Unusual morphology of Thysanoessa spinifera Holmes (Crustacea: Euphausiacea) at the Oregon coast, USA

Abstract

We report an unusual morph of the euphausiid Thysanoessa spinifera Holmes, 1900 collected off the Oregon coast, USA (June 2001). The organism is a juvenile with an unusual morphology, distinct from the original species description for juvenile and adults, with no rostrum and the presence of a small forwardly directed keel on the dorsal part of the cephalothorax. We define the distinction between intraspecific morphological variability at (1) population level (ecophenotype), and (2) individual variability in the natural gradient of morphology of euphausiids within the same population. It is unlikely that this specimen represents a new species of euphausiid, but has features of an unusual morphology as part of an extreme case of individual variability in T. spinifera or possibly an hybrid of T. spinifera and Euphausia pacifica.