Friend or foe? Development of odour detection, differentiation and antipredator response in an embryonic elasmobranch

Young animals, especially those developing within eggs, are extremely vulnerable to predation risk given their immobility, small size and limited functionality of developing sensory systems. Embryos from a range of taxa can detect predator cues and use antipredator responses to reduce risk; however, little is known about this capacity in elasmobranchs, especially regarding olfactory detection. Embryonic Port Jackson sharks (Heterodontus portusjacksoni) were exposed to elasmobranch and teleost odours across key developmental stages to investigate their capacity to detect and discern between cues. Oxygen uptake rates were measured as a proxy for antipredator response and to estimate their energetic costs. Earlier developmental stages exhibited limited responses, regardless of the odour, likely associated with an undeveloped sensory system. However, older shark embryos depressed oxygen uptake rates (i.e. crypsis responses) when exposed to teleost cues, but showed little response to elasmobranch cues. In contrast, hatchlings exhibited minimal responses to teleost cues but a significant increase in oxygen uptake rates when exposed to elasmobranch odours, indicative of a general stress response in preparation for escape. Collectively, our data suggest that embryonic sharks can differentiate between odour cues and elicit various responses, but this ability is limited by their developing sensory capacity.