Macoma biota, Arruda & Domaneschi, 2005

Arruda, Eliane P. & Domaneschi, Osmar, 2005, New species of Macoma (Bivalvia: Tellinoidea: Tellinidae) from southeastern Brazil, and with description of its gross anatomy, Zootaxa 1012 (1), pp. 13-22 : 15-21

Macoma biota new species

(Plate I, Figs. 1–5; Plate II, Figs. 6–8)

Holotype: Museu de Zoologia, Universidade de São Paulo ( MZUSP) 41183. (Plate I, Figs. 1–4).

Measurements: length 52 mm, height 36 mm, width 21.54 mm.

Paratypes: MZUSP 41184 , 23 º37'30.1? S; 45º23'57.1'' W, 4.IV.2001 (1 spec., Sta. 109Pp) ; MZUSP 41185 , 23º37'30.1'' S; 45º23'57.1'' W, 4.IV.2001 (2 spec., Sta. 69Pg) ; MZUSP 41186 , 23º37'27.9'' S; 45º23'55.0'' W, 29.VIII.2004 (1 spec.) ; MHN­BBI 546, 23º37'27.9'' S; 45º23'55.0'' W, 4.IX.2003 (3 spec.); MHN­BBI 547, 23º37'30.1'' S; 45º23'57.1'' W, 4.IV.2001 (1 spec., Sta. 68Pg); MHN­BBI 548; 23º37'27.9'' S; 45º23'55.0'' W, 23.VIII.2001 (1 spec. Sta. 189Pex); MHN­BBI 549, 23º37'30.1'' S; 45º23'57.1'' W, 4.IV.2001 (1 spec., Sta. 67Pg); MHN­BBI 550, 23º37'30.1'' S; 45º23'57.1'' W, 4.IV.2001 (1 spec., Sta. 70Pg); MHN­BBI 551, 23º37'31.3'' S; 45º23'53.9'' W, 4.IV.2001 (1 spec., Sta. 75Pg); MHN­BBI 552, 23º37'30.1'' S; 45º23'57.1'' W, 4.IV.2001 (1 spec., Sta. 108Pp); MHN­BBI 553, 23º37'30.1'' S; 45º23'57.1'' W, 4.IV.2001 (1 spec., Sta. 119Pex); MHN­ BBI 554, 23º37'27.9'' S; 45º23'55.5'' W, 22.VIII.2001 (2 spec., Sta. 137Pex); MHN­BBI 555, 23º37'27.9'' S; 45º23'55.5'' W, 23.VIII.2001 (2 spec., Sta. 170Pex); MHN­BBI 556, 23º37'27.9'' S; 45º23'55.0'' W, 10.IX.2003 (2 spec.); MHN­BBI 557, 23º37'30.1'' S; 45º23'57.0'' W, 4.IV.2001 (1 spec.).

Type Locality — Municipality of Caraguatatuba, Caraguatatuba Bay, São Paulo State, Brazil, in the intertidal zone of Praia da Cidade, a protected beach near the urban area (23º37'27.9'' S; 45º23'55.0'' W); fine and very fine sandy sediment, retaining large amounts of silt­clay (mean sediment grain size: 2.82Ø); 6.64% organic matter; 8.88% calcium carbonate; 34‰ S (interstitial seawater salinity). The specimens were buried near a bank of the burrowing bivalve Mytella charruana (d’Orbigny, 1842) , Mytilidae , as well as intermingled with this mytilid.

Description — Holotype and paratypes (n = 22). Shell ranging from 9.09 to 53.24 mm in length, 6.48 to 39.5 mm in height and 3.32 to 23.9 mm in width. Shell subovate­trigonal inflated, equilateral, inequivalve; left shell valve almost entirely regularly convex, except for a faint, wide radial depression following the posterior radial carina; right shell valve regularly convex over its anterior 2/3, slightly concave to form a wide, shallow groove below and following the posterior radial carina; valves gaping posteriorly and moderately flexed to the right. Left posterior radial carina faintly marked, delimiting a steeper posterior dorsal slope, the latter provided with a radial depression followed by a weak, dorsal radial elevation. Right posterior radial carina prominent along its entire length, delimiting a slightly convex posterior dorsal slope. Umbos central, pointed, orthogyrate. Anterior dorsal margin long, slightly convex, steeper ventrally; posterior dorsal margin roughly as long as the anterior, almost straight, descending more steeply ventrally; anterior margin broadly rounded; posterior margin short, obliquely truncated, forming an acute angle in its confluence with the broad, regularly convex, ventral margin. Exterior smooth, whitishopaque, with fine commarginal growth lines and microscopic radial striae on the anterior 2/3 of both shell valves. Periostracum thin, light brown, deciduous, best preserved over the newly formed shell margins. Both lunule and escutcheon elliptical­elongate, shallow, weakly defined; lunule widest and delimited by a faint ridge only on the right shell valve; escutcheon almost twice long as the lunule and best defined on the left shell valve. Ligament elongate, yellowish brown, protuberant; nymph sublanceolate, broader and steeper near the umbo, narrowing gradually posteriad. Right and left cardinal teeth fragile, easily broken, separated by a deep trigonal socket. Right cardinal teeth narrow, subtrigonal; anterior cardinal extending antero­ventrally from beneath the umbo; posterior cardinal thicker, single or slightly bifid, descending vertically along the umbo line. Left anterior cardinal narrow, subtrigonal, single or slightly bifid, descending nearly vertically along the umbo line, anteriorly inclined; left posterior cardinal laminate and appressed to the broad anterior margin of the nymph. Interior smooth, whitish, lustrous; anterior 2/3 crowded with fine radial striations; muscle scars well marked. Anterior adductor muscle scar elongate, pronounced in its ventral 2/3; posterior adductor muscle scar ovate, wider and shorter than the anterior one. Elbow­like pallial muscle scar touching the posterior adductor scar ventroposteriorly. Anterior pedal retractor muscle scar rounded­elongate, barely separated from the dorsal end of the anterior adductor scar; posterior pedal retractor scar trigonal, lying on the dorsum of the posterior adductor scar. Cruciform muscle scars sometimes conspicuous, protuberant; posterior scar double, unequal. Accessory adductor muscle scar present, attached to the end of the pallial line posteriorly. Pallial sinus deep, broad, roughly symmetrical on both shell valves, slightly deeper and broader on the left valve; dorsal limb arcs dorsad to form a rounded angularity at the umbo line; ventral limb short, detached and steeper, confluent with the anterior 1/4 of the pallial line.

Variation — Specimens from 9.09 through 14.2 mm in shell length have: shell almost equivalve; left and right valves regularly convex and posterior radial carina faintly marked; anterior dorsal margin straight, longer than the posterior dorsal margin; posterior margin rounded­angulate.

Gross anatomy — Mantle lobes thin, except at the thickened, three­folded ventral margins; middle fold fringed with short, digitiform tentacles arranged in a single row. Both left and right mantle lobes with a long smooth additional mantle fold that isolates a ventral channel for pseudofeces (waste channel of Kellogg 1915); left and right additional mantle folds extending from the mid­point of the pedal opening posteriorly up to the vicinity of the cruciform muscle, where the left additional fold ends and the right one bends abruptly dorsal and forward and extends anteriorly; anterior dorsal extension of the right additional mantle fold increasing in height as it proceeds anteriorly, with the free edge becoming lobulate and often intensively ramified. Siphonal organ present on each side of the proximal opening of the inhalant siphon; siphonal organs asymmetric, slightly cushiony at the base and bordered by a thin, plicate sheath at the free apical edge; left siphonal organ more developed, strongly plicate and fringed with ramified lobular projections. Mantle margins completely free ventrally, except at the posterior end where both right and left inner mantle folds fuse to each other and expand to form the long, extremely maneuverable inhalant and exhalant siphons. Inhalant opening fringed with six blunt lobes; exhalant opening smooth. Cruciform muscle composed of two thick bundles of muscle fibers; posterior portion of each bundle splitting into two distinct branches before attaching to the shell valves where they leave double, unequal posterior scars. A short, stout, herein­named “accessory adductor muscle” attaches to both shell valves ventrally between the anterior and posterior insertions of the cruciform muscle; accessory adductor elliptical in cross section, easily detachable from the pallial muscles and identifiable as a single, unique muscle. Anterior and posterior adductor muscles subequal; anterior adductor dorsoventrally elongated, pronounced in its ventral 2/3; posterior adductor ovate, wider and shorter than the anterior one. Pedal musculature composed of a pair of anterior protractors and paired anterior and posterior retractors; pedal protractors traverse the anterior adductor muscle from its posteroventral side and attach on to each the shell valves along with the adductor muscle. Foot large, axe­shaped, projecting anteroventrally through an extensive pedal opening. Ctenidia smooth, homorhabdic, eulamellibranchiate; outer and inner demibranchs equally long, extending from the umbonal cavity to the base of the siphons; outer demibranchs reduced at each side of the body to a single, completely upturned lamella; inner demibranchs complete in having both descending and ascending lamellae. Labial palps trigonal, large, wide, extending up to 2/3 the shell length; folded inner surfaces densely plicate, bordered dorsally by a wide, smooth trigonal area; unfolded outer surface of each palp bearing short, conical papillae more conspicuous and numerous on the outer palps; papillae arranged in two rows along the midventral half of the outer palps, and restricted to a single inconspicuous row on the inner palps.

Remarks — Among the species of Macoma known from western Atlantic, Eastern Pacific, and Arctic waters, only five (Table 1) closely resemble M. biota in shell outline and require comparative analysis. Conchological differences that distinguish these six species, as well as their shell similarities, are summarized in Table 1. Considering the shell morphology and outline, the western Atlantic M. constricta is the species most similar to M. biota .

Differences in form, depth, and asymmetry of the pallial sinus are the most useful shell characters in distinguishing M. biota from M. lama (Bartsch, 1929) , M. expansa (Carpenter, 1864) , M. inquinata (Deshayes, 1855) and M. lipara Dall, 1916 . The pallial sinus is deep, wide, and symmetrical on both shell valves of M. biota ; whereas it is strikingly asymmetrical in M. lama , moderately deep and not detached in M. expansa , deep, nearly reaching the anterior adductor muscle scars and not detached in M. inquinata , and moderately deep, detached and slightly asymmetrical in M. lipara . Table 1 lists additional minor conchological differences that help to separate M. biota from these other species.

Macoma biota cannot easily be separated from M. constricta based solely on their shell morphology and outline. Examination of the shape of the pallial sinus is required for proper species identification. Both species have a deep, wide, nearly symmetrical pallial sinus on both shell valves. However, M. constricta has a long ventral limb, closely parallel to and confluent with the pallial line far posteriorly, whereas in M. biota the ventral limb is short, steeper, and meets the pallial line in along the first quarter of this scar. The pallial sinus also extends up to and not infrequently merges with the anterior adductor muscle scar in M. constricta , whereas in M. biota the pallial sinus extends close to adductor muscle scar, without reaching it. Other conchological characters separating these two species are the hinge plate and periostracum. The right posterior, and the left anterior cardinal teeth are single or slightly bifid in M. biota , whereas they are markedly bifid in M. constricta . The sub­lanceolated nymph of M. biota is broader and steeper near the umbo, decreasing abruptly in height posteriorly, and the periostracum is brown, whereas M. constricta has a lanceolate­elongate, almost horizontal and evenly wide nymph, and the periostracum is olive­green to gray.

In addition to conchological characters, inspection of the gross morphology of M. biota and M. constricta revealed anatomical differences that confirm the identity of the former as a new Atlantic species of Macominae , and distinguish it from its most similar congener M. constricta .

Macoma biota has the ventral channel formed by the left and right additional mantle folds, both extending from the midpoint of the pedal opening to the vicinity of the cruciform muscle, where the left additional fold ends and the right one bends abruptly and extends antero­dorsally, growing wider, slender, not infrequently lobulate and intensively ramified in its free distal edge. In contrast, M. constricta has its ventral channel isolated by the right additional mantle fold only, which extends from the midpoint of the pedal opening posteriorly to end close to the cruciform muscle ( Narchi 2003). The presence of paired, asymmetrical siphonal organs, each attached to one side of the proximal opening of the inhalant siphon of M. biota , is another striking difference separating this new species from the closely allied M. constricta . The latter species has an unpaired, left siphonal organ that maintains a close functional relationship to the free, undulated terminal tip of the left inner labial palp ( Narchi 2003). The terminal tip of the left inner labial palp in M. constricta is produced into a thin sheath with four undulated plicae ( Narchi 2003); such a sheath and undulation are absent in all four labial palps of M. biota . Narchi (2003: 360) described the labial palps of M. constricta “as well developed, triangular and on the opposed faces there are large folds separated by deep grooves, while the outer faces are smooth”. Macoma biota has the outer, non­plicate surfaces of its labial palps provided with short, conical papillae. These papillae are more conspicuous and numerous on the outer palps, where they are arranged in two rows along the mid­ventral half of these organs, and restricted to a single, inconspicuous row on the inner palps. The extension of the inner and outer demibranchs forming the ctenidium is also a good character to separate M. biota from M. constricta . Both demibranchs are equally long, with filaments present along the entire length of the ctenidial axis in M. biota ; whereas in M. constricta the inner demibranch is longer and extends beyond the antero­dorsal limit of the outer demibranch. The presence in M. biota of a single short stout “accessory adductor muscle” that attaches to both shell valves ventrally, and the absence of such a muscle in M. constricta , completes the series of morphological characters in the mantle cavity which indubitably differentiate M. biota from its closest relation M. constricta .

Etymology — This species is named after the BIOTA/FAPESP Program, which aims to encourage scientists to carry out a large­scale effort to map Recent fauna, flora, and microorganisms, described and otherwise, marine, terrestrial, and freshwater, from the state of São Paulo, Brazil.