which can further facilitate the degradation of coral reefs. The effects of the extended phenotype of species likely influence heterogeneity across landscapes as well as temporal differences in community composition.

Landscape edges shape dispersal and population structure of a migratory fish.

Many freshwater organisms have a life-history stage that can disperse through seawater. This has obvious benefits for colonization and connectivity of fragmented sub-populations, but requires a physiologically challenging migration across a salinity boundary. We consider the role of landscape boundaries between freshwater and seawater habitats, and evaluate their potential effects on traits and developmental histories of larvae and juveniles (i.e., dispersing life-history stages) of an amphidromous fish, Galaxias maculatus. We sampled juvenile fish on their return to 20 rivers in New Zealand: 10 rivers had abrupt transitions to the sea (i.e., emptying to an open coastline); these were paired with 10 nearby rivers that had gradual transitions to the sea (i.e., emptying into estuarine embayments). We reconstructed individual dispersal histories using otolith microstructure, otolith microchemistry, and stable isotope analysis. We found that fish recruiting to embayment rivers had distinct dispersal and foraging histories, were slower growing, smaller in size, and older than fish recruiting to nearby non-embayment rivers. Our results indicate that landscape edges can affect dispersal capabilities of aquatic organisms, potentially leading to divergent life-history strategies (i.e., limited- versus widespread-dispersal). Patterns also suggest that dispersal potential among landscape boundaries can create heterogeneity in the traits of individuals, with implications for metapopulation dynamics.

Born at the right time? A conceptual framework linking reproduction, development, and settlement in reef fish.

Parents are expected to make decisions about reproductive timing and investment that maximize their own fitness, even if this does not maximize the fitness of each individual offspring. When offspring survival is uncertain, selection typically favors iteroparity, which means that offspring born at some times can be disadvantaged, while others get lucky. The eventual fate of offspring may be further modified by their own decisions. Are fates of offspring set by birthdates (i.e., determined by parents), or can offspring improve upon the cards they've been dealt? If so, do we see adaptive plasticity in the developmental timing of offspring? We evaluate these questions for a coral reef fish (the sixbar wrasse, Thalassoma hardwicke) that is characterized by extreme iteroparity and flexible larval development. Specifically, we monitored larval settlement to 192 small reefs over 11 lunar months and found that most fish settled during new moons of a lunar cycle (consistent with preferential settlement on dark nights). Settlement was significantly lower than expected by chance during the full moon and last quarter of the lunar cycle (consistent with avoidance of bright nights). Survival after settlement was greatest for fish that settled during times of decreasing lunar illumination (from last quarter to new moon). Fish that settled on the last quarter of the lunar cycle were ~10% larger than fish that settled during other periods, suggesting larvae delay settlement to avoid the full moon. These results are consistent with a numerical model that predicts plasticity in larval development time that enables avoidance of settlement during bright periods. Collectively, our results suggest that fish with inauspicious birthdates may alter their developmental trajectories to settle at better times. We speculate that such interactions between parent and offspring strategies may reinforce the evolution of extreme iteroparity and drive population dynamics, by increasing the survival of offspring born at the "wrong" time by allowing them to avoid the riskiest times of settlement.

Variability in relative importance of determinants of reef fish recruitment.

Multiple processes can act together to determine abundance of organisms and structure of communities. Recently, appreciation of this fact has motivated development of conceptual and statistical frameworks that quantitatively assess the relative importance of multiple causal factors. However, little consideration has been given to variability in the "importance" of processes through space and time (i.e. robustness), which represents another facet of a process's importance. Here, I focused on populations of a coral reef fish (Thalassoma hardwicke) and used an existing analytical method to assess the relative importance of initial population inputs (larval supply) and subsequent juvenile mortality in determining the average abundances of juvenile fish populations in different locations and times. The relative importance of processes varied significantly both temporally and spatially across a range of scales, and indicate a need for future assessments of relative importance to incorporate this variability.