Observations on the Shells of Some Fresh-Water Neritid Gastropods from Hawaii and Guam1 Geerat J. VERMEJJ Department of Biology, Yale University Abstract Observations on the fresh-water neritid prosobranch gastropods Neritina vespertina Sowerby 1849 and N. granosa Sowerby 1825 from Hawaii, and N. pul­ /igera conglobata von Martens 1879 and Septaria porce/lana (Linnaeus 1758) from Guam, have yielded a qualitative correlation between clinging ability of the animal and the degree of development of limpet-like shell characters. The hypothesis is put forth that the granular ornamentation on the shell of N. granosa, and possibly the presence of egg capsules on the shells of many fluviatile neritids (notably N. pu/ligera conglobata and S. porcellana) may create turbulence and minimize the effects of the strong current in which the animals live. Methods Shell dimensions were measured to the nearest tenth millimeter with Vernier calipers. Length was taken as the greatest distance from the apex to a point on the outer lip and usually coincides with the greatest linear dimension of the shell. Width is the greatest distance parallel to the outer edge of the parietal septum. Height is the greatest distance from a point on the dorsal surface to the plane of the opening of the shell measured perpendicular to the length and width dimensions. Attempts at quantitatively measuring the force by which the animal clings to the substratum and the resistance against shear were not successful, but the quali­ tative differences in these properties between the various species are striking. Names for the species discussed in this paper have been taken from Baker (1923), Kira (1962), and Rabe (1964), and have been confirmed and augmented by Drs. E. A. Kay of the University of Hawaii and H. K. Mienis of the Zoologisch Museum in Amsterdam. Observations I. Neritina vespertina Sowerby 1849. The shell of this species is superficially 1 Contribution No. 340, Hawaii Institute of Marine Biology, and Contribution No. 7, Micronesian Institute for the Natural Sciences, University of Guam. Micro 11csica 5 (1) : 155-162, 1969 (July ), 156 Micronesica much like that of the slipper limpet Crepidula, but the apical portion of the shell is produced into anterior and posterior wing-like extensions. Ornamentation consists of inconspicuous growth lines, but corrosion often causes the shell to be deeply pitted in the apical region. The shelf or parietal septum covers somewhat more than half the shell opening. Its outer margin is convex and smooth anteriorly and posteriorly, but concave and slightly serrated in the middle. The operculum is more or less typical of other Neritidae, but its abapical margin is uncalcified and membranous. Data presented in Table I indicate that the width to length (w/1) and height to length (h/1) ratios may increase with length. Table 1. Measurements of Neritina vespertina from Wainiha River, Kauai w h w/1 h/1 19 .5 17 .1 6.2 0.877 0.318 20 .7 18 .2 6.7 .831 .324 22 .9 21.0 7.3 .917 .318 24 .2 22.2 8.8 .917 .364 26.5 27.4 9.2 1.04 .347 26.6 26 .6 9.2 1.00 .347 28.0 27 . I 9.4 0.968 0.336 Specimens of N. vespertina were collected on the upper surfaces of large basalt boulders in a fairly gentle current just above the mouth of the Wainiha River on Kauai, Hawaii, with no other gastropods present. The clinging ability of the animal is considerable, but resistance to a shearing force is not great. Upon being removed from the substratum, the snail retracts slowly into its shell, the abapical margin -of the operculum jutting out beyond the plane of the opening of the shell in the process. When the animal is fully retracted, the plane of the operculum is flush with that of the adjacent portion of the parietal septum. 2. Neritina granosa Sowerby 1825. The shell is rather like that of N. ves­ pertina, but the apex is more prominent and located more posteriorly. The most striking feature of the shell is the surface ornamentation, which consists of closely set, rather low granules or pustules and irregularly placed concentric riblets. Small individuals and the older portions of larger specimens have rather ill-defined granules and a smooth outer margin, but the concentric riblets are well developed. In older individuals granules and riblets are prominent, especially near the growing margin. In these specimens the outer margin of the shell, particularly the anterior and pos­ terior edges, are distinctly crenulated. Most specimens have the concentric riblets worn away on the dorsal surface just abapical of the outer margin of the septum and in areas near the apex. The septum is similar to that of N. vespertina except that the outer margin is smooth throughout. The operculum of N. granosa has a wider abapical membra­ nous portion than in N. vespertina. Vol. 5. July 1969 157 Data presented in Table 2 indicate that the width to length ratio decreases with increasing length (p = 0.003 by the U-test when comparing shells greater than 27.o mm in length with smaller ones), but no trend can be observed in the height to length ratio (p = 0.223). Table 2. Measuremtns of Neritina granosa from Kaluanui River, Oahu w h w/1 h/1 22.9 21.0 8.7 0.917 0.380 25.1 22 .8 10 .0 .908 .398 26 .7 22.8 9.1 .854 .341 26.7 25 .2 9.6 .954 .359 26.9 22.8 9.3 .845 .346 26 .9 24.0 9.5 .892 .353 27 .8 23.0 10 . 1 .827 .363 27.9 24.5 10.4 .878 .373 28.5 25.2 10.4 .884 .365 29.6 24 .6 10 .8 .831 .365 30.2 26 .5 10 .6 .877 .351 30.4 27 .1 10 .8 .892 .355 30.7 27.6 12 .8 0.899 0.417 Specimens of N. granosa were collected below Sacred Falls in the Kaluanui River on Oahu, Hawaii; no other gastropods were present. The species appears to be markedly rheophile, for all specimens seen clung to surfaces of basalt boulders -directly exposed to the full force of the swift current, and not to surfaces where even a slight degree of shelter was available. About 80 % of the more than 40 snails observed in situ were oriented with the posterior margin pointed toward the streambed. The force with which N. granosa clings to the substratum and the resistance to shear­ ing are qualitatively much greater than in N. vespertina. Opercular movements and positions during and after retraction of the animal into its shell are similar in the two species. 3. Neritina pulligera (Linnaeus 1767) f. conglobata von Martens 1879. The shell of this species is less limpet-like than in the foregoing two species, being pro­ portionately higher (Tables 3 and 4) and having a somewhat more restricted aperture. The plane of the parietal septum lies at a greater angle to the plane of the opening of the shell than in the other species discussed here, and its juncture with the main portion of the shell is more rounded. The external surface of the shell is smooth except for the presence of irregular growth lines. All specimens observed, however, were covered with egg cases of their own species, particularly on areas of the dorsal surface closest to the substratum to which the animals cling. The presence of these capsules imparts a granular texture to the external surface of the shell. The outer edge of the septum is weakly serrated throughout and bears a shallow depression in the middle. The operculum functions ;is in N. vespertina, but it is ·essentially calcified throughout. 158 Micronesica Table 3. Measurements of Neritina p ulligera conglobata fro m swift current, Tarzan River, Guam. w h w/1 h/1 13 .3 11 .1 7.5 0.835 0.564 13.4 11.2 7.4 .836 .545 14.6 12 .0 7.5 .822 . 513 16 .8 13.1 8.5 .774 .506 17. I 13 .4 8.5 .784 .497 17 .6 12 .8 9.4 .727 .540 18.6 14.6 9.8 . 785 .527 19.3 13.7 9.5 .710 .492 19.7 14.8 10 .0 .752 .513 20.0 14.0 9.4 .700 .470 20.0 14.9 10.3 .741 . 511 21.1 15.0 10.1 .711 .479 21.2 14 .9 10.1 .703 .476 21.6 15.3 10.3 .709 .477 22.0 16.7 10.8 .764 .49 1 22.3 15.6 11.0 .700 .493 22.6 16 .0 10.8 .708 .478 23.6 17 .6 12.6 .746 .534 23.7 17. I 11.1 . 722 .468 24.0 17.2 12.6 .717 .525 24.4 18.3 12.8 .750 .525 24.4 18.6 12.6 .762 .516 25 .3 18.3 12.4 .723 .491 25 .9 17.7 13 .0 .683 .502 26.7 19 .1 12.8 .715 .479 26.7 19.4 13.6 .727 .509 27.7 19.6 13 .6 .708 .455 28.6 20.4 14.2 . 713 .496 30.2 21.6 14.6 0.715 0.483 The data in Table 3 indicate that shells greater than 16.0 mm in length are re­ latively lower and less circular than smaller individuals. Specimens of N. pulligera conglobata were collected together with two uni­ dentified thiarid gastropods from two locations several meters apart below Tarzan Fall s in the Tarzan River, Guam.