Heterobranchia, Burmeister, 1837

Subclass Heterobranchia View in CoL Infraclass Euthyneura

Superorder Eupulmonata Order Ellobiida Superfamily Ellobioidea Family Trimusculidae Genus Trimusculus Schmidt, 1818

Trimusculus pifius new species ( Figs. 36-37 View Figure 36 View Figure 37 ) https://zoobank.org/ D641C465-99A7-480A-BF99-4462C15B81D3

Types: Holotype MZSP 165668 View Materials . Paratypes: MZSP 100919 View Materials , 1 View Materials spm, MZSP 100993 View Materials , 2 View Materials spm, MZSP 165669 View Materials , 1 View Materials spm, all from type locality. BRAZIL. Bahia ; Off AlcobaÇa, 17°30′14″S 38°48′51″W, 10-20 m, MZSP 165670 View Materials , 7 View Materials shells (W.Vailant-Mattos col., 2017) GoogleMaps .

Type locality: BRAZIL. Bahia ; Cairu, Praia de Garapuá ( Morro de São Paulo), 13°29′32″S 38°54′21″W, ~ 10 m [Petronio Alves Coelho-Filho col., Petrobras, 2011] GoogleMaps .

Diagnosis: E Brazilian species of up to ~ 10 mm.Shell surface smooth. Apex located posteriorly, turned towards posterior. Radially pigmented.

Description: Shell ~ 5 mm, elliptic-rounded (~1.2 times longer than wide), amply conic (3-4 times longer than tall); anterior and posterior edges rounded, similar-sized ( Fig. 36A, C, K View Figure 36 ). Walls thin, slightly translucent. Color reddish beige to light brown, with 15-20 darker radial bands relatively uniform, interspaces ~ ⅓ of their width, their width in shell edge relatively uniform, being slightly wider posteriorly ( Fig. 36A, K, L View Figure 36 ). Sculpture absent, except for weak growth lines. Periostracum exceeding short distance beyond edges ( Fig. 36L View Figure 36 ). Apex arched, located in posterior third ( Fig. 36A, C, D, K, L View Figure 36 ). Protoconch sinister, white, translucent; of 2.5 smooth whorls in left side ( Fig. 36E, M View Figure 36 ); smooth, bulbed in right side ( Fig. 36F View Figure 36 ); width ~200 µm; border with teleoconch clear, orthocline. Profile with angle 120°, anterior surface convex, posteri- or surface weakly concave ( Fig. 36D, L View Figure 36 ). Internal surface glossy, muscle scars weakly visible.

Head-foot occupying most of ventral region of shell, except for dorsal region, below shell beak, occupying by pulmonary (pallial) cavity ( Fig. 37 View Figure 37 ). Head (he) flap-like, thick, wide (~ ⅔ of foot width) lateral edged, curved ventrally;pair of dark eyes (ey) immersed in integument,locat- ed lateral region of head ( Figs. 36H, I View Figure 36 , 37 View Figure 37 ). Mouth located in ventral side of head,ventrally covered by anterior edge of foot ( Fig. 36I View Figure 36 ). Three shell muscles ( Figs. 36G View Figure 36 , 37 View Figure 37 : sm), anterior pair relatively symmetrical, elliptical in section, located in each side of anterior region of pulmonary cavity, each one with ~¹/₂₀ of head-foot area; third shell muscle located in postero-right region of pulmonary cavity, with right half bulged, with area equivalent to other shell muscles, and narrow extension, working as postero-right edge of pulmonary cavity, extending to left length equivalent to bulged area. Mantle border (mb) thick; simple and colorless, possessing 6 long pallial papillae (mp), being pair anterior, locater some distance from each other over head; other 2 pairs respectively in middle-lateral and in postero-lateral corner of mantle edge. Pulmonary (pallial) cavity only opened in right side in pneumostome (pn), wide,flanked by antero-right and postero-right shell muscles; anal flap (fl) located at middle of pneumostome, in its ventral surface, with ~ ⅓ of its length, ~tice wider than long. Pallial floor smooth, planar, with 2 elevations, one in right region correspondent to reno-pericardial structures; another in right-posterior region, correspondent to visceral structures. Pericardium (pc) mostly locat- ed in pallial hoof, anterior and at right from gill; ~15% of pulmonary cavity area, ~twice longer than wide, auricle located closer to gill, ventricle more ventral. Gill (gi) locat- ed transversely, crossing right to left in posterior region of pulmonary cavity; its anterior region on mantle edge ( Fig. 37 View Figure 37 ), being wider at right, gradually tapering towards left up to left end of cavity. Both afferent gill vessel (af) and ctenidial (efferent) vein relatively wide. Auricle insertion in ctenidial vein in middle of it.

Etymology: The specific epithet is a Latinization Brazilian Portuguese word pífio, meaning weak, an allusion to the weakness of the shell and the small size of the species.

Distribution: Coasts of Bahia to Espírito Santo.

Habitat: From intertidal up to ~ 10 m, usually found sorting sediment.

Measurements (L, W, H in mm): Holotype ( Fig. 36 View Figure 36 A-I): 4.9 by 3.7 by 1.1. Paratype: MZSP 165669: 3.8 by 3.1 by 1.3. Paratype: 109109: 3.9 by 3.1 by 0.8.

Additional material examined: BRAZIL. Bahia; Cairu, Praia de Garapuá ( Morro de São Paulo), 13°29′32″S 38°54′21″W, MZSP 100909 View Materials , 1 View Materials spm, MZSP 100914 View Materials , 1 View Materials spm, MZSP 100959 View Materials , 3 View Materials spm, MZSP 101055 View Materials , 1 View Materials spm [Petronio Alves Coelho-Filho col., Petrobras, 2011] GoogleMaps . Espírito Santo; Guarapari, Praia do Morro , 20°39′42″S 40°29′41″W, MZSP 57677 View Materials , 2 View Materials shells [Simone col., 15.i.1982] GoogleMaps .

Remarks: Specimens of T. pifius are another also commonly identified as Nacella mytilina . Shells of it usually were mixed with Tectura iguypis and N. mirim , despite in being relatively easy to be separated from them. From the former in not being laterally compressed. From the later by the relatively monochromatic shell, and by the shell beach more curved and more posteriorly positioned. Trimusculus pifius is also usually larger than the other 2 species.

Trimusculus pifius differs from the other 2 congener species that occur in South America, T. goesi Hubendick, 1946 , from Mexico to Caribbean, and from T. peruvianus (Sowerby I, 1835) , from Mexico to Chile, in totally lacking radial sculpture in the shell, which is well developed in both. Besides, T. pifius is apparently smaller, as those species easily reach over 10 mm.

Trimusculus goesi has been reported in Brazilian coast (e.g., Rios, 1975), however, on those cases, every time if checked the sample actually was of a dorsal brachiopod valve.