17^ THE NAUTILUS 101(1):9-18, 1987 Page 9 Anatomy of Alaba and Litiopa (Prosobranchia: Litiopidae); Systematic Implications Richard S. Houbrick Department of Invertebrate Zoology National Museum of Natural History Smithsonian Institution Washington, DC 20560, USA ABSTRACT A. Adams, 1861 is frequently considered a close relative of both Alaba and Litiopa and has been grouped with Anatomical study of Litiopa and Alaba shows that these taxa them (A. Adams, 1862; Smith, 1875:538) or placed in its differ from other cerithiaceans by a significant number of syn- apomorphies. These two taxa have been variously assigned to own family, Dialidae (Hornung & Mermoud, 1928). Dall the Planaxidae, Litiopidae, Diastomidae, Rissoidae, Cerithi- (1889:258) suggested that Alaba was related to Bittium idae, and to a number of subfamilies of the latter family. Both Gray and Diastoma Deshayes. Other workers, such as genera are highly adapted to algal habitats and have a meso- VVenz (1938) and Franc (1968), have included Finella podial mucous gland on the sole of the foot that produces long, A. Adams, Alabina Dall, and Alaba with the Diasto- anchoring mucus threads preventing dislodgement from the matidae. A summary of the various taxonomic alloca- algae. They share similar taenioglossate radulae; many-whorled, tions of Alaba and Litiopa is presented in table 1. Most ribbed protoconchs; nearly identical palliai oviducts; egg masses; workers have referred the two taxa to the subfamily and planktotrophic larvae. Both genera stand apart from other Litiopinae and placed this group under the Cerithiidae cerithiacean groups in having long, tapered, epipodial tenta- Bruguière, 1789. cles. The morphological evidence points to a close relationship between the two taxa and also supports their inclusion in the family Litiopidae Fischer, 1885. MATERIALS AND METHODS Living specimens of Litiopa melanostoma Rang, 1829 and Alaba incerta (Orbigny, 1842) were studied at the INTRODUCTION Smithsonian Marine Station at Link Port, Ft. Pierce, The higher taxonomic assignment of many small species Florida during January, February, and April of 1986. of cerithiacean snails is controversial and frustrating. Litiopa melanostoma was collected offshore on Sargas- Convergent shell characters and lack of anatomical sum weed. Alaba incerta was collected from intertidal knowledge about the various taxa have resulted in an marine grass beds in St. Lucie Inlet, Florida, and on unstable classification. Moreover, many of the published shallow water grass beds around Peanut Island in Lake systematic opinions about genera and families of ceri- Worth, Riviera Beach, Florida. Living snails were main- thiaceans are based on vague, equivocal, conchological tained in petri dishes of sea water and relaxed in a 10% characters. I have discussed the taxonomic problems of MgCl2 solution for study under a binocular dissecting small, heterogeneous cerithiacean taxa elsewhere (Hou- microscope. Snails were preserved in Bouin's seawater brick, 1980:4-5, 1981:610-611). This paper deals with fixative, embedded in paraffin, sectioned at 7 ¡xm, and the anatomy of Litiopa Rang, 1829, Alaba H. and A. stained with Alcian blue-PAS and counterstained in he- Adams, 1853, and several related taxa, and presents an- matoxylin (Humason, 1962:269). Critical point dried an- atomical data for their natural systematic arrangement. imals extracted from shells, radular ribbons, and proto- Litiopa, while usually assigned to the family Litiopidae, conchs were examined under a Novascan-30 scanning has been thought to be related to the Planaxidae, Ris- electron microscope to determine microscopic anatom- soidae, or Cerithiidae by various authors. Alaba has like- ical features. Protoconchs of some Australaba and Fi- wise been referred to the Cerithiopsidae, Planaxidae, nella species were also compared with those of litiopids. Dialidae, Litiopidae, Diastomatidae, Cerithiidae, and to a number of subfamilies of the latter family. Bändel MORPHOLOGY (1984:55) has discussed the confusing literature regard- ing the placement of Alaba. The status of a few other Shell morphology: Members of both genera are small, genera such as Styliferina A. Adams, 1860, and Diffal- not exceeding 25 mm in length, and have moderately aba Iredale, 1936 remains uncertain. The genus Diala turreted, conical, thin, nearly transparent shells. The shell Page 10 THE NAUTILUS, Vol. 101, No. 1 Figures 1-3. Animal of Alaba incerta. 1. Operculum, showing sinuous attachment ridge, by transparency. 2. Ventral view, showing sole of foot and disposition of epipodial tentacles. 3. Side view of female, showing ciliated groove and ovipositor on right side of head-foot (bar = 1 mm), ag = albumen gland; eg = capsule gland; cgr = ciliated groove; cm = columellar muscle; ct = cephalic tentacle; dg = digestive gland; e = eye; et = epipodial tentacle; etl = epipodial tentacular lobe; k = kidney; m = mouth; me = mantle edge; mmg = mesopodial mucous gland; op = operculum; ov = ovary; ovp = ovipositor; pmg = propodial mucous gland; pt = propodial tentacle; r = rectum; sn = snout; sf = sole of foot. of Alaba (figure 11) differs from that of Litiopa (figure spiral thread from a band of subsutural plaits. Micro- 9) in being longer and in having irregularly distributed, scopic spiral lines lie between the axial riblets in Litiopa wide varices and a taller, more turreted apex. In Litiopa, (figure 8). A good figure of Litiopa melanostoma and the whorls are more inflated and fewer in number than its protoconch has been given by Okutani et al. (1983; in Alaba, and the shell is tan-yellow while that of Alaba 24, figs. 1-5). Robertson (1971:5, pis. 2, 3) presented is vitreous and white. Litiopa has a wider aperture and detailed figures of the protoconchs of Alaba incerta and a weak tooth at the base of the columella. Both taxa Litiopa melanostoma. The periostraca of both taxa are have tiny brown subsutural spots and flammules, but thin and transparent. The ovate, paucispiral opérenla Alaba also has weak, spiral bands of tan spots. Litiopa are completely transparent and have eccentric nuclei in and Alaha have nearly identical protoconchs comprising both taxa. Only the portions of the opérenla beneath the 3.5-4 whorls (figures 8, 10). Protoconch I is smooth (pit- nuclei are attached to the metapodia by fine sinuous ted under high magnification) while protoconch II is ridges (figure 1). This opercular attachment scar is clear- sculptured with numerous axial riblets divided by a thin ly depicted by Robertson (1971 :pl. 4, fig. 16), who noted R. S. Houbrick, 1987 Page 11 the similarity of the opercula of both taxa. A ridge-hke attachment scar is also described for Alaba goniochila by Kosuge (1964:36, fig. 6). External anatomy: Litiopa and Alaba are very active snails and move about quickly in the algae or on the underside of the surface film of the water, on which they glide shell down by means of their long foot. Ex- ternally, Litiopa is yellow with a pale foot while Alaba has a whitish base color flecked with olive brown and red spots. The slender cephalic and pedal tentacles are striped with reddish brown in Alaba. A conspicuous fea- ture in both genera is the long, narrow, active foot which is used to climb about algal filaments and fronds. A very deep, anterior propodial mucous gland lies at the lead- ing edge of the sole (figures 2, 3, 7, 15, pmg) and a large, mesopodial mucous gland is centrally located at the pos- terior of the sole (figures 2, 3, 7, 12, 15, mmg). The osb propodial mucous gland produces a sheet of mucus that moves posteriorly along the sole of the foot. Scanning electron micrographs of critical point dried snails show that the sole of the foot is covered with long, dense, cilia (figure 17). The mesopodial mucous gland is defined by a slit-like groove (figures 2, 3, 7, 12, 15, mmg) that pro- duces a strong mucus thread attaching the snail to the substrate or surface film of the water. If accidentally pulled or dislodged from their algal habitat, litiopids cling to the mucus thread in spider-like fashion, em- ploying the foot and mouth to crawl back and reestab- lish themselves. The mucus thread is quite strong and tensile. The mesopodial mucous gland occurs in both Figures 4, 5. Litiopid palliai oviducts (distal end on left; bar = 0.3 mm). 4, Litiopa melanostoma. 5. Alaba incerta genera, but is especially well-developed in the pelagic (dotted lines I and 2 represent cuts corresponding to diagram- snail, Litiopa. Neither of these anatomical features was matic cross sections 1 and 2). acg = anterior ciliated groove; noted by Kosuge (1964), although he depicted a longi- ag = albumen gland; eg = capsule gland; 11 = lateral lamina; tudinal groove on the sole. A. Adams (1862) was the first ml = medial lamina; odg = oviductal groove; osb = opening to note the mesopodial mucous gland in Alaba, but his to spermatophore bursa; osr = opening to seminal receptacle; comments on the gland and its use in the spinning of sb = spermatophore bursa; sg = sperm groove; sr = seminal mucus threads have been overlooked by subsequent receptacle. workers. He also remarked that Alaba was similar to Litiopa in this respect. A small, median, mesopodial mu- cous gland that opens by a short duct to the posterior on living snails and may be the result of contraction of portion of the sole also occurs in the Turritellidae (Ran- the tentacles. A. Adams (1862) described a pair of epi- dies, 1900:57). podial tentacles on each side of the foot and a posterior Another conspicuous feature in litiopids, and espe- pair in Alaba picta A. Adams, 1861. Kosuge (1964:36, cially in Alaba, is the presence of long epipodial tenta- figs. 1, 2) illustrated slender epipodial tentacles extend- cles along the sides of the foot (figures 2, 3, 15, 16, et), ing well beyond the shell margin of Alaba goniochila. as noted by A.