Background

From an anthropocentric perspective, coral reefs cover a variety of important ecosystem services such as coastline protection from the wave energy, provisioning of nutritive resources and economic gains from tourism (Ferrario et al. 2014; Moberg and Folke 1999; Spalding et al. 2017). However, the most important role played by this marine habitat is its ecological one. Despite representing only 0.1-0.5% of the ocean floor, coral reefs are in fact one of the most biologically diverse ecosystems on earth (Fisher et al. 2015; Moberg and Folke 1999) offering vital resources in terms of habitat, shelter and nutrition to numerous fish species.

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Because of this diversity, one can be easily intrigued by how species composition and abundances vary along the reef. There is no such thing as always encountering the same community and numbers. This begs the question of which factors better explain the species and family patterns along the reef. One of these potential factors was suggested to be coral reef complexity (Gratwicke and Speight 2005). Complexity means something intricate formed by many different connected parts. Therefore, to describe coral reef complexity, scientists have used different variables such as topographic rugosity, height of the substratum, substratum diversity, variety of refuges and so on (Gratwicke and Speight 2005). These variables have acquired a great interest, since hard corals (i.e., the Scleractinians), act as the main structural support of the reef with their calcareous skeletons (Boström-Einarsson et al. 2020). In fact the variables which were recently found best explaining the link between coral reef complexity and fish diversity/abundances, are coral rugosity and variety of growth forms (Gratwicke and Speight 2005). The hypothesis is that more complex reefs might support greater numbers of fish species at higher abundances because of a greater variety of available resources in terms of habitat, shelter, nutrition and so on. It was in fact observed that the loss of live coral causes the loss of structural complexity (Alvarez-Filip et al. 2009, 2011) which leads to a decline in coral-associated fish biodiversity (Graham et al. 2009).

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However, fish species diversity and abundances are not only affected by biotic factors but also by abiotic ones. Scientists found the depth of the water column to be one of the most important factors affecting reef species distribution. As a matter of fact different species might be restricted to different depths and ranges leading to changes in species composition along the gradient (Macdonald et al. 2016). And this is the same for corals since, among other requirements, different species require specific temperatures and levels of light penetration which depends on the water depth (Gratwicke and Speight 2005). Hence, studies have found a tendency for fish abundances and diversity to decrease with an increasing depth (Macdonald et al. 2016; Jankowski et al. 2015).

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Although these are only two of the potential factors playing a role in determining reef fish distribution, these combined suggest that shallow reef areas are those inhabiting a greater fish diversity at higher abundances (Gratwicke and Speight 2005). Such knowledge is important as these are also those areas most affected by some of the main threats that coral reefs are facing (Bongaerts et al. 2010; Bongaerts and Smith 2019; Gratwicke and Speight 2005). Because of that, we aimed to investigate if these trends are also encountered in the largest bay found in the Quesier area in the Red Sea. However, given the amount of time we were able to dedicate to the study, we decided to only focus on how coral complexity in terms of rugosity and number of growth forms affects fish families' diversity and abundances.