Influence of substrate orientation on feeding kinematics and performance of algae grazing Lake Malawi cichlid fishes

Abstract

Lake Malawi cichlids have been studied extensively in an effort to elucidate the mechanisms underlying their adaptive radiation. Both habitat partitioning and trophic specialization have been suggested to be critical ecological axes underlying the exceptional diversification of these fishes, but the mechanisms facilitating this divergence are often unclear. For instance, in the rock-dwelling mbuna of Lake Malawi, coexistence is likely tightly linked to how and where species feed on the algae coating all the surfaces of the rocky reefs they exclusively inhabit. Yet, although mbuna species often preferentially graze from particular substrate orientations, we understand very little about how substrate orientation influences feeding kinematics or feeding rates in any group of organisms. Therefore, for three species of mbuna, we quantified feeding kinematics and inferred the rates that algae could be ingested on substrates that mimicked the top, side and bottom of the algae-covered boulders these species utilize in Lake Malawi. A number of differences in feeding kinematics were found among species, and several of the kinematic variables were found to differ even within species when the fish grazed from different surface orientations. However, despite their preferences for particular microhabitats, we found no evidence for clear trade-offs in the rates that the three species were inferred to be able to obtain algae from different substrate orientations. Nevertheless, our results indicate microhabitat divergence linked to differences in feeding kinematics could have played a role in the origin and maintenance of the vast diversity of co-occurring Lake Malawi mbuna species.

Feeding ecology underlies the evolution of cichlid jaw mobility

“…Future work might seek to understand the indirect effects of body shape variation on feeding kinematics and the evolutionary trade‐offs that result. Another area of interest is intraspecific variation in feeding motions, whether due to ontogeny (Cook ), diet (Liem ), or the physical environment (Rupp and Hulsey ). In these types of studies or others, we believe that further development of the methods used in this article may lead to breakthroughs in our understanding of evolution and diversity in functional systems.…”

Dissecting a potential spandrel of adaptive radiation: Body depth and pectoral fin ecomorphology coevolve in Lake Malawi cichlid fishes

“…Deeper bodied Malawi cichlids generally have greater pectoral girdle heights, and this greater pectoral girdle height is also associated with larger pectoral fins. Importantly, Malawi pectoral fin divergence is related to ecological divergence since fish that feed from the benthos generally have larger fin areas and muscles and use their pectoral fins intensively when feeding from the substrate (Hulsey et al, ; Rupp & Hulsey, ). Although there are undoubtedly other internal structures that contribute to body depth, the ecological opportunities and selective environments operating on the mechanical and hydrodynamic forces influencing pectoral fin diversity are likely a major determinant of body depth divergence in Malawi cichlids.…”

“…Deeper bodied Malawi cichlids generally have greater pectoral girdle heights, and this greater pectoral girdle height is also associated with larger pectoral fins. Importantly, Malawi pectoral fin divergence is related to ecological divergence since fish that feed from the benthos generally have larger fin areas and muscles and use their pectoral fins intensively when feeding from the substrate (Hulsey et al, 2013;Rupp & Hulsey, 2014 There are several potential reasons for the observed evolutionary correlation between body depth and pectoral fin ecomorphology. As seems to be the most common inference in the literature, body depth could almost always be selected for directly and provide a performance advantage for the organism that is independent of its association with pectoral fin ecomorphology.…”

“…Both body depth and pectoral fin morphology could commonly influence critical aspects of organismal diversification such as habitat specialization (Geerlink, 1983;Gerstner, 1999;Fulton et al, 2001;Higham, 2007aHigham, , 2007bTobler et al 2008;Weese et al, 2012;Hulsey et al, 2013;Colombo et al, 2016), trophic convergence (Collar, Wainwright, & Alfaro, 2008;Krabbenhoft et al, 2009;Ruber & Adams, 2001;Rupp & Hulsey, 2014), and speciation (Elmer et al, 2010;Hendry & Taylor, 2004;Husemann et al, 2017;Pfaender et al, 2010). What also seems likely is that each trait individually influences a number of both independent and correlated behaviors across species that link morphology to species interactions (Brönmark & Miner, 1992;Bakker & Mundwiler, 1999;Hechter, Moodie, & Moodie, 2000;Wainwright et al, 2002;Pigliucci, 2003;Domenici et al, 2008;Blob et al 2010;Monteiro & Nogueira, 2010;Head et al, 2013;Price et al, 2015).…”

“…Critically, in both Malawi and Tanganyikan cichlids, larger pectoral fin muscles and fin areas are correlated and are convergently associated with feeding from the substrate as opposed to feeding in the water column (Colombo, Indermaur, Meyer, & Salzburger, 2016;Hulsey et al, 2013). Additionally, it has been experimentally shown that the number of pectoral fin beats closely tracks the number of bites Malawi cichlids take when scraping algae (Rupp & Hulsey, 2014). If body depth changes closely track the functional morphological divergence in cichlid pectoral fin morphology that has previously been shown to play a role in both trophic and habitat divergence in Malawi, this would be consistent with body depth arising merely as a constructional by-product of adaptive pectoral fin divergence (Barel, 1983(Barel, , 1984Hulsey et al, 2007).…”