Clenchiellidae D. W. Taylor, 1966

Ponder, Winston F., Fukuda, Hiroshi & Hallan, Anders, 2014, A review of the family Clenchiellidae (Mollusca: Caenogastropoda: Truncatelloidea), Zootaxa 3872 (2), pp. 101-153 : 104-111

Family Clenchiellidae D. W. Taylor, 1966

Clenchiellini Taylor, 1966: 181.

Description. Shell. Minute (1.1–2.3 mm in diameter); discoidal, planorbid-shape or depressed skeneimorph with wide umbilicus. Usually thick, opaque. Colour deep reddish brown, sometimes white. Distinct spiral cords on whorls; one genus ( Clenchiella ) with pair of strong spiral keels dorsally and ventrally. Aperture round, prosocline, with or without varix on outer lip.

Operculum. Near circular, multispiral, with central nucleus, horny, translucent, concave. Inner surface without (species of Clenchiella ) or with triangular, white, presumably calcareous, smear. Some species with low horny projection on central part of inner side.

Head-foot. Head with slender, long cephalic tentacles and cylindrical, bilobed snout. Cephalic tentacles, snout and foot with black pigment or colourless except for minute white spots. Cephalic tentacles long, parallel sided, with rounded tips, ciliated, with rather short stationary compound cilia at their tips and few ciliated ridges on outer proximal half of left tentacle (ciliation details not confirmed in some taxa). Anterior pedal mucous gland with long transverse slit on central part of anterior end of foot, gland cells extend to edge of foot, with large gland in median part. Sole slender, simple, with scattered mucous cells; metapodial pedal gland absent. Anterior end of foot indented; posterior end of foot pointed or rounded.

Mantle cavity. Long, occupies about two thirds of last whorl. Kidney opening small, in posterior-most corner of cavity. Kidney entirely behind mantle cavity, compact, consisting of mass of colourless, transparent cells. Ctenidium well developed, large, with row of 16 to 30 slender triangular filaments. Hypobranchial gland small, elongately-oval, transparent, situated on middle part of cavity along posterior end of rectum. Osphradium large, broad, rather long, elongately-oval, simple, transparent, containing conspicuous osphradial ganglion.

Digestive system. Mouth opens between pair of muscular lips into buccal cavity. Buccal mass slender, rather long, occupying most of snout. Radular sac very short and not visible laterally. Radula taenioglossate; cusps on all teeth narrow, sharp; cutting edge of central teeth about half width of tooth, lateral sides of central teeth with narrow extensions in their lower halves, ventral edge with U-shaped extension; dorsal edge distinctly concave; lateral teeth with well-developed basal tongue, long, narrow shaft on outer side, 3–4 times width of short cutting edge; inner marginal teeth with cusps on distal outer third of tooth, moderately expanded basally on inner side; outer marginal teeth narrow except for thin basal extension on outer side, with cusps on distal quarter of inner side, cusps slightly to markedly smaller than on inner marginals. Oesophagus in most species folded in retracted state, otherwise simple, opens widely into buccal cavity. Buccal pouches not visible. Salivary glands long, simple, club-like, located at dorso-lateral sides of buccal mass, not passing through nerve ring. Oesophagus simple, with few strong folds just behind nerve ring; enters stomach on right side at posterior end of posterior chamber. Stomach with single opening to digestive gland posterior to oesophageal opening. Digestive gland pale lemon-yellow, composed of two parts: anterior portion mass of small cells covering right lateral wall of anterior stomach and right ventral edge of posterior stomach; posterior part large mass of many small cells. Style sac large, elongately pyriform. Origin of intestine at right antero-ventral end of anterior chamber of stomach and tightly looped over middle part of style sac before continuing to rectum. Rectum forms 1–2 tight loops in middle of roof of mantle cavity. Oval faecal pellets queued in single file in intestine and rectum. Anus simple, slightly posterior to anterior mantle edge.

Male reproductive system. Testis extremely large, wide, pale yellow, consisting of more than 20 bundles of slender, long lobes. Coiled seminal vesicle arises from vas efferens in mid-ventral region of testis; tube of seminal vesicle narrow, highly convoluted. Posterior vas deferens runs from posterior end of seminal vesicle, forms straight duct that crosses over oesophagus and runs to posterior end of prostate gland behind posterior end of mantle cavity. Prostate gland very small, simple, rectangular. Anterior vas deferens arises from anterior part of prostate gland, passes straight across roof of mantle cavity and enters muscular wall of neck and from there enters base of penis. Penis situated on left dorsal side of head (except Coleglabra n. gen. with penis on right side), large for body size, simple in Colenuda n. gen. and Coleglabra but with distinct swellings in Clenchiella or lobe-like clusters of small glands in Coliracemata n. gen. on middle and distal parts. Penial duct almost straight to undulating, narrow throughout.

Female reproductive system. Ovary large, covering ventro-lateral area of digestive gland, consisting of large, white, regularly arranged tubules. Posterior oviduct wide, runs along oesophagus on right lateral edge; then bends toward posterior end of albumen gland before reaching end of mantle cavity. Beyond this latter bend, oviduct strongly curved making at least one distinct loop or fold. Seminal receptacles usually two (one in Coleglabra ), very small, slender sacs, at posterior end of albumen gland. Pallial oviduct very short, thick, oval to rectangular, slightly longer than high, about half within roof of mantle cavity; clearly divided (in gross dissection) into two approximately equal parts, opaque albumen gland posteriorly and transparent capsule gland anteriorly. Histology of oviduct glands complex (sensu Ponder 1988). Muscular ventral channel continued from coiled oviduct, runs along ventral edge of albumen gland. Bursa copulatrix large, oval to elongately oval, at left side on middle part of oviduct; posterior 1/4 to 2/3 on albumen gland, rest on capsule gland. Bursal duct short but distinct, rather wide, runs from anterior portion of bursa to anterior end of ventral channel just behind genital opening. Genital opening subterminal to middle of capsule gland.

Nervous system. Circum-oesophageal nerve ring streptoneurous, epiathroid, with ganglia moderately concentrated. Cerebral ganglia large, between posterior end of buccal mass and folded part of oesophagus; cerebral commissure short. Pleural ganglia separated from each cerebral ganglion by a constriction. Tentacular nerves thick, with large swellings at their bases, each divided into two nerves distally. Optic nerves arise from cerebral ganglion just below points of origin of tentacular nerves. Cerebro-pedal and pleuro-pedal connectives rather short, about equal in length; no obvious asymmetry. Buccal ganglia very small, ovate, joined by long commissures passing along oesophagus near posterior end of buccal mass. Right pleural ganglion about half size of cerebral ganglion, round, partially fused with right cerebral ganglion, being separated by distinct constriction. Supraoesophageal ganglion small, round; right pleural and supraoesophageal ganglia completely separated by long, distinct pleuralsupraoesophageal connective. Left pleural ganglion about two thirds size of cerebral ganglion, oval, partially fused with right cerebral ganglion, separated by distinct constriction. Suboesophageal ganglion larger or smaller than left pleural ganglion, oval to round, abuts left pleural ganglion. Pedal ganglia connected posteriorly to cerebral ganglia. Pedal commissure rudimentary or very short. Metapodial and propodial ganglia distinct, globose, connected with pedal ganglia by very short, thick connectives; thick pedal nerves arise from these ganglia to innervate foot. Metapodial commissure not observed. Statocysts small, located dorsally on inner posterior edge of pedal ganglion. Osphradial ganglion large, elongated, in and beneath osphradium. Visceral ganglion not examined.

Remarks. The family is characterised by a minute discoidal to skeneiform shell, which is typically spirally sculptured, a circular, multispiral operculum with a central nucleus, long cephalic tentacles with short stationary compound cilia distally, no pallial or metapodial tentacles, an extremely short pallial oviduct, large anterior bursa copulatrix and, typically, two seminal receptacles (one in Coleglabra n. gen.).

The taenioglossate radula is very similar to that of many other truncatelloidean families such as Cochliopidae , Elachisinidae , Tornidae , Caecidae , Calopiidae , some Hydrobiidae s.l. and some Iravadiidae , in the central teeth having a single pair of basal denticles and long, divergent, narrow lateral processes, lateral teeth having a narrow cutting edge and long outer process, and narrow outer marginal teeth. The penis is simple to glandular, but the presence of apocrine glands in one genus ( Coliracemata n. gen.) is a character shared only with a few other truncatelloidean families ( Tornidae , Cochliopidae , Caecidae , Calopiidae , some Iravadiidae and the genus Botriophallus Ponder, 1990 of uncertain affinities; Ponder 1988, 1990, 1999; Hershler & Thompson 1992; Table 2).

Circular multispiral opercula are found in a few other truncatelloideans (Table 2), although some families with taxa that have near circular apertures have a paucispiral circular operculum. As in most other truncatelloideans, the salivary glands do not pass through the nerve ring, there is no oesophageal gland and there is a short crystalline style in the style sac. The intestine runs across the middle part of the style sac in all the species dissected herein, a character seen in some other taxa (e.g., Elachisinidae Ponder 1985 , fig. 2A) and the rectum is folded on the roof of mantle cavity.

The female genital system is unusually short, with the glandular oviduct being not much longer than it is wide. There are two slender seminal receptacles located posteriorly in three genera, whereas one genus only possesses one. The bursa copulatrix opens anteriorly to the vicinity of the genital opening by way of the bursal duct. The general configuration of the female genital system is generally similar to that in iravadiids ( Ponder 1984), but differs in having a shorter capsule gland, in the seminal receptacles being located behind the albumen gland, not opening at the anterior edge, and in having a coiled oviduct (absent in iravadiids).

While the family does not exhibit any clear-cut autapomorphies, it possesses a unique combination of characters (see Table 2 for comparisons with similar family-group taxa) and does not fit comfortably with any other family-group taxon. In particular, clenchiellids differ from other related taxa in having a combination of a minute depressed shell with a spirally striate protoconch, a circular operculum with a central nucleus, a thickened coiled renal oviduct, an anterior bursa copulatrix and a short, triangular capsule gland. The foot is simple and there are no metapodial or pallial tentacles.

Taylor (1966) erected Clenchiellini within Cochliopinae to include Clenchiella and Carinulorbis Yen, 1949 (nom. nov. pro Carinorbis Yen, 1946, non Conrad, 1862), which he treated as a synonym. The type species of Carinulorbis is Carinulorbis planospiralis Yen, 1946 from the lower Eocene of Wyoming, USA. Later Taylor (1975) recorded and discussed this species and another undescribed taxon, which he also placed in Clenchiella . Both species were from the early Tertiary of the Powder River Basin, Wyoming and Montana, USA and these records have occasionally been referred to in other literature. Taylor (1975) correctly placed these American taxa in Cochliopidae , and some other members of that family are rather similar in shell morphology to clenchiellids, including Coahuilix Taylor, 1966 , the flat-spired Balconorbis Hershler & Longley, 1986 and the low-spired TABLE 2. (Continued)

Character Clenchiellidae Iravadiidae Calopiidae Hydrobiidae Cochliopidae Pomatiopsidae

and Tateidae

Thick-walled coiled Present Absent Absent Present Present Absent oviduct

Bursa copulatrix Anterior Anterior Posterior Posterior Posterior Posterior Bursal opening Subterminal to mid capsule Genital opening or Posterior ventral channel Posterior ventral channel Posterior to sperm tube Posterior to sperm tube

gland at genital opening ventral channel (in

Lantauia)

Seminal receptacles 1–2 at posterior end of 1–2 with duct at anterior Absent 1 usually present, 1 usually present 1 usually present

albumen gland end of albumen gland sometimes absent or

and extending posteriorly multiple

on left side of albumen

gland

Albumen gland Very short, rectangular Short to moderate; Short, wide Medium to long Moderately long to long, Medium (about same

rectangular to pyriform posteriorly twisted length as capsule gland) Female opening shape Small, subterminal to Short to long slit, Small, anterior Small, anterior to Terminal to subterminal Terminal, small position middle of capsule gland subterminal to mid subterminal

ventral

Shape of capsule gland Very short, triangular Moderate, to elongate, Moderate, tapering Ovoid to tapering Ovoid to tapering Elongate, tapering albumen gland ovoid to tapering

Sperm duct in female Ventral channel Sperm groove or ventral Ventral channel Ventral channel Sperm tube present, Sperm tube present.

channel Opens separately Opens separately or at

(posterior to anterior in common opening

mantle cavity, or to

pericardium) or at

common opening

References Herein Ponder 1984; Fukuda et Ponder 1999 Hershler & Davis 1980; Davis & McKee 1989; Davis 1967, 1979; Davis al. 1990; Fukuda & Davis et al. 1989; Davis et al. 1982; et al. 1986a, 1986c, 1994; Ekawa 1997 Hershler & Ponder 1998; Hershler & Thompson Davis & Kang 1990 Davis et al. 1988b; 1992

Ponder et al. 1989,

1991, 1999

… …continued on the next page TABLE 2. (Continued).

Apocrine glands on Absent Absent Absent Present or absent Absent Absent

penial glands Absent Present Absent Absent Present Absent

Penial stylet Present or absent Absent Absent Absent Absent Absent

……continued on the next page TABLE 2. (Continued)

Character Stenothyridae Elachisinidae Assimineidae Tornidae Hydrococcidae Falsicingulidae

Thick-walled coiled Present Present Present Present Weakly developed Absent

oviduct

Bursa copulatrix Posterior + accessory Middle Posterior Anterior or middle Middle Posterior anterior sperm pouch

Bursal opening Posterior to sperm tube Posterior ventral channel Posterior to oviduct Genital opening Separate opening to Copulation via pericardium

posterior mantle cavity

Seminal receptacles 1 posterior 2 posterior behind 1 usually present, absent 1–2 posterior 1 near posterior 1 posterior albumen gland or several in few genera

Albumen gland Medium (about same Short, rounded Very short to very long Short to very short, Elongate Medium length as capsule triangular to rounded gland)

Female opening shape Terminal, small Small, subterminal Usually terminal Small, subterminal Small, terminal Small, terminal

position

Shape of capsule gland Elongate, tapering Oval Elongate, often tapering Very short, triangular or Elongate, slightly Elongate, tapering

albumen gland bean-shaped tapering

Sperm duct in female Sperm tube present. Ventral channel Central to near ventral in Ventral channel Absent – direct opening Via pericardium and gono-

Opens separately to capsule gland to bursa pericardial duct posterior end of mantle

cavity Ponder 1985

References Kosuge 1969; Davis Marcus & Marcus 1963; Bieler & Mikkelsen 1988; Ponder 1982 Slavoshevskaya 1975, 1982;

et al. 1986b, 1988a Hershler 1987; Fukuda et Fretter 1956; Ponder Ponder 1985 al. 2006; Fukuda & 1994

Ponder 2003, 2004, 2005,

2006

Cochliopina Morrison, 1946 (see Hershler & Thompson 1992, figs 19, 16, and 22 respectively). While the shells of these taxa are convergent in shape with clenchiellids, their female genital morphology is very different as they possess the typical cochliopid twisted albumen gland, have a sperm tube and lack an anterior bursa ( Hershler & Thompson 1992).

Like Ioganzen & Starobogatov (1982), we treat this group as a family pending more detailed analysis. We justify this decision based on the combination of morphological characters that do not conform to any currently recognised family group taxon. In recent classifications, the clenchiellids have been included in Hydrobiidae sensu lato as a subfamily. However, the Clenchiellidae differ from the Hydrobiidae s.l. with regards to several important anatomical characters (see Table 2), notably the presence of stationary compound cilia on the distal ends of the tentacles, the female genital anatomy, apocrine glands (in one genus) and a multispiral operculum as opposed to paucispiral in the Hydrobiidae . These morphological distinctions from the Hydrobiidae are corroborated in a molecular study by Criscione & Ponder (2013), showing the clenchiellids as genetically distinct from the Hydrobiidae s.l. and sister to the Iravadiidae and Calopiidae (see Table 2 for comparisons of these and other family-group taxa).