Crab or lobster? Mental principles underlying the categorization of crustaceans by biology experts and non-experts

Object categorization is an important research topic not only in the field of biology but also in psychology. So far, however, the mental principles guiding categorization in biology have not been systematically investigated from the viewpoint of psychology. We focused our study on the shape-based categorization processes of decapod crustaceans, incorporating the perspectives of both disciplines in order to shed more light on this phenomenon. Thus, in parallel to the investigation of the principles of morphology-based categorization of decapods, we examined the mental representations of the concepts of crustaceans and the development of these representations in human memory. For this purpose, we created a set of standardized and modularized images of decapod crustaceans consisting of a series of morphed exemplars of crabs and lobsters, which served as a stimulus basis for investigating category judgments. The images were ordered linearly on a continuum changing from a 'crab' to a 'lobster' pole by systematically manipulating certain categorization-relevant features like the shape of the carapace and the angle of the claws. This precisely controlled material allowed us to systematically investigate the influence of qualitative and quantitative changes in specific features of decapods on people's category judgments. Moreover, we also analyzed how different levels of biological expertise and prior knowledge influence the aforementioned processes. We conducted a psychological experiment by using a cognitive performance test with groups of biology students at different stages of expertise as well as naïve non-experts. Our results show that category judgments in the case of crustaceans specifically depend on the morphological, shape-based form information in the presented image but not on the degree of prior knowledge about the to-be-categorized material. Thus, the form information is robust and dominant across different levels of expertise. These current findings allow conclusions about the nature of categorization processes for decapod crustaceans and the impact of perception-guided formation of mental concepts in human memory. This underlines the need to combine methodological and conceptual knowledge from biology and psychology to understand the principles of categorization as a research topic in scientific investigations.