The effects of cyclic thermal stress on oviparous shark development

Abstract

Globally, shark species have undergone a dramatic decrease in abundance due to the environmental degradation caused by anthropogenic activities and global warming. There are limited studies on the effects of global warming on embryological development of threatened tropical sharks. This study investigated the impact of cyclic thermal stress on the embryonic development of the grey carpet shark (Chiloscyllium punctatum), as well as the ability of grey carpet shark neonates to tolerate a hypoxic challenge following exposure to thermal stress. Detailed characterisation of all developmental stages observable through the egg case to the naked eye formed the basis for the investigation of thermal stress. The data clearly showed that embryos exposed to 28 °C in a cyclic manner, displayed a delay in the appearance of three developmental milestones: (i) the time pigmentation took to reach the pectoral fins; (ii) widening of the trunk region; and (iii) the internalization of yolk. The time to hatching was increased by 10 days and neonates were significantly shorter in total body length in response to cyclic thermal exposure (28 °C) compared to control egg cases only exposed to 24 °C. No morphometric changes were observed on a separate experiment on neonates reared at 24°C, then exposed for five days either to intermittent thermal stress at 32°C or control conditions 24°C. This short exposure to cyclic thermal stress did not alter their ability to withstand a hypoxic challenge at 30 days post-hatching. During the hypoxic challenge, neonates were observed to use ventilation depression and diminished their use of buccal musculature. The strongest morphometric predictor of the time to loss of righting reflex in response to the hypoxic challenge was the ratio of the span of the first dorsal fin to the total length of the neonate, which could reduce the amount of energy needed during escape attempts and exploratory behaviours observed during the hypoxia exposure. My research sheds light on the potential survival of this near threatened tropical shark. Future studies should explore the impact of higher cyclic temperatures on grey carpet shark embryo development to expand upon the understanding of the impact of cyclic thermal stress on shark embryos.