Rebecca J. Fox

Sediments and herbivory as sensitive indicators of coral reef degradation

Around the world, the decreasing health of coral reef ecosystems has highlighted the need to better understand the processes of reef degradation. The development of more sensitive tools, which complement traditional methods of monitoring coral reefs, may reveal earlier signs of degradation and provide an opportunity for pre-emptive responses. We identify new, sensitive metrics of ecosystem processes and benthic composition that allow us to quantify subtle, yet destabilizing, changes in the ecosystem state of an inshore coral reef on the Great Barrier Reef. Following severe climatic disturbances over the period 2011-2012, the herbivorous reef fish community of the reef did not change in terms of biomass or functional groups present. However, fish-based ecosystem processes showed marked changes, with grazing by herbivorous fishes declining by over 90%. On the benthos, algal turf lengths in the epilithic algal matrix increased more than 50% while benthic sediment loads increased 37-fold. The profound changes in processes, despite no visible change in ecosystem state, i.e., no shift to macroalgal dominance, suggest that although the reef has not undergone a visible regime-shift, the ecosystem is highly unstable, and may sit on an ecological knife-edge. Sensitive, process-based metrics of ecosystem state, such as grazing or browsing rates thus appear to be effective in detecting subtle signs of degradation and may be critical in identifying ecosystems at risk for the future.

Why pair? Evidence of aggregative mating in a socially monogamous marine fish (Siganus doliatus, Siganidae).

Many species live in stable pairs, usually to breed and raise offspring together, but this cannot be assumed. Establishing whether pairing is based on mating, or an alternative cooperative advantage, can be difficult, especially where species show no obvious sexual dimorphism and where the act of reproduction itself is difficult to observe. In the tropical marine fishes known as rabbitfish (Siganidae), half of extant species live in socially monogamous, territorial pairs. It has been assumed that partnerships are for mating, but the reproductive mode of pairing rabbitfish is currently unconfirmed. Using passive acoustic telemetry to track movements of fishes belonging to one such species (Siganus doliatus), we provide the first evidence that paired adult fish undertake highly synchronized migrations with multiple conspecifics on a monthly cycle. All tagged individuals migrated along the same route in three consecutive months and were absent from home territories for 2–3 days just after the new moon. The timing and directionality of migrations suggest that S. doliatus may form spawning aggregations, offering the potential for exposure to multiple reproductive partners. The finding raises fundamental questions about the basis of pairing, mate choice and partnership longevity in this family.

Rabbitfish sentinels: first report of coordinated vigilance in conspecific marine fishes

Sentinel behaviour is defined as a strategy of coordinated vigilance in which predation risk is reduced by one member of a pair or group adopting a posture of vigilance, thereby allowing the other member(s) to reduce their vigilance, typically whilst foraging (McGowan and Woolfenden 1989). Such behaviour has previously only been documented in mammals and birds (Wright et al. 2001; Ridley et al. 2013). During observations of rabbitfishes (Siganidae) belonging to the species Siganus vulpinus, S. corallinus and S. puellus at sites around Lizard Island, northern Great Barrier Reef, Australia (14 40¢S, 145 28¢E), we saw pairs of all three species engage in what appears to be coordinated vigilance whilst foraging (Figs. 1, 2). Typically, we observed one member of the pair terminating vigilance at the same time as the other member of the pair commenced its vigilance, implying the presence of a deliberate behavioural strategy, rather than chance occurrence (Fig. 2; ESM S1). To our knowledge this is the first report of behavioural observations that are consistent with a sentinel system in conspecific fishes. It is worth noting that the species observed here all forage in cryptic locations in the reef matrix such as under ledges or within crevices, where vision (and therefore vigilance) is particularly compromised. We hypothesise that, in visually occluded habitats such as coral reefs, pairing species may gain a benefit by employing a strategy of coordinated vigilance, and we suggest that rabbitfishes may provide an alternative model organism with which to examine competing hypotheses regarding the evolutionary basis for sentinel behaviour.

Environmental change mediates mate choice for an extended phenotype, but not for mate quality

Sexual cues, including extended phenotypes, are expected to be reliable indicators of male genetic quality and/or provide information on parental quality. However, the reliability of these cues may be dependent on stability of the environment, with heterogeneity affecting how selection acts on such traits. Here, we test how environmental change mediates mate choice for multiple sexual traits, including an extended phenotype–‐the structure of male‐built nests – in stickleback fish. First, we manipulated the dissolved oxygen (DO) content of water to create high or low DO environments in which male fish built nests. Then we recorded the mate choice of females encountering these males (and their nests), under either the same or reversed DO conditions. Males in high DO environments built more compact nests than those in low DO conditions and males adjusted their nest structure in response to changing conditions. Female mate choice for extended phenotype (male nests) was environmentally dependent (females chose more compact nests in high DO conditions), while female choice for male phenotype was not (females chose large, vigorous males regardless of DO level). Examining mate choice in this dynamic context suggests that females evaluate the reliability of multiple sexual cues, taking into account environmental heterogeneity.

Good parenting may not increase reproductive success under environmental extremes

For species exhibiting parental care, the way in which parents adjust care behaviour to compensate for environmental change potentially influences offspring survival and, ultimately, population viability. Using the three‐spined stickleback (Gasterosteus aculeatus) – a species in which males provide parental care by building and tending a nest and fanning the eggs – we examined how low dissolved oxygen (DO) levels affect paternal care, embryo development and survival. Although levels of nest tending were unaffected by DO level, we found that larger males fanned their embryos more under low oxygen conditions. This resulted in faster rates of embryo development within the clutches of these larger males, but reduced embryo survival at 7 days post‐fertilization compared to clutches of smaller males. Our results suggest that although parents may attempt to compensate for environmental change via alterations to care behaviour, their ability to do so can be dependent on parental phenotype. This sets up the potential for oxygen levels to act on the strength and direction of selection within populations. We discuss possible explanations for the surprising result that supposedly adaptive changes in care behaviour by large males (i.e. increased fanning) led to reduced embryo survival at 7 days post‐fertilization, and whether, as a consequence, acute environmental conditions may have the potential to overwhelm selection on sexual traits.