Polyschides noronhensis, Abstract, 2009

Simone, Luiz Ricardo L., 2009, Comparative morphology among representatives of main taxa of Scaphopoda and basal protobranch Bivalvia (Mollusca), Papéis Avulsos de Zoologia 49 (32), pp. 405-457 : 414-417

Polyschides noronhensis new species

( Figs. 16-22 , 75-86 )

Types: holotype MZUSP 88441 . Paratypes: BRAZIL. Fernando de Noronha Archipelago. Porto Beach , 3°50’06.67”S 32°24’10.00”W, 6 m depth , MZUSP 32011 , 3 specimens (Simone & Souza col. 20/vii/1999) .

Distinctive Description

Shell ( Figs. 16-20 ): Size about 8 mm, walls thin, color white, translucent; weakly curved; middle and anterior thirds almost straight, posterior third more curved ( Figs. 16, 17 ). Softly widening (tax of increase about 0.125 mm /mm of length), general form almost cylindrical. Maximum width about half of shell length, located between middle and anterior thirds. Outer surface smooth, glossy. Anterior aperture oblique (ventral region slightly more posterior) ( Figs. 16-18 ); weakly flattened dorso-ventrally; about 75% narrower than broader region of shell. Posterior aperture about half of anterior aperture size, rounded, weakly turned dorsally, normally pale-brown pigmented; four equidistant projections ( Figs. 19, 20 ); dorsal and ventral projections about 50% wider than lateral projections; each projection with about 3% of total shell length.

Main muscle system ( Figs. 75, 76 ): Pair of longitudinal muscles originating on ventral and lateral sides of base of apical flap, occupying about 3/4 of this base. Both longitudinal muscles running towards anterior, restricted to ventral side, touching one another along median line. Middle region of both muscles narrower than both ends ( Fig. 76 : lm). Both longitudinal muscles trifurcating in middle level of animal’s body; external branches thin, splaying along anterior pallial wall; middle branches splaying in foot base as longitudinal layer of foot retractors ( Fig. 79 : mv); internal branches as pedal retractor muscles ( Fig. 79 : mf). Each pedal retractor muscle running anteriorly thought haemocoel, flanking some intestinal loops ( Fig. 80 ), connecting with its pair at middle level of haemocoel; after this, running as single bundle up to retracted pedal distal tip.

Foot ( Figs. 78-80 ): Mostly hollow, looking like a retractile gastropod proboscis, with pair of above described pedal retractor muscles inserted in distal tip. Foot capable of enfolding along itself at about half its length ( Figs. 79 , 80 ). Foot with about 1/4 of animal’s volume and approximately 1/3 of total shell length in retractile condition; width about half of adjacent shell width. Foot tip with central foramen corresponding to its enfolding portion. Transition between foot and insertion of pedal retractor muscles unclear; solid portion of foot about 1/6 its length; from this up to region of splaying longitudinal muscles of haemocoel ( Fig. 79 : mv) simple, thin walls.

Mantle ( Figs. 75-78 ): Anterior edge thick, a pair of folds; outer fold relatively narrow, thickness equivalent to that of shell wall; inner fold similarly thickened and 5-6 times longer than inner fold, positioned inwards ( Figs. 75, 76, 78 : mb). Posterior mantle flap tall, about 1/8 of shell length ( Fig. 77 ). Low transverse septum present in base of posterior mantle edge ( Fig. 78 : ms).

Pallial cavity ( Fig. 78 ): About 8-9 transverse folds present at posterior level of foot-visceral mass ( Fig. 78 : vm), uniform, very narrow (width equivalent adjacent to mantle thickness), spaced by equivalent width (of folds); only present in ventral half. Pallial gland with oblique folds ( Fig. 78 : pl) white, located approximately in middle region of mantle cavity, occupying about 1/6 of shell length; ventral folds slightly longer than dorsal folds and more anteriorly located; anterior edge of folds rounded and slightly taller, posterior end unclear. Transverse fold located close to mantle edge, with approximately same height of outer fold of mantle edge ( Fig. 78 : pf).

Visceral mass ( Figs. 78-81 ): Middle region with about 1/10 of shell length containing renal structures. Posterior region of almost half of shell length containing practically only gonad ( Figs. 78 : go).

Circulatory and excretory systems ( Fig. 78 ): Pericardium totally absent. Kidney solid, antero-posteriorly short, but with about same shell width.

Digestive system ( Figs. 80-85 ): Moderate quantity of captacula ( Figs. 18 , 80 ). Conjunct of captacula about 2-3 times shell thickness. Oral tube inner surface three equidistant longitudinal folds; each one low, narrow (about 1/10 of oral tube width). Oral tube edge with four equidistant projections ( Fig. 81 : ot), lateral and ventral projection corresponding to inner folds end. Odontophore muscles ( Figs. 82-85 ): mj, pair of thin protractor muscles, only distinguishable in ventral side of tube connecting odontophore and esophagus ( Fig. 83 ); m4, main pair of dorsal tensor muscles shorter, about 2/3 of odontophore cartilages height ( Figs. 84, 85 ); m6, approximator of cartilages very narrow, about 1/6 of each odontophore cartilages thickness ( Figs. 84, 85 ); m7, absent. Odontophore cartilages (oc) somewhat rectangular, dorsal edge rounded, ventral edge bluntly pointed ( Fig. 85 ), about twice longer than tall. Radula with about 15 rows of radular teeth. Radular teeth ( Figs. 21, 22 ): rachidian (central) thin, with about 1/8 of total radula width. Pair of lateral tooth thick, with serrated cutting edge ( Fig. 22 ). Esophagus relatively wide (about half odontophore width) in its region close to mouth, gradually narrowing; inner surface simple, smooth. No differentiated stomach, neither clear separation between esophagus and intestine. Intestine very narrow (about 1/8 odontophore thickness); performing 3-4 simple loops ( Figs. 80, 81 ) of similar width, partially compressed between pedal retractor muscles ( Fig. 80 ). Anus a relative wide papilla ( Fig. 78 : an).

Genital system ( Figs. 78 , 81 ): Gonad somewhat triangular, with almost half of shell length; anterior region as wide as kidney, gradually narrowing towards posterior, ending at base of posterior mantle border. Gonad constituted for relatively long digital acini forming transverse folds; each acinus with almost half of gonad width and bifid distal end; gonadal folds highly protruding inside posterior half of pallial cavity, surrounding almost completely adjacent animal’s diameter. Gonoduct running towards anterior and right, crossing rectum ( Fig. 81 ).

Central nervous system ( Figs. 79-81, 86 ): Located surrounding esophageal connection to mouth, edging base of oral tube as a nerve ring. Pair of cerebral ganglia located just dorsal to mouth; each cerebral ganglion spherical, with approximate diameter equivalent to 1/10 of that of anterior shell aperture. Pair of pedal ganglia of equivalent size to cerebral ganglia, located in opposite side of cerebral ganglia, ventrally to mouth; form of each ganglion roughly spherical. Pedal commissure with about half each ganglion length ( Fig. 86 ). Cerebro-pedal connectives of similar length, lying on esophageal wall. Pair of statocysts located just posterior to pedal ganglia; volume of each statocyst about 1/3 of each pedal ganglion. Pair of buccal ganglia absent.

Measurements (respectively antero-posterior length, dorso-ventral maximal lateral inflation in mm; finally tax of increase in mm/mm of length): MZUSP 32011: #1: 8.0 by 1.1, 0.137; #2: 7.4 by 0.9, 0.122.

Distribution: Endemic to Fernando de Noronha Archipelago, Brazil.

Habitat: Sandy bottoms, about 6 m depth. Content of buccal tube several foranmifer testa.

Material examined: Types.

Discussion: Polyschides noronhensis possesses the shell character arrangement typical of the genus, as the maximum diameter located in the anterior third, and wide posterior aperture bearing four lobes. The new species differs from the other co-generic species from the region ( Caetano et al., 2006; Caetano, 2007) mainly by the shape of the shell. P. noronhensis is wider than P. tetraschistus (Watson, 1879) , and than P. xavante Caetano & Absalão, 2005 , from which it also differs by a straighter shape. the other two species have a more arched shell, P. noronhensis still differs from P. portoricensis ( Henderson, 1920) and from P. tetrodon ( Pilsbry & Sharp, 1897) in being narrower, particularly close to the anterior aperture, and in having a more uniform width along its length; P. noronhensis has the wider portion of the shell closer to the anterior aperture than the other two species, as it is in the anterior third in P. portoricensis and almost in the middle in P. tetrodon .

Figures

FIGURES 16‑29: Shell and radular aspects of Gadilidae: 16-22) Polyschides noronhensis; 16) Holotype shell, left view, length = 8.6 mm; 17) same, paratype MZUSP 32011, length = 7.8 mm; 18) Paratype MZUSP 32011#1, left view, detail of anterior aperture in profile with some captacula, scale = 0.5 mm; 19-20) Paratype MZUSP 46736, detail of posterior aperture in profile, right and left views respectively, scale = 0.25 mm; 21-22) Radulae of 2 specimens in SEM, scale = 50 µm; 23-29) Gadila braziliensis MZUSP 46736; 23) #3, right view, length = 7.5 mm; 24) #2, dorsal view, length = 7.9 mm; 25) #4, young specimen, length = 3.4 mm; 26) Radula in SEM, scale = 50 µm; 27) Same in light microscopy; 28) Odontophore, radular ribbon removed, ventral view, maximum diameter = 0.7 mm; 29) #2, left view, detail of posterior aperture in profile, scale = 0.5 mm.

FIGURES 75‑79: Polyschides noronhensis anatomy: 75) Whole left view; 76) Whole dorsal view; 77) Detail of posterior region, dorsal view; 78) Whole ventral view, mantle opened longitudinally along median line; 79) Haemocoel, dorsal view, mainly showing pedal musculature, foot and visceral sac opened longitudinally. Scales = 0.5 mm.

FIGURES 80‑86: Polyschides noronhensis anatomy: 80) Haemocoel, dorsal view, mainly showing pedal musculature, digestive tubes and part of nerve ring, foot and visceral sac opened longitudinally; 81) Digestive tubes, dorsal view, topology of some adjacent structures also shown; 82) Odontophore, ventral view, superficial layer of membranes removed; 83) Same, whole ventral view; 84) Odontophore, dorsal view, radula removed (only radular sac topology indicated), right m4 deflected; 85) Same, m3 sectioned transversally, both cartilages and m4 deflected; 86) Nerve ring, anterior view. Scales = 0.5 mm.