Larval traits carry over to affect post‐settlement behaviour in a common coral reef fish

Abstract

Most reef fishes begin life as planktonic larvae before settling to the reef, metamorphosing and entering the benthic adult population. Different selective forces determine survival in the planktonic and benthic life stages, but traits established in the larval stage may carry over to affect post-settlement performance. We tested the hypothesis that larval traits affect two key post-settlement fish behaviours: social group-joining and foraging. Certain larval traits of reef fishes are permanently recorded in the rings in their otoliths. In the bluehead wrasse (Thalassoma bifasciatum), prior work has shown that key larval traits recorded in otoliths (growth rate, energetic condition at settlement) carry over to affect post-settlement survival on the reef, with higher-larval-condition fish experiencing less post-settlement mortality. We hypothesized that this selective mortality is mediated by carry-over effects on post-settlement antipredator behaviours. We predicted that better-condition fish would forage less and be more likely to join groups, both behaviours that would reduce predation risk. We collected 550 recently settled bluehead wrasse (Thalassoma bifasciatum) from three reef sites off St. Croix (USVI) and performed two analyses. First, we compared each settler's larval traits to the size of its social group to determine whether larval traits influenced group-joining behaviour. Secondly, we observed foraging behaviour in a subset of grouped and solitary fish (n = 14) for 1-4 days post-settlement. We then collected the fish and tested whether larval traits influenced the proportion of time spent foraging. Body length at settlement, but not condition, affected group-joining behaviour; smaller fish were more likely to remain solitary or in smaller groups. However, both greater length and better condition were associated with greater proportions of time spent foraging over four consecutive days post-settlement. Larval traits carry over to affect post-settlement behaviour, although not as we expected: higher quality larvae join groups more frequently (safer) but then forage more. Foraging is risky but may allow faster post-settlement growth, reducing mortality risk in the long run. This shows that behaviour likely serves as a mechanistic link connecting larval traits to post-settlement selective mortality.

Risk-sensitive foraging does not explain condition-dependent choices in settling reef fish larvae

“…The samples used in this study were collected by Dingeldein & White (2016), and additional details of collection are provided there. Recently settled juvenile bluehead wrasse were collected using hand nets and clove oil anesthetic from three sites on the northwest shore of St. Croix, USVI (Fig.…”

“…Bluehead wrasse settle to the reef in approximately week-long pulses following a new moon (Caselle & Warner, 1996); collections for this study occurred during settlement pulses in July and August of 2012. Dingeldein & White (2016) described collecting two sets of fish: zero-day collections, in which larvae settling to a transect were collected on their first day after emergence onto the reef, and additional collections in which entire groups and solitary fish were selected haphazardly for collection after they had been on the reef for 1-4 days (age could be ensured because the transects were cleared of all fish on day 0, and tagging has shown that fish do not move between shelter crevices after emergence; White & Warner, 2007a). We used the latter set of collections to examine patterns of post-settlement growth.…”

“…At this time, juvenile bluehead wrasse are either solitary or form small groups of up to twenty. Fish that are larger at the time of settlement are more likely to be found in groups, and per-capita mortality declines with increasing group size (White & Warner, 2007a;Dingeldein & White, 2016). White & Warner (2007b) showed that fish in groups spend considerably more time foraging than solitary fish, but have somewhat slower post-settlement growth rates, likely due to competition.…”

“…These observations of behavior and growth suggest that smaller fish trade the safety of group membership for the opportunity for a faster growth rate (White & Warner, 2007a;White & Warner, 2007b;Dingeldein & White, 2016). Faster growth is a metric of (eventual) fitness in small immature fish.…”

“…We hypothesized that small fish may be more likely to remain solitary because of the potential for higher prey capture rates and higher growth rates. To test this hypothesis, we re-analyzed the dataset collected by Dingeldein & White (2016), who found an effect of size-at-settlement on the decision to join groups, but did not examine the post-settlement growth rates of the fish they collected. We analyzed the post-settlement growth rates (estimated from otolith growth rings) to test for differences between solitary and grouped fish in the (a) mean and (b) variance of both gut fullness and growth rates.…”