The influence of habitat complexity and symbiotic associations on predator-prey interactions between fishes and reef-dwelling rhynchocinetid shrimps

Abstract

Predators can affect prey by increasing their mortality, or may reduce the fitness of prey which modify their behaviours in response to predation risks (i.e. risk effects). Non-lethal effects of predators on prey behaviours are still poorly understood, although they may have cascading effects through food webs. This thesis examined the effect of habitat structure and symbiotic associations on the interaction between predatory fish and three rhynchocinetid shrimps: Rhynchocinetes typus in temperate rocky reefs in Chile, R. brucei in tropical rocky reefs in Hong Kong and Cinetorhynchus hendersoni in coral reefs in Malaysia.

Underwater observations in Chile revealed that R. typus was rarely found on substrata with simple structure where refuges were scarce or lacking, but the effect of substratum complexity on their abundance was weak, probably because low fish abundance (and hence predation risk) did not affect survival probability below a critical threshold of refuge availability. This was in accordance with the observation that shrimp and fish abundance were negatively correlated in managed areas where fishing was limited, but not in open-access areas, were fish were less abundant. In addition, shrimps tended to form large aggregations in large shelters that offered limited protection against predators. Overall, direct effects of predation on shrimp densities and population structure were weak in Chile, whereas indirect effects on shrimp distribution within reefs appear to have been mediated through behavioural responses.

Shrimp and other small decapods often associate with invertebrate hosts in order to gain protection from predators. In Hong Kong, where prolonged overexploitation of large predatory fishes has resulted in dominance of small fishes, R. brucei uses two alternative hosts: an urchin (Diadema setosum) and an anemone (Entacmaea quadricolor). Underwater surveys revealed that shrimps were rarely found outside crevices and holes unless associated with anemones and urchins. Experiments in the laboratory confirmed that, when given a choice, shrimps avoided open areas and associated with anemones and urchins which protected them against fish. Shrimps also imprinted upon the host species with which they were associated when collected from the field, and selected that host when given a choice.

In Malaysia, where fish were more abundant than in Chile or Hong Kong, predation risks on C. hendersoni were high during the day, and shrimps exhibited nocturnal activity seemingly to reduce predation risk. Shrimps preferred only the most complex substrata, which provided the best protection against predators, while substrata with simple structure and few refuges were avoided. Tethering experiments and field observations confirmed that diel behaviour and substratum selection were driven by predation risk.

Differences in fish abundance and diversity at the three study sites resulted in varying predation risks that influenced activity patterns and habitat choice by reef shrimps. These behavioural responses of prey to predation risks and their interactions with habitat structure have important implications for predicting how human disturbances, such as overfishing or habitat degradation, may disrupt predator-prey interactions and modify food webs.