Overhead light intensity preference of the broad-fronted mangrove crab Metopograpsus frontalis (Decapoda: Grapsidae) in dry and wet conditions

Collaborators:

Noah Martin, Hannah Krohn, Vicky Fong, Olivia Delrosso

Principal Investigator: Dr. Ian Tibbetts

2019.9

Abstract. Intertidal species are subject to both terrestrial and aquatic predation, and face the challenge of shifting vision due to changing water level. Overhead light intensity information is important for sensing terrestrial avian predators, which are a major source of mortality among intertidal crustaceans. In this study, we examined the differences in overhead light intensity preference of Metapograpsus frontalis, a mangrove crab common on the sheltered rocky shores in Stradbroke Island, in two experimental treatments ­––– exposed on dry sand and submerged under water. The collective of M. frontalis individuals in our study established a population preference for the low light intensity (340-390 lux) in wet treatment, but not in dry treatment (wet: p=0.0024, dry: p=0.69, population difference: p=0.034). This corresponded to the natural tidal behavior of M. frontalis, which hide under rocks during high tide and forage during low tide. Given this tidal pattern, it is likely that intertidal crustaceans tune their eye structure to mainly detect either terrestrial or aquatic predators, but not both. In the environment where their vision is compromised, they rely on shelters for predator avoidance. It is thus essential to maintain proper shelters for intertidal crustaceans and avoid clustering of human activity that might over-stress their vision in coastal area.

Additional Keywords: intertidal adaptation, tidal rhythm, predator-prey, crab behavior, compound eyes

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Acropora and Pocillopora size-frequency distributions reflect different coral recruitment strategies on Heron Reef

Collaborators:

Tatum Bernat, Skai Peterson, Allison Aplin, Rachel Parsons

Principal Investigator: Dr. Ian Tibbetts

2019.10

Abstract. The increasingly threatened coral reef system sustains the marine habitats by modulating a great diversity of organisms, and supports human society by their productivity and aesthetic values. Since corals are routinely killed by a variety of natural mortality sources even in the absence of major perturbation, a study on coral population structure on the less-disturbed Heron Reef is essential to understand the background mortality and recruitment rate and to allow better assessment of the health status of a reef. Our study surveyed healthy Acropora and Pocillopora colonies on the reef flat of Heron Reef, and found strong relationship between their distinct recruitment strategies, spawning and brooding, and their population structure. Significantly more Pocillopora (75.4%) than Acropora (24.6%) communities were found in the surveyed area, indicating different recruitment success rate due to their contrasting reproduction mechanism. Additionally, local variation in the size-frequency distribution of coral colonies revealed strong disturbances from severe tropical cyclones and varying growth conditions. Alarmingly higher frequency of tropical cyclones in the current century can potentially disrupt structural complexity of coral reefs and lowers the biodiversity of coral reef habitat.

Additional Keywords: coral reef survey, population structure, life histories, coral reproduction, scleractinian corals

Fieldwork
Fieldwork

Measuring coral sizes at Heron Island

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M.frontalis
M.frontalis

Broad-fronted mangrove crab

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Pocillopora
Pocillopora

Healthy coral

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Fieldwork
Fieldwork

Measuring coral sizes at Heron Island

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