Morphological development of Corydoras aff. paleatus (Siluriformes, Callichthyidae) and correlation with the emergence of motor and social behaviors

Abstract

ABSTRACT. Here we examine major anatomical characteristics of Corydoras aff. paleatus (Jenyns, 1842) post-hatching development, in parallel with its neurobehavioral evolution. Eleutheroembryonic phase, 4.3-8.8 days post-fertilization (dpf); 4.3-6.4 mm standard length (SL) encompasses from hatching to transition to exogenous feeding. Protopterygiolarval phase (8.9-10.9 dpf; 6.5-6.7 mm SL) goes from feeding transition to the commencement of unpaired fin differentiation, which marks the start of pterygiolarval phase (11-33 dpf; 6.8-10.7 mm SL) defined by appearance of lepidotrichia in the dorsal part of the median finfold. This phase ends with the full detachment and differentiation of unpaired fins, events signaling the commencement of the juvenile period (34-60 dpf; 10.8-18.0 mm SL). Eleutheroembryonic phase focuses on hiding and differentiation of mechanosensory, chemosensory and central neural systems, crucial for supplying the larval period with efficient escape and nutrient detection-capture neurocircuits. Protopterygiolarval priorities include visual development and respiratory, digestive and hydrodynamic efficiencies. Pterygiolarval priorities change towards higher swimming efficacy, including carangiform and vertical swimming, necessary for the high social interaction typical of this species. At the end of the protopterygiolarval phase, simple resting and foraging aggregations are seen. Resting and foraging shoals grow in complexity and participant number during pterygiolarval phase, but particularly during juvenile period.

The development of tactile social interactions in Corydoras aeneus larvae

“…The underlying neural command appears to be ballistic, and the reflex is unaffected by sensory information once it is initiated (Eaton and Emberley, 1991). The c-start is particularly important for larval fish, and larvae and adults of a related species, Corydoras paleateus, exhibit well-developed Mauthner cells and circuits, which trigger the c-start response (Rodríguez-Ithurralde et al, 2014). We have also observed adult Bronze Cory catfish undergoing a c-start response while responding to a threat stimulus (Riley et al, 2019a).…”

“…Morphological constraints seem unlikely, as there were a few c-starts in the adult fish stimulated in this study, as well as in adult fish in our previous work (Riley et al, 2019). It also seems unlikely that larvae would adopt a freeze threat response before the development of prominent anterior pectoral fin rays or armoured scutes (which occurs at >22 mm in Corydoras arcuatus, a related species of similar size (Sire, 1993)) because they have very little protection from predation (Rodríguez-Ithurralde et al, 2014) and would rely on camouflage alone. Therefore, it seems that the most likely explanation for the change in response to tactile stimulation is that older larvae were less likely to perceive tactile stimulation as a threat.…”

“…Many behaviours necessary for group coordination, including social attraction (which leads to group cohesion), develop in groups alongside physical development in individuals. In the catfish Corydoras paleatus, the development of group cohesion and aggregative behaviours is highly correlated to anatomical developmental stage (Rodríguez-Ithurralde et al, 2014). The development of cohesive group behaviour with age has also been documented in zebrafish (Danio rerio), which begin to form increasingly cohesive shoals as individuals age (Buske & Gerlai, 2011).…”

“…The fact that Bronze Cory catfish respond to potential threats with a c-start response, but also interact tactilely with conspecifics without a c-start response, suggests that Bronze Cory catfish may develop a tolerance to tactile stimulation during development. Developing this tolerance would allow larvae to respond to tactile stimulation as a potential threat during early development (while larvae are most vulnerable and have not developed social cohesion) and eventually interact tactilely with conspecifics later in development as social aggregation behaviours develop (Buske & Gerlai, 2011;Rodríguez-Ithurralde et al, 2014).…”

The role of tactile interactions in flight responses in the Bronze Cory catfish (Corydoras aeneus)

“…A scoring protocol was developed by RJR and ERG, and both scorers cross‐scored three videos to ensure consistency; all subsequent scoring was performed by RJR. Body lengths are a common measure of cohesion in fish aggregations (Katz, Tunstrøm, Ioannou, Huepe, & Couzin, ; Rodríguez‐Ithurralde, del Puerto, & Fernández‐Bornia, ) and we chose two body lengths as our measure of cohesion because this is a distance at which individuals can readily nudge each other.…”