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: Stosicia, 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.

HONDURAS. Roatan Is., East side of entrance to Carib Bight. Abundant (with S. houbricki) as crabbed specimens under 10 to 20 cm of rubble. E. F. García! 12/93 [thirteen shells]. Seven shells in García Collection 13902; six shells in Lee collection.

Stosicia houbricki Sleurs, 1996 [fig. 6, paratype]

        Stosicia fernandesgarcesi Espinosa and Ortea, 2002 [fig. 5, holotype]

Description

    Protoconch blunt, globular, 1.25 whorls; teleoconch 4 to 5 whorls, teleoconch sculpture, coloration, and luster similar to S. aberrans but with fewer and sometimes less erect axial ribs (8 to 14; median 12 on the penultimate whorl), spirals regular and consistent in each specimen, but varying in number (four to six above the prevarical suture), elevation, and breadth from conspicuous to rather indistinct, making reticular sculpture less apparent in shells with the latter character [see figs. 3 right, 5, 6, 7]; usually more convex whorls, more impressed sutures (early sutures not narrowly channeled as in S. aberrans); labrum differs in lacking denticles, produced beyond the pre-labral varix - not reflected, and having posterior notch near the suture,. Pre-labral varix much narrower yet more expanded, well-defined, and abrupt at its origin, which appears demarcated by an axial crease. Size (n=20): 3.14 to 4.08 mm; L/W ratio variable; mean: 2.11.

    The synonymy of the treated taxa is generally straightforward, but Rissoa corilea "d'Orbigny" G. B. Sowerby II, 1876, here considered a synonym of Stosicia aberrans, warrants some review.  The type illustration [fig. 4] is accompanied by a scale line 5.8 mm. long, and Cuba is given as the habitat. Tryon (1887; pp. 363-364) considered Sowerby's name a simple misspelling of d'Orbigny's Rissoa caribaea and used d'Orbigny's figure (1842[?]; t. 11, bis fig. 32) to illustrate the two taxa. However, that action misidentifies Rissoa corilea. Although Sowerby's illustration and text feature the anterior canal, there is no such character in the type figure of Rissoa caribaea d'Orbigny, who gives (1842; pp. 21-22) the size of his shell as "2 millim." and mentions "deux carènes... bandes brun... Bouche ovale" which clearly relate to the species presently known as Simulamerelina caribaea (d'Orbigny, 1842), which is not even a congener [but see De Jong and Coomans, 1988 and Redfern, 2001 for differing interpretations at the species level]. Rosenberg (2003 et a priori) was the first to point out this peculiar synonymy.

    The taxonomic and zoogeographic relationships between these two Stosicia species and among all the congeners invite speculation as to the evolutionary scenario of their origin and deployment. Clearly S. aberrans and S. houbricki are quite closely related; the illustrations of these and other Stosicia in Sleurs (1996) attest to this propinquity.  It therefore seems likely that the two share a common ancestor.  I looked high and low for any fossil evidence of the presence of Stosicia (in any of its conceivable nomenclatorial iterations) in the western Atlantic but could find none.  Thus, it also seems likely that the ancestor was probably an immigrant from either the Eastern Atlantic or the Indo-West-Pacific in post-Miocene times, likely not long ago at all.  On analysis of protoconchs, if the immigrant ancestor were closer to R. aberrans, it would have planktotrophic larvae with the ability to disperse over large oceanic expanses unlike the case with S. houbricki with its inferred lecithotrophic (crawl-away) larvae.  What little we know of the ancestral forms in Stosicia (e.g., S. planaxoides Grateloup, 1838, a senior synonym of the type species; see Sleurs) suggests that they had multispiral protoconchs (planktonic larvae) like that of S. aberrans, yet many of the Recent taxa from the Indo-Pacific (and S. houbricki) have paucispiral protoconchs, which infer a lecithotrophic larval habit (crawl-away young).  Which came first in our evolutionary scenario?

    If we look at other groups in which sympatric pairs of closely-related marine snails having utterly different protoconchs much as do our two species, a few instances in the Caribbean fauna leap to mind in which the teleoconchs are actually even more similar: Alvania auberiana (d’Orbigny, 1842) / A. faberi De Jong and Coomans, 1988, Retilaskeya emersonii (C. B. Adams, 1839) / R. bicolor (C. B. Adams, 1845), and Iniforis turristhomae (Holten, 1802) / I. casta (Hinds, 1845).  In these cases of so-called didymous species pairs, it is not entirely clear to me which is the ancestral and which the derived condition, but, in a group taxonomically close to the Stosicia, Verduin (1977) looked at European Rissoa species and concluded that the lecithotrophic species was the descendant (apomorphic), and the planktotrophic was ancestral (plesiomorphic).  Similar conclusions were reached in a wider taxonomic and/or paleontological context in Ficus (Smith, 1945), Trophon (Bouchet and Warén, 1985), Nassariidae (Martinell and Cuadras, 1977), Neogastropoda in the early Tertiary and Volutidae in the Cenozoic to Recent (Hansen, 1983), Terebridae (Bouchet, 1981), and Turridae (Bouchet, 1990). To my knowledge, no instances of the opposite pattern of evolution has been reported.  In fact, Strathman (1978) concluded that plankotrophic development was ancestral to the lecithotrophic condition among all marine invertebrates with such life histories, and that the reversal of this process was much less frequent, even unlikely. A relatively limited geographic distribution of lecithotrophic species and their tendency to be less long-lived (in a geological sense) than their planktotrophic congeners has been noted by several workers including Jablonski (1982, 1986) and Hansen (1983).

    Thus, taking into account the evidence above and considering the substantial range S. aberrans and probable recent appearance of Stosicia in western Atlantic waters, we may create a scenario of planktonic larval waifs swimming into the Caribbean, aided by the Gulfstream from the eastern side of the Atlantic (where its parent stock subsequently perished) or, more likely, based on geochronology, invading America from the Indo-West Pacific and traversing the straits that were to become the isthmus of Panama sometime in, say, the late Pliocene epoch.  It then dispersed widely and rapidly. Somewhat later a mutant population, the new species S. houbricki, arose in the northern Caribbean. That new species spread on a lesser scale, now occupying a smaller geographic range with perhaps its most prodigious leap being across the Gulf Stream to southeastern Florida. It probably has very similar habits except for its reproductive strategy. Having crawl-away young may endow that new species with a selective advantage which allows it to prosper yet not outcompete the sympatric (?syntopic) S. aberrans, the plesiomorphic form borne as a juvenile in the plankton and thus more akin to, quite possibly conspecific with, the immigrant ancestor.  Likely the same pattern of speciation accounts for the deployment of the eight lecithotrophic species of Stosicia in the Indo-Pacific, half of which have limited ranges [see: Stosicia species].  However, four of these non-planktotrophic species [see: Stosicia incisa (Laseron, 1956)] have rather wide geographic distribution possibly reflecting an earlier origin in geologic time, a factor not unexpected in a basin where the genus is known to have a fossil record dating to the Miocene.

Bibliography

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Acknowledgements:

    The author thanks Bill Frank for technical and editorial assistance - in particular for his fine digital photography and other work on the illustrations. Paul Callomon, John Chesler, Dr. Emilio García, William G. Lyons, Bob Pace, Richard E. Petit, Colin Redfern, Dr. Gary Rosenberg, John Slapcinsky, and Peggy Williams deserve my thanks for provision of specimens, records, or images.

Epilogue:

    A few years after the above report was posted, more and better specimens of western Atlantic Stosicia came to light, including some shells collected in Honduras by frequent web site  contributor, Emilio García. After close study of this additional material, it became clear that the shells of S. houbricki and S. fernandezgarcesi consistently differed and that Emilio's shells were distinct from those two taxa and S. aberrans. Appropriately, the fourth western Atlantic congener was dubbed Stosicia garciai Rolán, Fernández-Garcés and Lee, 2009.

Rolán E., R. Fernández-Garcés, and H.G. Lee, 2009. The genus Stosicia in the Caribbean (Caenogastropoda, Rissoidae) with the description of a new species. Basteria 73(1-3): 1-8.