Methods for Fish Biology, 2nd edition

Chapter 6: Morphological Descriptions and Taxonomy

Diego F. B. Vaz, Christopher M. Martinez, Sarah T. Friedman, and Pedro P. Rizzato

doi: https://doi.org/10.47886/9781934874615.ch6

Morphology, the study of biological form, involves examination of structure, shape, color, and all other physical attributes of organisms. It can be used to describe the features within an individual organism but also provides useful information about variation among populations, species, families, and higher taxonomic levels. The ability to evaluate morphological variation, either descriptively or quantitatively, is fundamental to many fields of biology, including physiology, biomechanics, developmental biology, and systematics.

The morphology of fishes varies enormously, and virtually any feature can be used as a diagnostic character, helping us to distinguish one taxon (species, genus, or any other taxonomic group) from another. However, in order to reliably identify and propose diagnostic characters, it is crucial to differentiate taxonomically meaningful morphological differences from other forms of variation. Grande (2004) and Hilton and Bemis (2012) characterize four main types of morphological variation: ontogenetic, variation that occurs over development within an individual; sexual dimorphism, differences between males and females within the same species; individual, variation between representatives of a single species (i.e., at the population level); and last, phylogenetic (or taxonomic), which consists of morphological variation characterizing differences among species (or other taxonomic levels of interest). Ontogenetic, dimorphic, and individual variation in morphology all occur within the same species and therefore need to be carefully considered when seeking characters that provide distinguishing features at the species level or higher.

Traditionally, morphology has played an important role in taxonomy, the science of classifying living organisms. The main focus of taxonomy is to identify differences among related taxa and to assign characters that facilitate their recognition. As suggested above, this process is based on evolutionary relationships among organisms, and it is the task of systematics to unravel these relationships. Clearly, taxonomy and phylogenetic systematics are tightly linked and overlap broadly in scope. Early on, taxonomic characters were based mostly on morphology, but the incorporation of molecular data has driven much of the progress in systematics in recent years (Betancur-R. et al. 2013, 2017; Near et al. 2013; Arcila and Ortí 2022, Chapter 5, this volume). The limitations of morphology for systematics arise in cases like cryptic species, which are difficult to distinguish based solely on morphological characters. Molecular approaches have proven very useful for identification in instances like these (Tornabene et al. 2016). Nonetheless, morphology maintains an important role in taxonomy, and as you will see in this chapter, also provides vital context to investigate the ecological and lifestyle diversity that we see in fishes.