Mytilida, Ferussac, 1822

Order Mytilida View in CoL Superfamily Mytiloidea

Family Mytilidae Genus Brachidontes Swainson, 1840

Brachidontes ynous new species ( Figs. 49-50 View Figure 49 View Figure 50 ) https://zoobank.org/ AE6AA3AF-E95F-4664-B185-10EF4FC6BC4B

Types: Holotype MZSP 165671 View Materials . Paratypes: MZSP 105355 View Materials , 40 View Materials spm from type locality. BRAZIL. Espírito Santo ; Trindade Island (ProTrindade Project),Farrilhões, 20°31′22.4″S 28°19′52″W, MZSP 109572 View Materials , 25 View Materials spm (02.vi.2012), Tartarugas, 20°31′01.3″S 29°17′56.9″W, MZSP 115081 View Materials , 7 View Materials spm (Abbate col., 2013), Enseada da Cachoeira , 20°30′57.1″S 29°20′15.2″W, MZSP 108114 View Materials , 3 View Materials spm (J. MendonÇa col., 2012), Ponta do TÚnel, 20°31′36.8″S 29°18′14.3″W, MZSP 109914 View Materials , 3 View Materials spm (J. MendonÇa col., 2012) GoogleMaps .

Type locality: BRAZIL. Espírito Santo ; Trindade Island, Enseada do Lixo , 20°31′43.5′S 29°19′28.1″W [17.ii.2012] .

Diagnosis: Endemic species from Trindade Oceanic Island, living subtidal, isolated. Shell smaller than 10 mm. Color mostly light. Sculptured by relatively uniform, delicate, numerous radial cords. Hinge with 4 teeth anterior to ligament, 12-15 taxodont-like teeth posterior to ligament.

Description: Shell up to 10 mm, outline rather trapezoid-mytiliform. Anterior region 2-3 times smaller than posterior region; ~1.5 times longer than heigh; ~2 ( Fig. 49F View Figure 49 ) to ~1.3 ( Fig. 50E, K View Figure 50 ) times longer than wide. Color beige to light brown ( Figs. 49B, G, F View Figure 49 , 50A, B, F View Figure 50 ), with some few reddish ( Fig. 50L, M View Figure 50 ) and brownish specimens ( Fig. 50D, E View Figure 50 ). Umbones weakly protruded, subterminal, anterior.Sculpture relatively uniform,narrow,numerous, radial cords; located close from each other (interspaces smooth, ~ ⅓ of cords width); from umbo up to posteri- or, ventral and anterior edges, forming weak crenulate borders ( Figs. 49A, H View Figure 49 , 50G, H View Figure 50 ); cords slightly wide in some specimens, ~40 per valve ( Fig. 49B, G View Figure 49 ), up to very narrow, ~60 per valve ( Fig. 50F View Figure 50 ); cords rarely dichotomic, except for those located in strip between umbo and corner of ventral-posterior edges ( Figs. 49B View Figure 49 , 50B, D, L View Figure 50 ). Anterior edge narrowly rounded, sometimes as beak anterior to umbo ( Fig. 50 View Figure 50 F-H, J), sometimes as soft- er slope ( Figs. 49A, B, G, H View Figure 49 , 50A, B, D, L View Figure 50 ); dorsal edge straight; posterior edge amply rounded; ventral edge straight ( Figs. 49A, B, G, H View Figure 49 , 50A, B, D View Figure 50 ) to weakly convex ( Fig. 50F, G, H, L View Figure 50 ). Inner surface glossy, translucent. Hinge with ligament occupying ~ ⅓ of its length, from umbo beak towards posterior ( Fig. 49A, D, H, J, G, H View Figure 49 ); set of 4 teeth located anterior to ligament, weakly oblique, slightly uniform, except for more posterior tooth, being larger in right valve ( Figs. 49C View Figure 49 , 50J View Figure 50 ), and very oblique in left valve ( Figs. 49I View Figure 49 , 50I View Figure 50 ); set of 12-15 aligned, taxodont-like teeth posterior to ligament, being anterior teeth slightly smaller than posterior teeth ( Figs. 49D, J View Figure 49 , 50G, H View Figure 50 ). Scars of adductor muscles very weak; scar of anterior adductor muscle antero-posteriorly elongated, located in ventral region of anterior edge ( Figs. 49A, H, J View Figure 49 , 50G, H, I View Figure 50 ), with ~¹/₃₀ of inner valve area; scar of posterior adductor muscle united to that of pair of posterior retractor muscles of foot, located from posterior region of umbonal cavity up to posterior region of hinge, both occupying ~¹/₁₀ of inner valve area. Pallial line simple.

Mantle lobes (ml) translucent, except for middle and dorsal regions, fulfilled by whitish gonad ( Fig. 49L View Figure 49 : go). Mantle edges (mb) relatively thick, unpigmented; fusion between both lobes only between incurrent (ih) and excurrent (ex) canals ( Fig. 49M View Figure 49 : un); both canals lacking siphons or papillae, only simple edges ( Fig. 49M View Figure 49 ). Gill (gi) very large, almost as large as pallial cavity; inner (id) and outer (od) demibranchs of equal length and width ( Fig. 49K, L View Figure 49 ). Foot very small, peduncle-like ( Fig. 49K View Figure 49 : ft). Anterior adductor muscle from flattened to elliptic in section ( Fig. 49K, L View Figure 49 : aa), located in antero-ventral region. Posterior adductor muscle large, located as posterior end of visceral mass ( Fig. 49 View Figure 49 K-M: ap). Pair of anterior retractor muscles of foot ( Fig. 49K View Figure 49 : ar) narrow, long, ~ ⅓ of visceral mass length, running from foot base up to umbonal cavity, flanking ventral surface of local visceral mass. Pair of posterior retractor muscle ( Fig. 49K View Figure 49 : rr) with insertion as large as anterior adductor muscle, just anterior to it. Strong cruciform-like pallial muscle ( Fig. 49M View Figure 49 : pm) in region between incurrent and excurrent muscle.

Etymology: The specific epithet is a Latinization of the native Guarani word yno′õ, meaning island, or insular, an allusion to the endemism of the species to the Trindade oceanic island.

Distribution: Endemic from Trindade Island.

Habitat: Under rocks, from intertidal up to ~ 16 m. Shell usually covered by incrustations, rare specimens with valves exposed.

Measurements (respectively L, H in mm): Holotype ( Fig. 49 View Figure 49 A-J): 8.4 by 5.5. Paratypes: MZSP 105355#1 ( Fig. 50 View Figure 50 A-C): 8.3 by 5.5; MZSP 105355#2 ( Fig. 50 View Figure 50 D-E): 6.3 by 4.4; MZSP 109572#1 ( Fig. 50 View Figure 50 F-K): 7.6 by 6.2; MZSP 109572#2 ( Fig. 50 View Figure 50 L-M): 7.6 by 5.6.

Additional material examined: BRAZIL. Espírito Santo; Trindade Island (ProTrindade Project), Andradas , 20°30′50″S 29°18′20″W, MZSP 101235 View Materials , 3 View Materials spm (J. MendonÇa col., 24.vii.2011) GoogleMaps ; Farrilhões, 20°31′28.9″S 29°19′34.8″W, MZSP 104352 View Materials , 2 View Materials spm (Abbate col., 16.ii.2012), MZSP 104445 View Materials , 1 View Materials spm (Abbate col., 31.i.2012), MZSP 104419 View Materials , 3 View Materials spm (Abbate col., 21.ii.2012), Enseada do Lixo,20°31′43.5′S 29°19′28.1″W, MZSP 105356 View Materials , 6 View Materials spm (17. ii.2012), MZSP 105376 View Materials , 25 View Materials spm (21.ii.2012), MZSP 108238 View Materials , 1 View Materials spm, Ilha da Racha , 20°30′26.5″S 29°20′48″W, MZSP 108108 View Materials , 5 View Materials spm, Calheta, 20°30′26.1″S 29°18′44.2″W, MZSP 108122 View Materials , 12 View Materials spm, 20°30′37.6″S, 29°18′28.1″W, MZSP 109617 View Materials , 3 View Materials spm (12.vi.2012), 20°30′18.72″S 29°18′31.67″W, 16.7 m, MZSP 162509 View Materials , 1 View Materials spm (24.v.2022), MZSP 121639 View Materials , 7 View Materials spm (26.x.2014), Ponta do Nordeste , 20°29′46.4″S 29°20′35.4″W, MZSP108136 View Materials , 4 View Materials spm,Parcel dasTartarugas, 20°31′01.3″S 29°17′56.9″W, MZSP 108398 View Materials , 13 View Materials spm (11. vii.2012), Enseada do Meio , 20°29′32.3″S 29°20′32.6″W, MZSP 108409 View Materials , 9 View Materials spm, Ponta do TÚnel, 20°31′36.9″S 29°18′14.3″W, MZSP 108533 View Materials , 2 View Materials spm (19.vi.2012), Ponta Norte, 20°29′18.7″S 29°20′18.3″W, MZSP 162608 View Materials , 5 View Materials spm, MZSP 162499 View Materials , 6 View Materials spm (26.v.2022), Praia do Noroeste , 20°29′46.4″S 29°20′35.4″W, MZSP 162607 View Materials , 7 View Materials spm, MZSP 162580 View Materials , 8 View Materials spm (28.xi.2017), MZSP 162595 View Materials , 2 View Materials spm (06. viii, 2018), 9 m, MZSP 162538 View Materials , 15 View Materials spm (30.vii.2018), Orelhas, 20°29′40.2″S 29°20′32.9″W, MZSP 162597 View Materials , 1 View Materials spm (07.viii.2018), MZSP 121604 View Materials , 5 View Materials spm (01.xi.2014), MZSP 162592 View Materials , 8 View Materials spm (06.xii.2017), 9 m, MZSP 162529 View Materials , 1 View Materials spm (08. viii.2018) GoogleMaps ; Martin Vaz Island , 20°31′36.9″S 29°18′14.3″W, MZSP 108429 View Materials , 3 View Materials spm (30.vi.2012) GoogleMaps .

Remarks: Despite the Brachidontes that occur in Brazilian coast are far in being taxonomically well-known, the morphology, conchology and lifestyle of B. ynous are sufficiently different to be convincing that it is a new species. It appears to be endemic of Trindade oceanic island, a very remote place, ~ 2,000 km off the coast of Espírito Santo. It is the only species that does not occur in aggregates, i.e., it does not make colonies covering rocks like other local species do. On contrary, it is usually found isolated, on the hidden surface of the rocks. Also, it differently occurs much deeper, up to ~ 15 m, while the other congener species are intertidal. Three species of Brachidontes are reported to Brazilian coast ( Rios, 2009), but mostly possibly they are all misidentifications (personal observation), two of Caribbean species [ B. exustus ( Linné, 1758) and B.solisianus (d′Orbigny, 1842) (presently in the genus Mytilaster Monterosato, 1884 )], and one of an Argentinian species [ B. rodriguezii (d′Orbigny, 1842) ]. Sometimes, the Argentinian B. darwinianus (d′Orbigny, 1842) is also reported to Brazil (e.g., Rios, 1975). All these species have some synonyms, but only 2 have type locality in Brazil [ Mytilus exiguus Dunker, 1875 – Santa Catarina; and M. janeirensis Dunker, 1866 – Rio de Janeiro], both considered synonym of M. solisianus . From these species, the only one that have the richness of teeth in the hinge is B. exustus (Simone et al., 2015: figs. 55-58), as well as the Caribbean B. modiolus ( Linné, 1767) , a very similar species. Brachidontes ynous differs from these two species in being much smaller (they easily reach 30 mm, while B. ynous rarely reach 10 mm), by more rounded outline (they are much more elongated), in lacking yellow pigment, in having the radial sculpture much more dense (while the sculpture of those species have wid- er interspaces), and by the anterior set of teeth being curved, instead of forming a straight platform with 1-2 more teeth, projected beyond the umbonal level. The other species lack these teeth or have them much less developed; additionally, they are dark-brown pigment- ed, with coarser sculpture.