Ecomorphology of the eyes and skull in zooplanktivorous labrid fishes

Abstract

Zooplanktivory is one of the most distinct trophic niches in coral reef fishes, and a number of skull traits are widely recognized as being adaptations for feeding in midwater on small planktonic prey. Previous studies have concluded that zooplanktivores have larger eyes for sharper visual acuity, reduced mouth structures to match small prey sizes, and longer gill rakers to help retain captured prey. We tested these three traditional hypotheses plus two novel adaptive hypotheses in labrids, a clade of very diverse coral reef fishes that show multiple independent evolutionary origins of zooplanktivory. Using phylogenetic comparative methods with a data set from 21 species, we failed to find larger eyes in three independent transitions to zooplanktivory. Instead, an impression of large eyes may be caused by a size reduction of the anterior facial region. However, two zooplanktivores (Clepticus parrae and Halichoeres pictus) possess several features interpreted as adaptations to zooplankton feeding, namely large lens diameters relative to eye axial length, round pupil shape, and long gill rakers. The third zooplanktivore in our analysis, Cirrhilabrus solorensis, lacks all above features. It remains unclear whether Cirrhilabrus shows optical specializations for capturing planktonic prey. Our results support the prediction that increased visual acuity is adaptive for zooplanktivory, but in labrids increases in eye size are apparently not part of the evolutionary response.

Ecomorphology and resource use by dominant species of tropical estuarine juvenile fishes

“…The literature has placed more emphasis on fish assemblages in lakes (Adite & Winemiller, 1997;Piorski et al, 2005), rivers (Willis et al, 2005), streams (Mazzoni et al, 2010) and reservoirs (Teixeira & Bennemann, 2007). As a result, some investigators have proposed a series of relationships between different morphometric measurements, to infer about resource use: body/caudal fin and locomotor movements for the search and capture of prey (Webb, 1984); mouth gape, length of the intestine tract and the orientation of the mouth together with the presence of barbels were significantly correlated with the size, the type and the vertical position of the food, respectively (Piet, 1998); between the shape of the otoliths and depth (Volpedo & Fuchs, 2010); larger eyes for sharper visual acuity; reduced mouth structures to match small prey sizes; and longer gill rakers to help retain captured prey in zooplanktivorous fishes (Schmitz & Wainwright, 2011). However, for coastal areas of northeastern Brazil, few studies that address ecomorphological predictions have been performed in teleosts (Gomes et al, 2003;Medeiros & Ramos, 2007;Piorski et al, 2007).…”

Diet and ecomorphological relationships of four cichlid species from the Cuiabá River basin

“…There is a close relationship between the food size and spacing of the gill rakers, because the function of the gill rakers is to help retain captured prey in zooplankton (Schmitz & Wainwright, 2011). Thus, the longer and closer together are the gill rakers, the smaller the food size, and this relationship was observed in C. australis.…”

“…The ecomorphological approach, with a fairly high predictive power, is very useful when a rapid ecological assessment is required (Hugueny & Pouilly, 1999). For fishes, the mouth opening is considered as one of the most important food intake characteristics, however other structures, including eye diameter and body height, head, and snout length (Ward-Campbell & Beamish, 2005) and gill rakers (Schmitz & Wainwright, 2011) can drive physical constraints on prey size. Another structure that has been studied is the pharyngeal jaws, which may impose a functional constraint on piscivory via pharyngeal gape in cichlids (Burress et al, 2015).…”

Adaptive radiation in labrid fishes: A central role for functional innovations during 65 My of relentless diversification

“…Like many other lineages of reef fishes, labrids have evolved specialized zooplankton feeding typically associated with high‐aspect ratio pectoral fins, swift swimming behavior (Wainwright et al. ), and an overall reduction in the size of feeding structures (Schmitz and Wainwright ). The labrichthyine wrasses contain the only origin of dedicated coral mucus feeding in labrids (Huertas and Bellwood , ), a relatively recent novel trophic niche among reef fishes (Fig.…”

“…In the case of the slingjaw wrasse, Epibulus, piscivory is associated with a major novelty in the jaw mechanism that gives this genus the most protrusible jaws known in ray-finned fishes (Westneat and Wainwright 1989). Like many other lineages of reef fishes, labrids have evolved specialized zooplankton feeding typically associated with high-aspect ratio pectoral fins, swift swimming behavior , and an overall reduction in the size of feeding structures (Schmitz and Wainwright 2011). The labrichthyine wrasses contain the only origin of dedicated coral mucus feeding in labrids (Huertas andBellwood 2017, 2018), a relatively recent novel trophic niche among reef fishes (Fig.…”