Description, external morphology, and natural history observations of Nebalia hessleri, new species (Phyllocarida: Leptostraca), from southern California, with a key to the extant families and genera of the Leptostraca

A new and relatively large species of leptostracan crustacean, Nebalia hessleri, is described from enriched sediments and detrital mats off southern California. The new species is characterized by its size, possession of ''normal'' (versus lobed) eyes, rectangular and unpaired subrostral keel, acute dentition of the posterior pleonite borders, and caudal furca approximately twice the length of the telson. Clark's Nebalia pugettensis (Clark, 1932) is herein declared a nomen nudum. The new species differs from specimens at Friday Harbor, Puget Sound, Washington, in the form of the epimeron of the fourth pleonite, the dentition along the posterior border of the fifth through seventh pleonites, the relative length of the telson and caudal furca, and size. Coloration may also serve to distinguish N. hessleri from other species if egg-bearing females are available; eggs of N. hessleri are cream or gold colored. The new species differs from a currently unnamed sympatric species that occurs in adjacent sand flats (Vetter, in press) primarily in the morphology of the first antenna, which is greatly reduced in the sand-flat species, and the eye, which has unique dorsal and ventral corneal protrusions in the sand-flat species. Selected aspects of the external morphology of the new species are illustrated via scanning electron microscopy, highlighting a previously unappreciated diversity of spines and setal types. Based on these photographs, some limbs are suggested as having sensory functions. Selected features of the Friday Harbor specimens also are illustrated via SEM. Limited notes on feeding behavior and oxygen level tolerances are provided, based on preliminary laboratory observations. Finally, we include a morphology-based key to identification of the currently recognized families and genera of the Leptostraca.