Abstract

    The marine gastropod genus Stosicia is represented by two Recent species in the tropical western  Atlantic Ocean. The conchology, taxonomy, nomenclature, zoogeography, and evolution of S. aberrans (C. B. Adams, 1850) and S. houbricki Sleurs, 1996 are discussed.  Several new records for these two taxa, including the first U. S. occurrence for the latter, are reported. Key words: Stocisia, western Atlantic, Rissoidae, Florida.

Introduction

    In April, 1850 Charles Baker Adams (1814-1853) named Rissoa aberrans based on a shell collected in Jamaica, possibly by himself, during a visit in the winter of 1848-49, just five years before he died of yellow fever on his third Caribbean field trip.  It appears that Prof. Adams, writing from his desk at Amherst College, selected the trivial name because of a conchological character atypical for the genus in which he placed the taxon, an anterior (siphonal) canal in the aperture.  He wrote "This species connects the genus with those Cerithia, in which the canal is reduced to a notch."  In fact this feature calls to mind Cerithium lutosum Menke, 1828, which at first glance might be mistaken for a giant specimen of Adams' species.  A century later Clench and Turner (1950) illustrated the holotype (and, for that matter, Adams' species) for the first time [fig. 1]. The shell is worn and is missing its apex.

    For a century and a third this four to six mm white shell was occasionally mentioned in the conchological literature, where it was moved from Rissoa to Alvania to Rissoina by authors.  Ponder (1984) saw that it was related to a group of rissoids which was represented principally in the Indo-West-Pacific in the Recent and in geologic history from the lower Miocene of Europe (type species Rissoa buccinalis Grateloup, 1828) and the central Pacific, Stosicia Brusina, 1878.  He placed it with the genera Rissoina and Zebina in the Rissoininae, a subfamily of the Rissoidae.  Stocisia aberrans (C. B. Adams, 1850) was said to be the only member of its genus surviving in the Atlantic Ocean. Unfortunately he appears to have lacked intact material as he strongly implied the species had a paucispiral protoconch (vide infra).

    Sleurs (1996) reviewed the Recent species of Stosicia and included 16 species including five previously unknown taxa, two of which he named.  A second Caribbean species, reported only from Belize, was added to the roster, S. houbricki - all the others were from the Indo-West-Pacific or adjacent parts of Australia.

    In the late 1990's I recognized a single shell in a lot of Stosicia aberrans collected in Broward Co., Florida as that of a distinctive congener [fig. 3, right]. In ignorance of Sleurs' work, in May, 2003 I began to prepare a report describing this new taxon, and the present study is the outcome of that effort.

Materials and Methods

    A search of the western Atlantic molluscan literature, including electronic media, was undertaken.  Dry material from my personal collection, that of John Chesler (Plantation, FL), Dr. Emilio García (Lafayette, LA), Peggy Williams (Tallevast, FL), and the Florida Museum of Natural History (FLMNH) was studied using a Swift Stereo eighty microscope at 10 to 40 X.  Measurements were performed using an eyepiece gauge calibrated to a stage micrometer (nearest 0.01 mm). Whorl counts were performed according to Pilsbry (1939, p. xi). Records from the Academy of Natural Sciences Philadelphia (ANSP) were confirmed by examination of digital images provided by Dr. Gary Rosenberg. Digital images of Dania specimens of Stosicia aberrans and S. houbricki in my collection were made with a Kodak DC290 digital camera using 3x and 7x diopters. The images of Rissoa aberrans, Stossichia serrei, and Stosicia houbricki (paratype) were taken from the original descriptions using a HP 5370C ScanJet flatbed scanner. The pair of S. houbricki from Roatan in my collection were scanned directly with the same device. The image of Rissoa corilea was provided by Dr. Rosenberg, who scanned the original figure; Colin Redfern likewise provided the type figure of Stosicia fernandesgarcesi.  All images were edited with JASC Paint Shop Pro software by Bill Frank.

Results

Stosicia aberrans (C. B. Adams, 1850)
        Rissoa aberrans C. B. Adams, 1850 [fig. 1]
        Rissoa corilea "d'Orbigny" G. B. Sowerby II, 1876 [fig. 4]
        Stossichia Serrei Bavay, 1922 [fig. 2]

Description

    Protoconch with acuminate apex, conical, 2.25 whorls, smooth; teleoconch about six whorls, sculptured with 18-21 (median 19) axial ribs on the penultimate whorl, strong from initiation, weakening on the anterior half of the body whorl; these are crossed by slightly narrower spiral ridges which form rather regular nodules at the intersections; three on the spire whorls, faint initially, stronger on following whorls, usually three (but up to five) on the penultimate, and eight to 12 (median eleven) on the body whorl, faint secondary spirals not easily seen.  Aperture ovate with roundly angulate columellar aspect, thickening anteriorly near the narrow, deep, and short anterior channel; no posterior canal; labrum with three evenly-spaced denticles arising a short distance within, not reaching the labral margin, which is slightly reflected.  Immediately before the labrum is a broad, thick, but diffuse varix bearing the spiral ridges. Color, snow white to light brown; solitary live-collected shell is homogeneous light chocolate brown; fresher shells somewhat glistening. Size, based only on intact adult shells (n=56): 3.98 to 6.05 mm [but see Grenada record below]; L/W ratio variable; mean: 2.24.