Redescription of Hiatella meridionalis d ’ Orbigny , 1846 ( Mollusca , Bivalvia , Hiatellidae ) from Argentina

The redescription of Hiatella meridionalis (d’Orbigny, 1846) is provided as first attempt to improve the systematics of the genus in the regions of Atlantic and western Pacific. This reanalysis is based on specimens collected in the vicinity of the type localities and is based on detailed morphology of samples that some researches consider a single, wide ranging species. From the morphological characters, the more interesting are: a high quantity of papillae at incurrent siphon; the retractor muscles of siphon divided in two bundles; the small size of the palps; the muscular ring in the stomach; and the zigzag fashion of the short intestinal loops. These characters distinguish the species from the other hiatellids so far examined. Type material of the species was examined, by first time illustrated, and the lectotype is designated.

1994).The dissimilarity regards not only the shells, but also size, as some populations have specimens growing to more than 40 mm, while others the specimens barely reach 10 mm.It is also regards the bathymetry, there are samples collected intertidal, and others in deep waters.Besides, the geographic range of some species is also extraordinary, occurring from the Arctic to the Antarctic seas, through Mediterranean Sea, almost entire tropical areas, with records in the east and west coasts of the Atlantic, and east coast of the Pacific; normally in disjunct populations.With this data in mind, two conclusions are possible: (1) an astonishing environmental complacency of a small IntROductIOn There is considerable confusion in the taxonomy of the Mediterranean, Atlantic and western Pacific hiatellids.As their shells are highly irregular, it is difficult to find conchological characters for resolving the problem.As related below, a more conservative terminology has been applied by several authors, considering every sample as belonging to a single species.Considerably dissimilar specimens identified under the same epithet: Hiatella solida (Sowerby, 1834) or H. arctica (Linnaeus, 1767), are commonly found in collections, and even in the literature (e.g., Rios, bivalve, possessing a fantastic power of dispersion, or (2) a misidentification of conchologically similar separated species.

MAtERIAL And MEtHOdS
A complete list of examined material is presented after Description.The MZUSP specimens, containing individuals of every sizes, were all dissected.The studied material was collected during scallop fishing, as an epizoic on the shell of live scallop Zygochlamys patagonica (King and Broderip, 1832) in grounds 90-130 m depth.The specimens were fixed in 70% ethanol, unrelaxed.They were dissected by standard techniques, with the specimen immerse in fixative under a stereomicroscope.All drawings were made with the aid of a camera lucida.Serial sections of 5 µm of the middle portion of the whole animal were done also using standard histological procedures, stained with Mallory.Voucher material of a previous study (Narchi, 1973), deposited in Museu de Zoologia da USP, was also examined anatomically for basing the comparison and discussion of the presently studied sample, at least with specimens from Brazilian coast.A complement of the morphological knowledge of the Brazilian and other samples will be published elsewhere; however, the current literature (mainly Narchi, 1973) Bastida et al. 1992:696, Ciocco et al. 2005:1272(non Sowerby, 1834).Hiatella arctica : Pouliot, Bourget & Fréchette 1995:280 (non Linnaeus, 1767).
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.Ante- rior 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).
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 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).
Geographic range: Argentina coast.

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 .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.18A-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 .The type material figured in this paper (Fig. 18) and the young specimens  show the tendency of the shell to be flatter, with the umbos weakly more protruded and pointed 20,21), features that are gradually modified to an inlaid umbos and a more obese fashion in larger specimens 22).The two pairs of radial dotted cords can even bear projected spines in some young, 3-4 mm specimens .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 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 10- 14 :
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.

FIguRE 18 :
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.