Heterochrony in Two Sculpins of the Genus Psychrolutes: Growth Outpaces Skeletal Development in an Apex Predator among Ichthyoplankton

In fishes, development is a plastic process, even among closely related, sympatric taxa. Selective pressure manifests itself through morphological divergences in early life. Of particular interest in the present study is heterochrony: alterations in developmental rates of change. Examinations of skeletal development in the Soft Sculpin, Psychrolutes sigalutes, and Tadpole Sculpin, Psychrolutes paradoxus, revealed marked developmental differences between species. While P. paradoxus quickly develops an adult skeleton and settles in adult habitat, P. sigalutes undergoes a remarkably protracted transformation (the transition from larval to juvenile characters) over a period of three to four months. It reaches sizes in excess of 40 mm (75% of adult size) before settling. While the earliest phases of life focus on the development of feeding elements and a transition from larval/anguilliform locomotion to caudal/subcarangiform propulsion, the two species diverge quickly in postflexion development. With the exception of feeding elements, the skeleton of P. sigalutes remains largely incomplete to sizes beyond 25 mm in length. By shunting resources typically dedicated to skeletal growth towards the development of mass alone, P. sigalutes quickly grows larger than any cohabitant zooplankton. Hiding by the day and hunting by night, P. sigalutes relies on its mass and inertia to chase down smaller prey hampered by the viscosity of water. The seasonality of P. sigalutes gives it ample time to grow before the majority of larval fishes hatch. Psychrolutes sigalutes may be a specialized ichthyoplankton predator, responsible for the earlier settlement times of cohabitant larvae.