Testing Historical Hypotheses of Morphological Change: Biomechanical Decoupling in Loricarioid Catfishes

Abstract

The "decoupling hypothesis" has been proposed as a mechanistic basis for the evolution of novel structure and function. Decoupling derives from the release of functional constraints via loss of linkages and/or repetition of individual elements as redundant design components, followed by specialization of one or more elements. Examples of apomorphic decoupling have been suggested for several groups of organisms, however there have been few empirical tests of explicit statements concerning functional and morphological consequences of decoupling. Using the loricarioid catfishes, we tested one particular consequence of decoupling, the prediction that clades possessing decoupled systems having increased biomechanical complexity will exhibit greater morphological variability of associated structures than outgroups having no such decoupled systems. Morphometric procedures based on interlandmark distances were used to quantify morphological variance at three levels of design at successive nodes in the loricarioid cladogram. Additional landmark-based procedures were used to localize major patterns of shape change between clades. We report significantly greater within-group morphometric variance at all three morphological levels in those lineages associated with decoupling events, confirming our predictions under the decoupling hypothesis. Two of 12 comparisons, however, yielded significant variance effects where none were predicted. Localization of the major patterns of shape change suggests that disassociation between morphological and functional evolution may contribute to the lack of fit between variance predictions and decoupling in these two comparisons.

Biting disrupts integration to spur skull evolution in eels

“…A number of evolutionary innovations are thought to weaken functional integration among structures. These include the emergence of redundant function in non-overlapping sets of structures 15,16 , structural duplication [17][18][19][20] , mechanical decoupling 18,19,21 and, similar to what we describe here, behavioural transition (also referred to as functional transition), which is a shift in one or more of the functions executed to meet a biological demand. Each of these innovations can break up ancestral functional relationships among structures and permit new trait combinations to arise without disrupting the ability to meet a biological demand.…”

To feed or to breed: morphological constraints of mouthbrooding in coral reef cardinalfishes

“…Organismal form is often related to its function and may be assumed to reflect the adaptive significance of the structure [1]. Over the past few decades, integrative approaches to organismal form and function have greatly enhanced our understanding of ecological and evolutionary relationships among species and assemblages [2 -5].…”

“…Functional decoupling-the duplication and redundancy of individual design elements-has contributed to increased diversity in many groups through the release of functional constraints [1,3,16]. Perhaps the best-documented example of functional decoupling is the development of a pharyngeal jaw apparatus in several fish families [2,17,18].…”

Phylogenetic signal and major ecological shifts in the ecomorphological structure of stream fish in two river basins in Brazil

“…There has been little discussion regarding the role of key traits in the diversification of Neotropical fishes, but certain changes (e.g., morphological changes in Loricariidae to algae scraping) underlie the exceptional diversity of the most species-rich taxa (Schaefer & Lauder, 1996;Albert et al, 2011). In this study, the most important ecomorphological divergence (in Siluriformes/Characiformes) established a primary division in both habitat and the use of food resources within the stream habitats.…”

“…For instance, the evolutionary success of loricariids at the continental scale may be largely due to the central role of algivory (Schaefer & Lauder, 1996;Albert et al, 2011). Algivory involves a highly derived set of anatomical and physiological specializations, such as in the oral and pharyngeal jaws, digestive and respiratory organs, and life-history and behavioral traits .…”