Myoforceps aristatus ( Dillwyn, 1817 )

Simone, Luiz Ricardo L. & Gonçalves, Eric Pedro, 2006, Anatomical Study On Myoforceps Aristatus, An Invasive Boring Bivalve In S. E. Brazilian Coast (Mytilidae), Papéis Avulsos de Zoologia (São Paulo) 46 (6), pp. 57-65 : 58-63

Myoforceps aristatus ( Dillwyn, 1817)

( Figs. 1-18 )

For synonymy see Turner & Boss (1962:106). Complement:

Lithophaga (Myoforceps) aristata : Dall, 1898:800; Turner & Boss, 1962:105-108 (pls. 69-72); Abbott, 1974:434 (fig. 5083); Merlano & Hegedus, 1994:52 (fig. 52).

Lithophaga aristata : Morton, 1993:609-619 (figs. 1-6 + pls. 1-2); Redfern, 2001:202; Valentich-Scott & Dinesen, 2004:343-344 (figs. 9-11).

Types: See information by Turner & Boss (1962:106).

Description

Shell ( Figs. 1 -10): Description given by Turner & Boss (1962) adequate. Characterized by posterior extensions crossing, with left valve possessing an inferior projection ( Figs. 2, 4, 6 ), and right valve a superior projection ( Figs. 3, 5 ), like coring fingers ( Figs. 1, 7, 8 ), however, some specimens are contrary (Figs. 9, 10). No clear pallial sinus detectable.

Main muscle system ( Figs. 15 , 16 ): Anterior adductor muscle dorso-ventrally flattened, antero-posteriorly elongated (length about 1/5 of total shell length); located in ventral-anterior corner of valves, close to valve edges. Posterior adductor muscle relatively small, about half of anterior adductor muscle size; somewhat rounded in cross-section; located far from valve edges, positioned between middle and posterior thirds of animal length, and between dorsal and middle thirds of animal height. Pair of anterior foot retractors thick, originating in middle level of anterior region of valves, in an area equivalent to 3/4 of that of anterior adductor muscle and relatively far and totally detached from this adductor muscle; extending toward posterior and ventral up to pedal base, approximately in middle region of ventral animal edge. Pair of posterior foot retractors almost symmetrical to anterior retractors; originating just dorsal to posterior adductor muscle in an area equivalent to 1/4 of that of this adductor muscle; extending toward anterior and ventral, inserting in pedal base just posterior to anterior foot retractor insertion.

Foot and byssus ( Figs. 11, 15 ): Foot relatively small, of about 1/5 valve length, and approximately 4 times longer than wide. Foot base located about in middle region of ventral surface of visceral mass. Byssal furrow extending all along foot ventral surface, in median line, relatively deep (about half of foot thickness). Byssus not seen.

Mantle ( Figs. 11, 13 ): Mantle edges of both lobes mostly free from one another. Three folds of mantle edge similarly sized, circular in section, well-separated from one another. Siphonal region restricted to posterior end, marked by gradual muscular enlargement of mantle thickness. Incurrent and excurrent siphons separated by thick transverse connection between mantle lobes, of about 1/6 animal length. This connection possessing a flap extending toward anterior ( Fig. 13 : uf), projecting inside infrabranchial pallial cavity, dorso-ventrally flattened, narrowing gradually to bluntly pointed tip, length equivalent to that of siphons. Inner mantle edge folding abruptly, becoming larger and wider from middle level to incurrent siphonal base ( Fig. 13 : pf); anterior end of this enlargement shortly projecting inwards, tip rounded; remaining region of this enlargement uniform; in siphonal base abruptly expanding, becoming part of incurrent siphonal wall. Incurrent siphon anatomically open along ventral edge, but edges of both sides contacting each other, producing a tube. Incurrent siphonal edges smooth and thick. Excurrent siphon totally fused with incurrent siphon, being only internally separated, as described above, by septum-like, transverse branchial septum; dorsal wall totally fused, anatomically forming a tube. Transverse, low flap in dorsal and lateral regions of base of excurrent siphon, at same level of posterior end of branchial septum. Excurrent siphonal tip with smooth edges. Pairs of pallial glands that secrete a calcium-binding mucoprotein for boring into calcareous ( Jaccarini et al., 1968) whitish; anterior pair located in anterior end of mantle, at short distance from mantle edge, just anterior to origin of both anterior retractor muscles of foot; posterior pair located in posterior-ventral corner, at short distance from mantel edge ( Fig. 16 : mg)

Gonad filling most regions of mantle lobes, being very thick dorsally ( Figs. 11, 12 ), becoming thinner ventrally, reaching region close to mantle edges (more details below).

Pallial cavity ( Figs. 11-14 , 18 ): Palps relatively small, of about 1/20 valve area; antero-posteriorly elongated. Palp external surface smooth. Palp internal surface ( Fig. 14 ) with about 20 transverse folds; more distal folds, close to free end, gradually becoming oblique and longer toward proximal end; proximal to connection of hemipalps folds gradually becoming smaller, suddenly disappearing at a distance equivalent to half of hemipalp free region. Inner palp folds with rounded ends, at some distance from palp edges, producing smooth margin. With a smooth furrow between hemipalps. Gills (ctenidia) occupying about 3/4 of pallial cavity ( Fig. 11 ); inner and outer demibranchs of approximately same size; gill anterior end just posterior to origin of anterior retractor muscle of foot, rounded; gill posterior end with about same shape as anterior end, at level posterior to posterior adductor muscle. Posterior end of gill connected to middle region of posterior adductor muscle ventral surface by short, weakly muscular, suspensory stalk ( Figs. 13, 15 , 18 : gp). Gill filaments connected with each other by aligned rows of cilial junctions. Ventral edge of each demibranch bearing shallow food groove ( Fig. 12 : fg). Inner lamella of inner demibranch and outer lamella of outer demibranch of approximately same size, connected respectively to visceral sac and to mantle via cilial junctions; outer lamella of inner demibranch and inner lamella of outer demibranch about twice longer than inner lamella if inner demibranch and outer lamella of outer demibranch, connected to dorsal structures via tissue junctions ( Fig. 12 ). No clear vessels present in demibranch extremities, except in region between demibranchs connected to pericardium ( Fig. 12 : pc). No clear connection between lamellae of each demibranch. Flap extending from septum between siphons (described above) ( Fig. 13 : uf) lying between posterior regions of left and right gills.

Visceral mass ( Figs. 11, 12, 15 , 16 ): General form somewhat triangular (foot positioned as ventral angle). Both pairs of foot retractor muscles forming ventral bor- der of visceral sac ( Fig. 15 : fm, fr). Digestive diverticula (gland) greenish-beige in preserved specimens, occupying almost entire middle and anterior thirds of visceral sac. Gonad occupying dorsal region of middle and anterior thirds of visceral sac, extending throughout mantle lobes as described above. Reno-pericardial structures (described below) occupying posterior third of visceral sac. Visceral integument poorly muscular.

Circulatory and excretory systems ( Figs. 15 , 16 ): Heart occupying little more than half of entire reno-pericardial volume. Auricles connected directly to gills by pericardium in their middle-posterior quarter ( Fig. 12 : pc). Each auricle weakly triangular, its posterior surface with irregular, relatively large lobes ( Fig. 15 : au); anterior surface simple. Auricular connection to ventricle narrow and lateral to ventricle. Ventricle surrounding intestine, about as long as pericardium. Kidneys solid, whitish, located in lateral surfaces of reno-pericardial posterior region. Kidneys amply opened to supra-branchial chamber, by long fissure of somewhat same length as kidney; inner folds of renal glands exposed through renal aperture.

Digestive system ( Figs. 16, 17 ): Palps described above ( Fig. 16 ). Mouth elliptical, located at short distance from median end of palp folds; flanked by smooth surfaces, anterior and posterior tips relatively thick ( Fig. 14 : mo). Esophagus short and wide, of about 1/10 shell length, passing between anterior pedal retractor muscles, close to their anterior end; esophagus totally free from anterior adductor muscle. Esophageal inner surface smooth. Transition of esophagus and stomach marked by oblique, deep furrow ( Fig. 17 : gt), located in ventral and left surfaces; inner surface of this furrow with low, narrow, well-spaced folds parallel to furrow longitudinal axis. Stomach occupying about 1/3 of visceral sac volume and 1/4 of total animal length; located at anterior region of visceral sac, below umbos, just posterior and dorsal to esophagus ( Fig. 16 : st); general form elliptical. Inner surface ( Fig. 17 ) mostly smooth; low, broad, arched fold located at left side ( Fig. 17 : gf), as continuation of posterior edge of furrow located between esophagus and stomach, anteriorly low, posteriorly weakly taller and rounded, close to left duct to digestive diverticula. Small sorting area in dorsal-right surface ( Fig. 17 : sa), opposed to previous described fold; composed by oblique, low, narrow folds close to each other. A pair of narrow ducts to digestive diverticula ( Figs. 16, 17 : dd), located at anterior region of gastric ventral surface, one on each side. Gastric shield of about 1/4 gastric surface ( Fig. 17 : gs); located at left, slightly dorsal and posterior to left duct to digestive diverticula. Intestine and style sac totally fused and nearly indistinguishable; both separated from gastric chamber by low, transverse fold ( Fig. 17 , preceding ss). Style sac extending directly toward posterior, narrowing gradually, reaching anterior surface of posterior adductor muscle; length slightly longer than that of stomach. Intestine continuing after style sac end, marked by abrupt 180° curve ( Fig. 16 ); extending dorsally, slightly at right along style sac surface; close to stomach posterior surface, performing wide loop to left, returning toward posterior, extending more dorsal and away from preceding loop; passing through pericardium, and along dorsal surface of posterior adductor muscle. Anus a low, simple, small aperture located at anterior region of adductor muscle ventral surface ( Figs. 13 , 16, 18 : an).

Genital system: Not seen in total detail. Gonad, as described above, covering dorsal region, reaching mantle lobes close to mantle edges ( Figs. 11, 12, 15 : go). Color cream or brown in preserved specimens. No genital apertures observed.

Central nervous system: Cerebral and pedal ganglia not seen in detail. Visceral ganglia ( Fig. 18 : vg) located just anterior to ventral surface of posterior adductor muscle; widely fused to one another in median line ( Fig. 13 ); volume approximately 1/10 of posterior adductor muscle. Cerebro-visceral connectives relatively thick, extending through digestive diverticula.

Measurements (in mm): MZSP 48274 #1 ( Figs. 1-4 ): 14.8 length by 5.1 height; MZSP 48275 #1 ( Figs. 5-7 ): 11.1 by 4.2. FMNH 311641: 24.1 by 17.7 (Figs. 9, 10).

Distribution: North Atlantic: from Portugal to Senegal; from North Carolina to Venezuela, including the Gulf of Mexico. Now introduced to southeastern Brazilian coast. For occurrence in Pacific Ocean see Turner & Boss (1962).

Habitat: Boring calcareous substrata, mainly shells of other mollusks, from intertidal to 5 m depth.

Material examined. UNITES STATES OF AMERICA. Florida. Florida Keys, Monroe, Looe Key coral reef, 8 m depth, 24°32.8’N 81°24.8’W, 1 specimens, FMNH 311641 (Sta. FK-260; Bieler & Mikkelsen col. 10/viii/1999). BRAZIL. Rio de Janeiro; Búzios, Ossos Beach, MZSP 48275 , 1 specimen (Simone et al. col., 19/iii/2005); Arraial do Cabo, Porcos Island , 22°57’35.5”S 41°59’47.7”W, MZSP 48276 , 7 specimens [Simone et al. col., 19/iii/2005, in Thais haemastoma (Linné, 1767) ]. São Paulo; Ubatuba, MZSP 48274 , 10 specimens [Iris L.A. Álvares col.; iv/2005, in Nodipecten nodosus (Linné, 1758) ] .

Figures

FIGURES 1-8. Myoforceps aristatus shells: 1-4) MZSP 48274 #1 (from Ubatuba, SP), 1) dorsal view; 2) left view; 3) right valve, inner view; 4) left valve, inner view; total length = 14.8 mm; 5-8) MZSP 48275 #1 (from Arraial do Cabo, RJ); 5) right view; 6) left view; 7) ventral view; 8) posterior view, showing characteristic crossed posterior projections; total length = 11.1 mm; 9-10) FMNH 311641 (from Florida), ventral and right views, specimen with contrary crossed posterior projections, total length = 24.1 mm.

FIGURES 11-15. Myoforceps aristatus anatomy: 11) whole right view, right valve and mantle lobe removed (except for a short portion related to the siphons); 12) transverse section in middle region of gill, with some adjacent structures also shown; 13) posterior region, right view, with special emphasis on siphonal structures, with excurrent siphon sectioned longitudinally and most right structures removed, pallial flap covering siphons removed; 14) right palp, ventral view, both hemipalps deflected to show inner surfaces; 15) whole right view, with most right structures removed, and special emphasis on visceral structures and main muscles seen as in situ. Scales = 1 mm.

FIGURES 16-18. Myoforceps aristatus anatomy: 16) whole right view, semi-diagrammatic representation of digestive tract and topology of main muscles, mucous gland (mg) and pericardium; 17) stomach right view, longitudinal section along gastric right wall; 18) ventral region of posterior adductor muscle (pa), with most integument and right gill removed, and topology of some adjacent structures shown. Scales = 1 mm.