Heleobia brucutu, Simone & Oliveira, 2021

Simone, Luiz Ricardo L. & Oliveira, George Vita de, 2021, A new species of the micro snail genus Heleobia (Caenogastropoda, Cochliopidae) from Bahia, Brazil, Papéis Avulsos de Zoologia (Pap. Avulsos Zool., S. Paulo) 61, pp. 1-7 : 2-6

Heleobia brucutu new species

( Figs. 1-26 ) http://zoobank.org/ B8DB24F5-FBA3-4062-89E7-A31BB559F087

Types: Holotype MZSP 151288 ( Figs. 1-3 ).

Paratypes from type locality: MZSP 151289, 50 specimens,MZSP 152202, 35 specimens,MZSP 152203,1 metalized shell ( Figs. 13-15 ).

Type locality: Brazil, Bahia, Iraquara, Lagoa da Pratinha, Pratinha River, ParaguaÇu River basin. Elevation 642 m, 12°21′10″S, 41°32′24″W [Angela Maria Zanata, George Vita & Rafael Burger col. 29/xi/2016) ( Fig. 2 ).

Etymology: The specific name is in apposition and refers to the epithet of the better-known fish predator, Astyanax brucutu .

Diagnosis: Shell minute (up to 2.5 mm), slightly turriform, suture well-marked, whorls profile slightly round- ed. Jaws small, reduced. Insertion of visceral vas deferens close to origin of pallial vas deferens. Penis lacking glands, with terminal papilla and lateral-terminal projection. Pallial oviduct with elongated bursa copulatrix and seminal receptacle originated close from each other. Egg capsules attached externally to adult shells.

Description

Shell: ( Figs. 1-6, 9-16 ) size ~ 2 mm; elongate, slightly conic-turriform, ~65% longer than wide, weakly umbilicate; whorls blandly convex, rounded, suture shallow but well-marked ( Figs. 13-15 ); adult ~5-6 whorls. Sculpture absent except for weak growth lines ( Figs. 2, 5, 14 ). Color pale beige, opaque ( Fig. 1-6, 9-11 ), periostracum thin, mostly eroded in older regions ( Figs. 3, 6, 9 ). Protoconch ( Fig. 9, 14,17 ) of ~ 240 µm, with~1.5 smooth whorls;color white.Teleoconch of ~4 whorls. Spire angle ~55-65°,with blunt apex. Aperture oval, peristome continuous, weakly attached to penultimate whorl ( Figs. 1, 4, 9, 10, 13, 16 ); no callus; ~40-50% of total shell length, ~60 of shell width; profile almost orthocline, ~15° with longitudinal shell axis ( Fig. 4, 13 ); inner lip smoothly concave; outer lip and interior third on inner lip performing continuous semi-circle ( Figs. 9, 10, 16 ), relatively thick; anal region angled in ~85; incurrent region widely rounded. Umbilicus very narrow, mostly covered by middle region of peristome inner lip.

Head-foot ( Figs. 19-20, 24 ): Relatively short, relatively narrow; mostly unpigmented, with pale brown spots in lateral edge of foot and snout. Mesopodium sole thick, short, slightly wider than long (fs). Anterior furrow of pedal glands (pg) deep, restricted to anterior edge. Opercular pad elliptic, terminal, occupying most of posterior foot dorsal surface. Head bulbous, with ~80% of food width; pair of cephalic tentacles (te) simple, thick, with ~half foot length. Eye (ey) large, at outer tentacles base; occupying more than half width of tentacles base. Snout as long as tentacles; ~double tentacles width (sn); anterior slightly bilobed. Columellar muscle thick, ~¾ whorl, simply curving to right (cm).

Operculum ( Figs. 7-8 ): Corneous, thin, translucent, pale yellow-beige, flexible, paucispiral. Outline semi-circular, ~1.2 longer than wide. Nucleus subcentral. Occupying entire shell aperture ( Fig. 1 ).

Mantle organs ( Figs. 23, 26 ): Pallial cavity ~1 whorl in length. Mantle edge simple, weakly thick wide; unpigmented. Osphradium (os) short, elliptical, simple; length ~10% pallial cavity length; 3-times longer than wide; located in anterior-left corner of cavity, close to gill and from mantle border; osphradial satellite fold (of) present, with ~half of osphradium width. Gill (gi) elongated, narrow, with ~70% of pallial cavity length and ⅙ of cavity width; anterior end blunt, posterior from mantle edge; width relatively uniform along length; filaments ending at anterior edge of pericardium. Gill filaments tall, narrow, distal tip blunt (gi); relatively few filaments (~12). Between gill and rectum wide distance equivalent to ¼ of pallial cavity width. Hypobranchial gland inconspicuous. Rectum narrow, with small posterior curve (concavity right), up to ⅛ of pallial cavity width; bearing aligned series of transversely disposed, elliptical, white fecal pellets (fe), easily seen by translucence; rectum slightly displaced from right mantle cavity edge. Anus simple,shortly siphoned, located short distance from mantle border at ~10% of cavity length. Genital ducts running along right-posterior edge, relatively massive, described below.

Visceral mass ( Fig.23 ): Length ~3 whorls.Most structures color pale beige to white, local mantle dark pigmented in superior half. Reno-pericardial (ki, pc) organs as anterior structure. Stomach (st) compressing anteriorly small reno-pericardial structures; stomach of ~0.5 whorl, with almost entire adjacent whorl width. Digestive gland light beige, of ~2.5 whorls,mostly posterior to stomach.Gonad (ts) running along columellar surface of each whorl when mature. Other genital and digestive details below.

Circulatoryandexcretorysystems ( Fig.23 ):Pericardium rounded, located longitudinally between stomach and left-posterior corner of pallial cavity (pc), volume ~¹/₂₀ of that of visceral mass; auricle (au) anterior, small, just posterior to posterior end of ctenidial vein (cv); ventricle (ve) posterior, simple. Kidney (ki) minute, weakly smaller than pericardium, located in right side of pallial cavity posterior end; inner tissue scanty, mostly hollow, white. Nephrostome (ne) small, transverse, located close middle distance between pericardium and adjacent rectum.

Digestive system ( Figs. 21-23 ): Mouth (mo) in ventral surface of snout. Buccal mass occupying entire snout inner surface and ~¼ of haemocoelic volume. Oral tube (ot) conic, wide, with ~¼ buccal mass length. Pair of jaw plates ( Figs.21-22 :jw) small,laterally located; each one as long as wide, obliquely disposed, very thin, translucent. Pair of buccal ganglia relatively large (bg). Odontophore spherical, with ~half of buccal mass volume.

Odontophore muscular arrangement ( Figs. 21, 22 ): m2, pair of small protractor muscles of buccal mass, originating in ventral surface of haemocoel, in posterior level of odontophore, running dorsal-posteriorly,inserting in latero-posterior surface of odontophore; m4: pair of main dorsal tensor muscles of radula, surrounding externally both cartilages, relatively narrow; m5, pair of auxiliary dorsal tensor muscles of radula,originating between m4 and cartilages in posterior-median region, running medially and anteriorly, inserting in middle region of radular sac; m6, horizontal muscle, connecting ventral edge of both cartilages, along ~75% their length, ~half cartilages width; mj, air of jaw and peribuccal muscles, originating in outer-lateral region of cartilages,running anteriorly splaying in oral tube and having main lateral band ( Fig. 22 ) up to lateral region of mouth. Radular sac ~twice longer than odontophore ( Fig. 21 : rs); radular nucleus (rn) weakly widening.

Radula ( Fig. 18 ): Typical for the family; rachidian tooth occupying ~20% of radular ribbon width, ~double width than high, cutting edge concave,strongly curved, central cusp relatively large, 4 pairs of secondary cusps gradually smaller towards lateral; no basal cusps except for pair of lateral thickening, possessing rounded nodule in median side as large as first secondary cusp; lateral tooth with main region triangular, with ~double rachidian height, 60% its width; 4 cusps similar to central cusp of rachidian, similar-sized; inner and outer lateral tooth similar shaped with each other, being outer tooth ~30% narrower than inner marginal tooth, same length as lateral tooth and ~20% narrower, general form as a rod curved inwards at tip, tip flattened, bearing ~8 inner small terminal cusps of similar size from each other; each cusp small, sharp pointed. Salivary gland absent. Esophagus simple, zigzagging along haemocoel ( Fig. 21 : es), slightly broad- er in its middle region, lacking apparent glandular areas. Stomach dimensions and position above described (visceral mass); possessing main gastric chamber posterior ( Fig. 23 : st), posterior surface rounded. Style sac narrow ( Fig. 23 : ss), with ~¾ remaining gastric width, and double its length. Esophagus (es) inserting in posterior-left region of stomach, very narrow; intestinal (in) origin on left side of posterior level of style sac;duct to digestive gland (dd) almost as broad as esophagus, located in middle of posterior-ventral stomach region, running towards posterior. Intestine (in) slightly narrower than esophagus; running surrounding ventrally style sac anterior surface up to right posterior end of style sac, in this region fecal pellets appearing, subtly curving towards anterior and dorsal running to pallial cavity ( Fig. 23 : in). Rectum and anus above described (pallial cavity).

Genital system: Male ( Figs. 23-24 ): Seminal vesicle (sv) very narrow, highly convolute, located in columellar region of visceral mass ventral-posterior to stomach, col- or pale grey-iridescent; narrow, straight portion of vas deferens, ~½ whorl in length, inserting in prostate subterminally ( Fig. 23 : sv), just posterior-left level of prostate. Prostate with ~ ⅕ pallial cavity width, slightly dorso-ventrally flattened, white ( Fig. 23 : pt). Pallial vas deferens originating from prostate ventral-middle region, opposite to visceral vas deferens insertion ( Fig. 23 : vd); after short distance penetrating in floor of pallial cavity up to of penis base, running immersed in integument ( Figs. 20, 24 : vd). Penis ( Figs. 19, 20, 24 : pe), simple, curved, elongated (~¼ of pallial cavity) base broad, gradually tapering up to bluntly pointed tip. Penis duct seen by translucence, running straightly along central penial region, up to penis distal tip ( Fig. 24 : pd); penial terminal papilla (pi) with penial aperture distal, simple, small. Female ( Fig. 26 ): Ovary size and localization similar to those of testicle. Visceral oviduct very narrow, running along middle level of columellar surface of visceral mass ~1 whorl (vo). Visceral oviduct inserting at base of seminal receptacle (vo). Seminal receptacle (sr) small (~¹/₁₀ of entire pallial oviduct), balloon-like, inner lumen pigmented dark, arched. Internal, broad duct originating in receptacle base (gd), sigmoid, running immersed in albumen gland up to opposed side in right-posterior region. Albumen (ag) and capsule gland (cg) as continuation from each other, white, without clear separation. Capsule gland tapering gradually towards anterior up to terminal atrium. Terminal atrium (ta) of ~ ⅙ of pallial oviduct, conic, walls weakly thick. Female pore (fp) sessile, papilla-like, aperture longitudinal, turned ventrally, located close to and slightly posterior to anus (an).

Central nervous system ( Fig. 25 ): Nerve ring located just posterior to buccal mass, with pedal ganglia weakly more anterior than remaining ganglia. Each cerebral ganglion (ce) oval, with size equivalent to esophageal section; cerebral commissure narrow, ~half-length of each ganglion. Pleural ganglia (pp) with ~ ⅕ size of cerebral ganglia, located just ventral to them fused with cerebral ganglia by constriction. Each pedal ganglion (gp) as large as cerebral ganglion, somewhat spherical; pedal commissure narrow, minute. Cerebro-pedal and pleuro-pedal connectives narrow, about as long as pedal ganglia. Subesophageal ganglion (su) almost as large as pleural ganglion, located short distance from right pleural ganglion. Pair of statocysts (sy) located in ventro-anterior side of pedal ganglia.

Measurements (length and width in mm): Holotype 2.7 by 1.6 ( Figs. 13-15 ). Paratypes MZSP 151289 (dissected specimens) #1 ( Figs. 1-3 ): 2.6 by 1.4; #2 ( Figs. 4-6 ): 2.5 by 1.4; #3 ( Fig. 9 ): 2.0 by 1.2; 10-11, #4, frontal and right views (L 2.4 mm); #4 ( Figs. 10-11 ): 2.4 by 1.4.

Distribution: Only known in the type locality.

Habitat and ecological notes: Heleobia brucutu sp. nov. has a very restrict distribution, so far known from a small stretch of the Pratinha stream, at the Pratinha lake, near the Pratinha cave entrance ( Fig. 3 ). Details about the

type locality were previously discussed by some authors ( Lima & Gerhard, 2001; Zanata et al., 2017). The Pratinha Lake have a maximum depth of around 1.5 m and possesses substrate composed mainly of sand, rocks and muddy. At this place empty shells and living individuals of H. brucutu sp. nov. composes small substrate patch- es, mainly on the sandy bottomed portions of the substrate. The aquatic vegetation is composed mainly of Cabomba sp. The Pratinha Lake possesses high water transparence, with around 8 m of horizontal visibility ( Lima & Gerhard, 2001). Heleobia brucutu sp. nov. occurs syntopically with the bioinvader snail Melanoides tuberculatus (Müller 1774) ( Oliveira et al., 2019) . The new species is the main food resource of the Astyanax brucutu , the endemic and Critically Endangered durophagous “lambari” of the Pratinha river ( Zanata et al., 2017).

Development: in some specimens (~10%), small spherical egg capsules were found attached to the shell ( Figs. 11, 12 ). A small protoconch-bearing embryo was seen by translucence inside each capsule ( Fig. 12 ).

Material examined: Types.

Figures

Figure 18. Heleobia brucutu sp.nov.: radula drawings.Scale bar = 100 µm.

Figures 1-17. Heleobia brucutu sp. nov., hard parts: (1-12) paratypes MZSP 151289: (1-3) #1, frontal, dorsal and right views (L 2.6 mm); (4-6) #2, frontal,dorsal and right views (L 2.5 mm); (7-8) operculum,outer and inner views (L 0.7 mm);(9) #3,frontal view (L 2 mm); (10-12) #6, frontal,right view (L 2.7 mm) and detail of egg capsule with young specimens inside; (13-15) holotype, SEM,frontal, right-slightly apical,right-slightly frontal views (L 2.7 mm);(16) paratype with 3 egg capsules attached,with embryo inside,frontal view (L 2.1 mm);(17) same,detail of apical region.

Figures 19-26. Heleobia brucutu sp. nov., anatomical drawings:(19) head-foot,male, right-slightly dorsal view;(20) same,dorsal view; (21) foregut,right view; (22) buccal mass,dissected,radula removed,odontophore deflected downwards,dorsal view,buccal hoof expanded upwards,ventral view;(23) pallial cavity hoof, ventral view,and visceral mass partially uncoiled,most structures as in situ, digestive gland portion ventral so stomach (st) removed, anterior region of visceral vas deferens (vd) sectioned; (24) head, male, focusing penis; (25) nerve ring, dorsal view; (26) pallial oviduct, ventral view, topology of some adjacent structures also shown.Scale bars = 0.25 mm.