Functional studies of the anatomy of some neritid prosobranchs

Abstract

The reduced right half of the pallial complex of Nerita tessellata, N. fulgurans, N. versicolor, N. peloronta and Neritina virginea comprises a hypobranchial gland, a vestigial gill and the opening of a duct which bears the same relationship to this gill as that of the left kidney to the functional gill. The duct leads from the region of the genital duct corresponding to the renal section of mesogastropods. The posterior part of the glandular genital duct lies in the visceral mass; its relationships are identical with those of the left kidney. The anterior part develops alongside the rectum within the anterior pallial vein. The left half of the pallial complex of Nerita includes a hypobranchial gland absent in Neritina and Theodoxus. The floor of the mantle cavity of Nerita forms an accessory respiratory surface, receiving blood from the anterior aorta. Its blood spaces communicate with the haemocoel and the parabranchial vein. This vein, also present in Neritina and Theodoxus, drains the mantle skirt. In Nerita its importance increases when the snail is out of water and the mantle cavity is used as a lung. The buccal mass of Nerita fulgurans has the same basic organization as that of Monodonta lineata: differences are related to specializations in the neritid radula requiring greater versatility of movement during feeding. It is protruded by lateral and ventral protractor and buccal depressor muscles originating on the body wall and inserted on the posterior radular cartilages. The horns of the anterior cartilages are orientated to the feeding ground by an array of levator muscles unparalleled in the trochid, and by the buccal fold which also spreads the radula and ensures its maximal sweep. The radular membrane is pulled outward by protractor muscles originating on the posterior cartilages and associated with a cartilage in the posterior wall of the sublingual pouch. Food is gathered on the back stroke of the radula which is drawn into the buccal cavity by a powerful retractor muscle associated with a radular diverticulum. The diverticulum is absent in trochids which have muscles for direct retraction of the buccal mass. The absence of salivary glands in neritaceans may be related to the development of the buccal fold; glandular buccal pouches and sublingual glands may compensate for their loss. The oesophageal glands extend through the anterior oesophagus to the buccal cavity; owing to their separation from the mid‐oesophagus they are unaffected by torsion. The stomach is a voluminous sac, simplified in that the caecum is lost or vestigial and that the major typhosole ends at the junction of the style sac with the proximal region. The gastric shield forms a tooth, very prominent in Neritina, which can protect the openings of the oesophagus and ducts of the digestive gland.

Major adaptive radiation in neritopsine gastropods estimated from 28S rRNA sequences and fossil records

“…Baker 1923;Fretter 1965;Thompson 1980) seem to have only limited value in clade diagnosis. For example, limpets have evolved from coiled ancestors at least three times in Phenacolepadidae, Neritidae (Septaria) and Neritiliidae (an undescribed genus; figure 3); both concentric and paucispiral opercula occur in the Helicinidae, Neritiliidae and Neritidae; the radula lacks its central field in the primitive Neritopsidae as well as in the advanced Neritiliidae; and the diotocardian heart (retention of both right and left auricles) is found not only in the Neritopsidae but also in the Neritiliidae, Neritidae and Ceresinae of Helicinidae (e.g.…”

“…Although the Neritopsina is widely accepted as a distinct clade with a number of apomorphies in anatomy, embryonic development and sperm ultrastructure, much controversy exists concerning the group's relationships with the other major gastropod clades (e.g. Yonge 1947;Fretter 1965;Koike 1985 Van den Biggelaar & Haszprunar 1996;Harasewych et al 1997;Ponder & Lindberg 1997;Sasaki 1998;Winnepenninckx et al 1998). We therefore selected three species as outgroups to represent all the possible sister groups of the Neritopsina (table 1): i.e.…”

Mineralisation in the teeth of the gastropod molluscNerita atramentosa

“…Although the benefit of the sandpaper approach to biomineralisation in individual teeth may be short-lived. this is compensated for by the radula in many molluscs, including Nerita, having a relatively rapid rate of tooth turnover (Runharn 1962, Fretter 1965.…”

“…As in other neritids, the major lateral teeth are considerably enlarged and serve as the main scraping implements (Baker 1923, Fretter 1965, with the working teeth exhihiting extreme wear and being lost prior to adjacent teeth of the same row. Throughout the radula, thc increasing colour observed at the light microscope level corresponds to a relative increase in the amount of sulfur.…”

“…Additional hardening of these teeth is achieved by the incorporation of biomineralised granUles. The less robust marginal teeth exhibit little wear, and arc only responsible for sweeping food into the mouth as the radula is retracted (Fretter 1965, Steneck & Watling 1982. The inner mature marginals arc coloured (and presumably tanned), and mineralised to a level comparable to that which occurs relatively early in the development of the major lateral teeth.…”