Coral architecture affects the habitat choice and form of associated gobiid fishes

Abstract

Gobiid fishes of the genus Gobiodon live in strong association with certain reef-building corals that vary considerably in size and architecture. These fishes hence are excellent model systems for studying evolutionary adaption to specific microhabitats. Using a sample of Gobiodon histrio and G. rivulatus and their most important host corals (Acropora digitifera and A. gemmifera) from the northern Red Sea, we assess (1) how corals that are occupied by gobies differ in their architecture from colonies that are not occupied and (2) how fish body shape is associated with the architecture of their host coral. Fish body shape was assessed by geometric morphometric techniques. Coral measurements included colony size, branch length (BL), and interbranch as well as branch tip distance of adjacent branches, for which we applied a new and non-destructive measurement technique based on casts of two-component epoxy resin. The most important factor influencing the occupation of corals was a BL of more than 5 cm. The distance between coral branches was clearly related to the width of the fishes and hence constrained overall fish size. G. histrio and G.rivulatus differ in adult body shape as well in their allometric development of lateral body compression, resulting in different maximum body sizes attainable in the restricted interbranch space of corals. The strong dependence of coral-associated fishes on large coral colonies with specific architectures increases the extinction risk of fishes within deteriorating coral reefs.

Microhabitat characteristics influence shape and size of coral-associated fishes

“…These 2 factors combined offer better protection against predation, also explaining the high occupation rate (up to 86%) of A. digitifera colonies in the investigation area (Schiemer et al 2009). Untersteggaber et al (2014) discovered that the mean BLs of corals occupied by adult fishes was greater (> 5 cm) than in unoccupied colonies. Although narrow-branched corals support a greater abundance of organisms (Vytopil & Willis 2001), IBD seems to limit maximum size and growth rate in fishes (Munday 2001).…”

“…To measure the IBD close to branch bases, a new and non-destructive measurement technique employing casts of 2-component epoxy resin (Reef Construct © ) was used (Untersteggaber et al 2014). Cylindrical pieces of the mixed resin were individually size-adjusted underwater and then inserted between 2 coral branches with strong stainless steel forceps with graded tips.…”

“…After the casts had hardened (~2 h), their width was measured on a fixed height (7 mm) with a digital sliding caliper to the closest 0.01 mm. The 7 mm height value equals half of the average head depth of adult specimens of Gobiodon, which is the widest part of the body and considered to be constrained most by interbranch distance (Untersteggaber et al 2014). Data were processed in Microsoft Excel 2010 for Windows.…”

“…Interbranch distance, defined as the distance between adjacent branches, represents the space where strictly coral-associated invertebrates and fishes spend most of their lives. This interbranch distance may differ among coral species and may change with colony size, and it is also a physical constraint for the body size and growth of associated organisms (Sebens 1987, Munday 2001, Vytopil & Willis 2001, Untersteggaber et al 2014. As coral-associated fishes are faced with numerous challenges while acquiring food and avoiding predation in their spatially constrained habitat, locomotive performance is crucial for their survival (Fulton 2010).…”

“…Among other Acropora species, they favor the species A. digitifera and A. gemmifera on shallow reef flats of the Gulf of Aqaba. Untersteggaber et al (2014) revealed that certain coral parameters such as branch length (BL) or interbranch distance (IBD) determine whether a colony becomes inhabited or not and affect maximum fish size. While BL was the most relevant factor for occupation, IBD is a constraint for the widest dimension of the body (= maximum head width).…”