Spatial distribution of invertebrates

Spatial distribution of invertebrates in

Red Sea coral reefs

Maxime Clerc, Alvaro Sobrino & Arnaud Strubin

Introduction

Coral reefs provide a very heterogeneous habitat that can host a vast array of species, from fishes to invertebrates. The complex coral topography leads to variable light exposure and bares cavities that offer good shelters for marine species.
Marine invertebrates have been little studied compared to fishes, even though they play a key role in the ecosystem. They can be detritivorous and necrophagous, which means they clean the water by filtering it, and feed on dead animals.

The aim of our project was to describe the spatial distribution of coral reef invertebrates, and to start building a database that will be completed over time by new teams of students.

Methods

Data collection

We recorded the spatial distribution of invertebrates every 2 meters along the edge of the coral reef using waterproof slates. Every time we recorded whether the animals were (i) in the Light or in the Shade, and (ii) Sheltered in the coral or visually Exposed.
For each point, we only took data on one individual of each taxon of invertebrates, which means we could not take data on two clams at the same point. The observations were done in two separate field sites in Egypt: Dahab and Ras-Mohamed.

Arnaud Strubin collecting data

Statistics

We used chi-square tests to assess whether a taxon preferred light or shade, and to be exposed or sheltered. These tests were meant to clarify the results we obtained in Fig 2.

Results & Discussion

We can see in Table 1 that the Dahab site has slightly more diversity than Ras-Mohammed. However, the general biodiversity is similar. The difference in the observed diversity could be easily explained by the fact that we spent more time exploring Dahab than Ras-Mohammed.

In Figure 1, we see that ‘Light/Exposed’ is the more represented condition, followed by ‘Light/Shelter’ and ‘Shade/Shelter’, the least represented being the ‘Shade/Exposed’ condition.
It is again easily explained by the fact that ‘Light/Shelter’ condition for an animal is the easiest to notice. The two middle conditions, ‘Light/Shade’ and ‘Shade/Shelter’ mean that the animal is concealed either or both in the shade and in a shelter, making them hard to see. The ‘Shade/Exposed’ condition is likely to be the least represented because it is a quite paradoxal condition: it is hard to find a place which is both exposed and in the shade.

In Figure 2, we can see that clams seem to have a high preference for exposed habitats (p=0.0005), and a moderate preference for light (p=0.041). This could be because they can protect themselves against predators, and do not need shelters, because of their shells. The light preference coud be explained by the need for light of the symbiotic zooxanthella.

Anemones seem to have a preference for light, however, this is not significant (p=0.28) This apparent preference could make sense because they also have symbiotic zooxanthella.

Brittle stars had a very strong preference (p=0.0046) for sheltered spots, as we never found any exposed. Whether the habitat was lit or not seems not to have any effect on them. They prefer shelters because they are a territorial species, and they defend their crevices against other brittle stars.

Sea urchins showed a strong preference for sheltered patches (p=.0001). This can be explained by the fact that these echinoderms dig holes in corals when they feed on algae.

Christmas tree worms did not yield any significant results with the chi-square test, and they seem to be found everywhere. This could be explained by their fast and efficient hiding mechanism when something approaches too close, making them able to live everywhere.