Hiatella meridionalis (d’Orbigny, 1846)

Simone, Luiz Ricardo L. & Penchaszadeh, Pablo E., 2008, Redescription of Hiatella meridionalis d'Orbigny, 1846 (Mollusca, Bivalvia, Hiatellidae) from Argentina, Papéis Avulsos de Zoologia (São Paulo) 48 (14), pp. 119-127 : 120-127

Hiatella meridionalis (d’Orbigny, 1846)

( Figs. 1-24 )

Saxicava meridionalis d’Orbigny, 1846:521 (pl. 81, figs. 21-22; published in 1847).

Hiatella solida : Bastida et al. 1992:696, Ciocco et al. 2005:1272 (non Sowerby, 1834).

Hiatella arctica : Pouliot, Bourget & Fréchette 1995:280 (non Linnaeus, 1767).

Types: BMNH 1854.12.4.672, 6 valves (examined, Fig. 18 ). Specimen Figs. 18 A-B here designed as lectotype, the remaining specimens are the paralectotypes.

Type locality: “ Malvinas (Falkland) Is. and Patagonia, Argentina ”.

Redescription

Shell ( Figs. 1-8 , 10, 12 , 18-24 ): Outline somewhat rectangular ( Figs. 2-5 ). Outer surface irregular, whitish, with strong growth lines and concentric undulations. Periostracum yellowish, transparent, thick, usually eroded close to umbos, extending beyond calcified shell, along siphons. Umbos weakly protruded, flat, located between middle and anterior thirds; young specimens with umbos slightly more protruded and pointed ( Figs. 18 E-M). Young specimens sometimes possessing one or two pairs of posterior radial cords, normally interrupted somewhat uniformly, forming successive spines, pointing posteriorly ( Figs. 19, 20 ), or pustules ( Figs. 18G , 21, 23 ), or a mixture of both ( Fig. 22 ); this posterior sculpture disappearing in larger specimens ( Figs. 1-4 , 18 A-D). Ligament external, located posterior to umbos along about 1/7 of shell length ( Figs. 1, 7, 8 , 24 ). Hinge with single short cardinal tooth in each valve, just below umbos, each one with a fold-like platform base ( Figs. 6-8 , 18B, D , 24 ); anterior tooth in right valve ( Figs. 5, 7 ). Gap narrow between both valves in posterior (siphonal) region ( Fig. 6 ). Inner surface whitish, glossy (Figs, 3, 5, 24). Scar of anterior adductor muscle elliptical; located close to ventral-anterior corner of valve, at some distance from shell edge; scar area about 1/20 of valve surface. Posterior scars of posterior adductor muscle and anterior retractor muscle also elliptical, located close to dorsal edge of valves, between middle and posterior thirds of dorsal edge; area about double of anterior scar. Pallial line with shallow sinus ( Fig. 24 ). Mantle: Thin, whitish, of uniform thickness ( Figs. 10, 12 ). Edges of mantle lobes mostly fused, a single pedal open (further that of siphonal aperture), situated in middle-anterior region of ventral edge ( Fig. 12 : po), corresponding to about 1/4 of shell length. Siphon muscular root divided into 2 bands ( Fig. 10 : sm): 1) ventral band narrower, bordering ventral edge of mantle along about half of its length and 1/3 of valve height; 2) dorsal band about half shorter than ventral band, with equivalent width of ventral band, attached to shell just ventral to posterior adductor muscle.

Siphons: Incurrent siphon about half fused along its length to excurrent siphon in basal region ( Fig. 12 ). Outer surface almost entirely covered by periostracum ( Figs. 9 , 10 : pe). Incurrent siphon slightly broader than excurrent siphon; tip bearing several series of small papillae ( Fig. 9 : mp), each papilla with slender, long, filiform base, and weakly expanded, balloon-like tip. Excurrent siphon tip somewhat similar to incurrent one, except for bearing less numerous papillae, restricted to a single series. Both siphons totally separated from each other by a muscular internal septum, started in posterior end of gills ( Fig. 11 ), attached to gills by cilia.

Main muscles and foot: Adductor muscles relatively small. Anterior adductor muscle situated close to ventral-anterior edge of shell ( Figs. 10, 12, 13 , 24 : am), with about 1/25 of valve area. Posterior adductor muscle situated at some distance from posterodorsal shell edge ( Figs. 10, 12, 13 , 24 : pa); about 1.5 times larger than anterior. Foot relatively small (about 1/6 of visceral volume), byssal furrow running longitudinally along ventral surface, on median line, along about 1/4 of ventral edge of visceral mass ( Figs. 12, 13 ). Byssal gland running in dorsal and lateral regions of this furrow ( Fig. 16 : bg). Foot anterior region weakly projected; posterior region broader due to byssal gland ( Figs. 12, 13 ). Pair of anterior retractor muscle of foot narrow, slender ( Fig. 13 : fr); originating dorsal, at some distance from anterior adductor muscle ( Figs. 10, 12, 13 ), origin with about 1/3 of anterior adductor muscle, and separated from that by a distance about 1/3 of shell height; inserting in anterior region of foot. Pair of posterior retractor foot muscles broad and thick, runs straight towards dorsal ( Fig. 13 : fm); origin just anterior and dorsal to posterior adductor muscle ( Figs. 10, 12, 13 ) with area approximately 80% of that of posterior adductor muscle; insertion in posterior end of byssal furrow, as continuation from byssus ( Fig. 13 ).

Pallial cavity: Occupying about 75% of shell inner volume, covering about 85% of lateral surface of visceral mass. Gills length about 90% of that of shell; gills height about 80% of that of shell. Outer demibranch somewhat triangular, anterior end narrow, increasing gradually, becoming broader in its middle region towards posterior, of same width of inner demibranch ( Figs. 11, 12 ). Inner demibranch somewhat rectangular, width uniform along its length ( Figs. 11, 12 ). Anterior half of inner demibranch connected to visceral mass by cilia, posterior half connected to with inner lamella of other inner demibranch forming an anatomical separation between infra- and supra-branchial chambers ( Fig. 11 ) by ciliary connection. Anterior region of inner demibranch not covered by outer demibranch, introduced between both hemipalps ( Fig. 15 ). Food groove running in outer edge of inner demibranch ( Fig. 15 ).

Circulatory system: Heart relatively small ( Fig. 13 : pc) (about 1/15 of visceral mass), positioned between gonad and pair of posterior retractor muscles of foot. Auricles triangular, each connected to ctenidial (efferent) vessel ( Fig. 14 ) and directly to gill at about 1/4 of gill length, in their middle region. These ctenidial veins narrow in both sides anterior and posterior. Ventricle surrounding intestine, anterior aorta dorsal and posterior aorta ventral ( Fig. 14 ).

Digestive system: Palps relatively small (about 1/15 of demibranch area), triangular ( Figs. 11-13 , 15 ). Outer hemipalps slightly larger than inner hemipalps ( Fig. 15 ). Inner surface with 8-10 broad, transversal folds; each fold with rounded ventral end, at short distance from palp ventral edge; folds successively smaller towards distal and posterior. Folds faint in dorsal end at some distance from palp dorsal edge ( Fig. 15 ), forming a smooth inner margin both dorsal and ventral in both hemipalps. Mouth broad and ample, both lips with thick edge; situated somewhat posteriorly to anterior adductor muscle ( Figs. 11 , 15 ). Esophagus broad, with about 1/4 of shell length ( Fig. 16 ), inner surface smooth ( Fig. 17 ). Stomach occupying about 1/3 of visceral volume, covered in both sides by pale green digestive diverticles ( Fig. 16 ). Esophageal aperture into stomach protected in ventral side by a narrow rim ( Fig. 17 : rm), and in dorsal side by tall, long, sigmoid fold, transversally furrowed ( Fig. 17 : gt). This fold extending in both sides, surrounding anterior sides of aperture to digestive diverticles. Gastric shield with about 1/4 of inner gastric area, located in its left-posterior region. Tall fold surrounding gastric shield ventral edge, extending along gastric dorsal wall in level of intestine origin ( Fig. 17 ). Aperture to digestive diverticles multiple, situated around 2 shallow cavities by side of esophageal aperture; these cavities separated from esophageal aperture by gastric transversal typhlosole, and from each other by space equivalent to about 1/4 of gastric width. Between gastric chamber and intestine a sphincter, marked by a narrow layer of circular muscle immerse in gastric walls ( Figs. 16, 17 : sp). Intestine and style sac anatomically combined; both separated internally by a pair of tall longitudinal folds; right fold originating in gastric fold surrounding gastric shield; left fold originating abruptly ( Fig. 17 : ty). Intestine narrowing gradually towards posterior-ventral region of visceral mass; suddenly towards anterior, bordering byssal gland; zigzagging in anterior region of foot, crossing to right side of its first loop, running towards dorsal close to stomach; in this region bearing a curve, running towards posterior, crossing through pericardium, and surrounding dorsal and posterior surface of posterior adductor muscle ( Fig. 16 ). Anus simple, somewhat projected ( Figs. 13 , 16 : an).

Genital system: Gonad pale cream in color, located surrounding visceral structures, including digestive diverticula; filling almost entire visceral sac, except ventral region, at some distance from foot ( Figs. 10, 13 ). No gonoducts detectable. Pair of genital orifice located anterior and close to nephropores, separated from them by a distance equivalent to 1/20 of shell length.

Main ganglia of nervous system: Not seen in details. Pair of visceral ganglia large, close one another, located in ventral surface of posterior adductor muscle, close to adjacent region of posterior foot retractor ( Figs. 11, 13 : vg).

Habitat: Epizoic on the shell of live bivalve Zygochlamys patagonica , between 83 and 189 m depth ( Bastida et al., 1992).

Measurements (length, height and width in mm): MZUSP 61525, #1 ( Figs. 1-5 ): 9.5 by 5.3 by 5.4; #2: 7.9 by 4.5 by 4.2 mm.

Geographic range: Argentina coast.

Material examined: Lectotype and paralectotypes (see above). ARGENTINA; Buenos Aires; Off Mar del Plata, 39º04’S 55º44’W to 39º29’S 56º04’W, MZUSP 61525, 5 specimens (31/viii/1998), MZUSP 61526, 33 specimens, MACN 36596, 326 specimens (05/ix/2001).

DISCUSSION

In the original description of Hiatella meridionalis, d’Orbigny (1846) figured a specimen with a pair of radial, posterior cords, forming somewhat uniform dots. This pattern is not shown in grown specimen, neither in the type specimens ( Fig. 18 ). Nevertheless, the grown specimens tend to have a slightly uniform carina ( Figs. 1 , 18A – the here designed lectotype), forming a slope between the posterior and middle thirds of the valves. However, the pattern of two pairs of dotted cords is exhibited by some young specimens ( Figs. 19-23 ). The flat shape of specimen figured by d’Orbigny (1846) and its size (4 mm sic.) show that the specimen he examined was young. This is corroborated by the exam of the type material ( Fig. 18 ), in such the larger specimens have about 4 mm ( Figs. 18 A-D). The finding of young specimens with similar conchological features of d’Orbigny (1846, pl. 81, fig. 21) specimen, allows that they can be considered the same species ( Figs. 19-23 ). The type material figured in this paper ( Fig. 18 ) and the young specimens ( Figs. 19-23 ) show the tendency of the shell to be flatter, with the umbos weakly more protruded and pointed ( Figs. 18 E-M, 20, 21), features that are gradually modified to an inlaid umbos and a more obese fashion in larger specimens ( Figs. 1-5 , 18 A-D, 22). The two pairs of radial dotted cords can even bear projected spines in some young, 3-4 mm specimens ( Figs. 19-21 ). These conchological patterns have not been found in specimens collected from other localities so far examined.

The anatomical characters of Hiatella meridionalis are similar to those of congener species where the anatomy is known ( Hunter, 1949; Yonge, 1971; Narchi, 1973). It, however, differs from Brazilian sample of H. solida (sensu Narchi, 1973; personal observation), here therefore designed as H. cf solida , in having smaller sized palps, a stomach with a less developed dorsal hood, middle region of intestine not coiled (only performing a zigzag), and mainly by the greater quantity of papillae in the incurrent siphon ( Fig. 9 ). It is important to emphasize that all examined specimens of H. meridionalis and the Brazilian samples of the H. cf solida have an equivalent number of siphonal papillae inside each sample, which demonstrates that such character is not highly variable and a suppose important character for species distinction. As only preserved specimens were available, the study on the inner surface of the stomach is somewhat precluded. Although some differences are detectable among the inner surface of examined specimens with some known hiatellids ( Purchon, 1958; Narchi, 1973), e.g., absence of clear sorting areas, a deeper analysis of the differences is not performed here.

Hiatella meridionalis occurs much deeper, around 110 m depth, than the Brazilian H. cf solida , which is intertidal. H. meridionalis differs from Hiatella sp. ( Yonge, 1971) in having more developed anterior and posterior retractor muscles of the foot, by smaller adductor muscles, by larger pedal aperture in mantle, and by different fashion of siphonal papillae. One interesting character of H. meridionalis is the pair of retractor muscles of siphons divided into 2 bundles ( Fig. 10 ). This feature is not usually shown in the siphon-bearing bivalves, but its significance in taxonomy is so far speculative, as it needs to be proved to occur in other hiatellids.

Hiatella meridionalis still differs anatomically from species of the genus Saxicavella Fischer, 1878 ( Scott, 1994) in having longer siphons, in lacking papillae surrounding the siphonal base, foot lacking heel, less developed pallial musculature and longer outer demibranch. A deeper additional analysis on the morphological differences among the hiatellid genera is provided by Yonge (1971).

A revision of the genus Hiatella is still in progress, and several morphological differences have been found among geographically distant samples of the genus, indicating that there are actually several species. This paper brings the base for this revision, with a more complete description of one of the species, in MZUSP

do

Pnblicações

de

Seção

such further studied species will be, gradually, added. Although even the artificial transportation have been advocated for explaining the wide range of this bivalve (e.g., Orensanz et al., 2002), in a single-species scenario, the preliminary results have shown that samples from different regions constitute isolated species. as explained above, in this paper Hiatella meridionais is, then, taxonomically defined. The remaining species will be defined in complementary papers.

Figures

FIGURE 18: Hiatella meridionalis type material, 6 valves successively shown in external and internal views; specimen A-B here designated as lectotype; the remaining specimens are the paralectotypes. Courtesy of the Natural History Museum, London. Scale = 1 mm.

FIGURES 1-9: Hiatella meridionalis: 1-5) a specimen MZUSP 61525; 1) dorsal view; 2) left valve, outer view; 3) same, inner view; 4) right valve, outer view; 5) same, inner view; 6) another specimen of same lot, ventral view, main concern to inner view of hinge; scale = 2 mm; 7) detail of hinge, right valve, same specimen of figs 1-5; 8) same, left valve; 9) detail of siphonal area showing siphonal papillae (mp), some papillae and periostracum (pe) are lost and damage because of dissection. Scales = 1 mm.

FIGURES 10-14: Hiatella meridionalis anatomy: 10) left view of an animal with left valve just removed; 11) whole specimen, ventral view, left mangle lobe partially removed, gills deflected and partially separated from each other in posterior region, showing a portion of suprabranchial chamber, siphons and posterior adductor muscle seen by their ventral base; 12) whole specimen, left view, left valve, periostracum and part of left mantle lobe removed; 13) isolated visceral mass, left view, most structures seen by transparency; 14) detail of pericardium region, left view, left pericardial wall removed. Scales = 1 mm.

FIGURES 15-17: Hiatella meridionalis anatomy: 15) detail of palps region, ventral view, palps partially deflected; 16) whole digestive system, left view, seen as in situ, some adjacent structures also shown; 17) stomach, dorsal view, dorsal gastric wall partially removed and deflected to right, esophagus and intestine partially opened longitudinally. Scales = 0.5 mm.

FIGURES 19-24: Hiatella meridionalis young specimens MACN 36596 showing two pairs of posterior radial cords with spines; 19- 20) specimen #1, (broken) right and left valves respectively; 21) specimen #2, left valve; 22) specimen #3, right valve and a portion of left valve; 23) specimen #3 in situ on scallops; 24) left valve, inner view, muscular scars artificially dotted, MZUSP 61525. Scale = 2 mm.