Spatial patchiness in the sea cucumber Pachythyone rubra in the California Channel Islands

Abstract

Pachythyone rubra is a small, direct-developing sea cucumber, with a limited geographic range in central and southern California. Surveys from 1996 to 1998 in the Santa Barbara Channel revealed spatial patchiness, with high densities immediately adjacent to low or zero densities in similar habitat. To investigate causes of this patchiness, I transplanted and followed survival and recruitment of P. rubra in sites that had high and low densities of P. rubra. This transplant experiment revealed that survival and recruitment of P. rubra vary spatially, along a gradient in environmental conditions, and that these differences may be caused by predation, sedimentation, and food supply. Laboratory predation experiments identified two predators: the lobster Panulirus interruptus and the sea star Pycnopodia helianthoides. Field observations and trials with freeze-dried P. rubra pellets suggested that fish do not eat P. rubra. Population growth of P. rubra was highest in the western region of the Channel Islands that receives cool, nutrient and phytoplankton-rich water suitable for P. rubra feeding and reproduction. Patchiness of P. rubra in this area may result from spatially variable, but intense, predation by P. helianthoides. The eastern region of the Channel Islands has warmer water with less nutrients and phytoplankton, which may be less suitable for P. rubra feeding and reproduction but lacks P. helianthoides, resulting in lower population growth but potentially more stable patches over space and time.