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Ecology

How Coral Size Influences Fish Size

Article: Wehrberger F and Herler J (2014) Microhabitat characteristics influence shape and size of coral-associated fishes. Marine Ecology Progress Series 500:203-214. Doi: 10.3354/meps10689

Introduction

Coral reefs are a complex network of branching structures, often times constraining the inhabitants that live there. Several fish species find a host coral and spend their lives residing inside that colony’s network of interlocking branches, but several constraints of the coral limit the fish’s morphological adaptations. Things such as coral size, growth, and shape all effect the path a fish has to take in order to avoid predators and search for food. For instance, a fish living in a 3-dimensional coral colony will evolve to have a laterally compressed body shape (Fig. 1) in order to nimbly squeeze through the coral’s maze of branches, but a fish living on the surface of a large flat coral will have a depressed body shape (Fig 2). Researchers from the University of Vienna, Austria set out to discover how the skeleton and body shape of a fish develop differently in different coral environments.

Fig. 1 Compressed body shape

Fig 2. Depressed body shape

 

 

 

 

 

Methods

Five distinct experiments were conducted in order to answer the question of how coral size affects fish morphology. Researchers set off for Dahab, Egypt and dove the reefs of northern Red Sea (Fig. 3) in order to take fish samples and coral measurements. Two species of gobies were selected, Gobiodon histrio and Gobiodn rivulatus, and their host corals, Acropora digitifera and Acropora gemmifera, were selected. The first experiment took a total of 64 coral colonies and measured them by taking casts of the corals. The casts could then be taken back to the lab so that measurements of each coral branch could be individually measured without harming the actual coral. A total of 93 juvenile and adult gobies (of both species) were examined and geometric morphometric analyses was performed in order to understand the different body shapes and skeletal growth patterns between adults and juveniles (Fig. 4).

The second experiment was done in order to understand how exactly coral size governs fish size and flexibility. An aquarium maze was set up using PVC pipes to mimic coral growth patterns. Sixty-one fish were then observed going through the maze in order to see how fast they swam through straight tunnels, as well as how sharp of a turn could they physically make.

Thirdly, a long-term observational study was conducted. Corals were measured, tagged, and then re-measured after 6 months. The same was done with fish, although changes in the fish’s social status (breeding pair or single adult) were also noted. And finally, fishes from large coral colonies were transposed to small coral colonies and vise versa. Differences in fish size before and after the transport were measured.

Fig 4. Position of 36 landmarks for geometric morphometric analysis to capture body shape differences in coral-associated gobiid fishes

Fig 3. Northern Red Sea where field work was conducted

 

 

Findings and Significance

As to be expected, field observations showed that larger coral colonies hosted larger fish and provided a more suitable home for paired fish species, rather than single adults. However, it was shown that the same fish species will indeed grow larger when hosted in a larger coral colony, meaning small corals limit the growth of fish. The size of corals isn’t the only factor that determines fish occupation and size, however. Fish tend to prefer taller coral structures as opposed to the shorter branching corals. This is most likely to avoid predation.

Strong competition for habitats may be the main reason the fish examined in this experiment evolved to have a very compressed body shape, for larger fish size equals higher rank in fish hierarchy. Fish length does not always help functionally (such as living in a tight coral locked home), however. There is certainly a trade-off between living in a tight coral colony, which has lower risk to predation, to living in a larger coral colony, where fish can grow to be larger, thus increasing their hierarchical rank.
Understanding factors governing coral size, fish size, and space preference are important to understanding how changes in coral composition will affect coral inhabitants. Many factors, such as coral bleaching, increased algal coverage, and disease, all contribute to the degradation of coral reefs. Altering the habitat structure of coral dwelling fish species will undoubtedly affect not only fish livelihood, but also their growth and movement patterns.

 

Picture Credit: Marsha Ward. “Compressed and Depressed fish body shapes” photo. Earth Education and Research: Testing Hypothesis 28 March 2014. <http://www.mbari.org/earth/mar_tech/EITS/ob_deep/od_back.html>

 

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