Hydrobiologia 350: 127130, 1997. 127c

1997 Kluwer Academic Publishers. Printed in Belgium.

Histology of gonads and cephalic brood pouch of the marine gastropodPlanaxis sulcatus of Karachi Coast (Pakistan)Muzammil Ahmed & Ghazala SiddiquiCentre of Excellence in Marine Biology, University of Karachi, Karachi-75270, PakistanReceived 25 April 1996; in revised form 15 April 1997; accepted 7 May 1997

Key words: Gonadal histology, Planaxis, Karachi

Abstract

This histological study of 141 specimens of adult Planaxis sulcatus from Karachi showed the presence of only femalegametes in gonadal follicles, supporting the conclusion of earlier workers that the species may be parthenogenetic.The gonadal follicles of several snails were occupied by an unidentified trematode parasite.

Introduction

Large populations of the snail Planaxis sulcatus arefound in the high tide zone of some elevated rockyshores along the coast of Pakistan, bordering the North-ern Arabian Sea. A fairly large population inhabits thesupratidal zone of Buleji rocky ledge. The habitat issomewhat protected on an otherwise exposed shore.The snail occupies a height of about 3 meter above0-tide mark. At Pacha, a rocky shore located 45 kmnorthwest of Buleji, the snail lives on a more elevat-ed semi-exposed platform. During ebb tides the snailsgather into small clusters around depressions, holesand crevices of rocks and boulders.

Only a few Pakistani workers have focussed atten-tion on the biology of this species. Barkati & Ahmed(1982) described the breeding habits and developmentof juveniles hatched in the laboratory. They also report-ed that all the developmental stages are passed withinthe maternal body and that the embryos are retained inthe pallial oviduct where they reach the crawling stage.Zehra & Perveen (1987) confirmed this ovoviviparity.Subsequently Zehra & Perveen (1988) reported thepresence of only females in the population studied andsuggested that this species may be parthenogenetic.

Planaxis sulcatus is widely distributed along Indo-Pacific rocky shores. Thorson (1940) reported theabsence of males from a Persian Gulf population, sug-gesting that it was parthenogenetic. Bandel (1976),Ponder (1980) and Boss (1982) briefly described

the brood pouches and the embryos contained there-in. Rhode (1981) examined the population dynamicsof P. sulcatus and its trematode parasites. Recent-ly, Houbrick (1987) studied in detail the comparativeanatomy and reproductive biology of the planaxids,P. sulcatus, Fissilabia decollata, Hinea brasiliana,Angiola lineata and Supplanaxis nucleus. He reportedthe presence of males in a New Caledonian popula-tion and rejected the possibility of parthenogenesis inplanaxids.

In view of the above mentioned contrasting obser-vations, the objective of the present study was to deter-mine through histology, whether any males occur inthe Karachi area population of P. sulcatus.

Material and methods

A total of 151 specimens of P. sulcatus were stud-ied histologically. The collections were made monthlyfrom Buleji (18 km from Karachi), from July 1992 toJuly 1993 excepting April. One collection was made inMarch 1994 for the study of the cephalic brood pouch.All specimens were measured and the animals wereextracted by gently breaking open the shell. Gonadalportions of 141 specimens, and the cephalic regions of10, were fixed in Davidsons Fixative (Shaw & Battle,1957), wax embedded, sectioned at 7 m and stainedwith Delafield haematoxylin and eosin. Egg sizes weremeasured from the histological sections prepared in the

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Figure 1. Planaxis sulcatus. A: T.S. of the gonad, showing the nuclei (N) of mature oocyte (OC). 200.B: A multicellular embryo in the brood pouch. 1000C: Section through upper part of the gonad, showing mature oocytes (OC) and developing embryos (EM). 200.D: T.S. through the cephalothorax, showing the chambered brood pouch filled with numerous developing embryos (EM) and miniature snails(MS). 100.

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Figure 2. T.S. showing follicles occupied by trematode parasites (TP). Two oocytes are also seem. 200.

month of July , which was earlier reported by Barkati& Ahmed (1982) as the peak month of reproductiveactivity in P. sulcatus.

Results

Histological sections of the gonads of 141 specimens ofP. sulcatus ranging in size from 1022 mm, indicatedthe presence of only female gonadal tissue (Plate 1A)throughout the year. This material consisted of onlymature looking eggs with large nuclei.The mean size of100 eggs (oocytes) from 10 snails was 56.08m 9.44in July, 1992.

The cephalic brood pouch, which is located in thehead and foot region of the snails, was found in histo-logical sections to consist of many chambers (Plate 1D)which were filled with embryos in various stages ofdevelopment, ranging from early embryos (Plate 1B)to miniature snails (Plate 1C & D). Nurse eggs werealso seen.

A by-product of the study was the detection of atrematode parasite in the gonadal tissue. Seventeensnails were found infected in July, September andOctober. A maximum of 13 (36.11%) infected snailswere observed in July, 1992. The trematode occupied50 to 90% of the gonadal area, so that all gonadal fol-

licles showed curved bodies of one or more parasites;in some cases only an occassional follicle containedeggs (Plate 2), owing to the occupation of the entirefollicular area by trematodes.

Discussion

During the present histological study no male speci-men was found in the Buleji population of P. sulcatusat any time during the year. All histological sectionsshowed the presence of female gonadal tissue only.Thus the Buleji population may be parthenogenetic assuggested by Zehra & Perveen (1988), confirming thereport of Thorson (1940) whose findings led earlierworkers (Morrison, 1954; Kay, 1979; Boss, 1982) tosuggest that all members of planaxidae are partheno-genetic. Houbrick (1987), however, regarded this as amisconception, but he did not rule out the possibili-ty of parthenogenesis in some remote populations ofP. sulcatus such as those of the Persian Gulf.

In the present study, the cephalic brood pouch ofP. sulcatus contained developing embryos and juvenilesnails. A similar observation was reported by Thor-son (1940) from the Persian Gulf. He found smallsnails in the maternal brood pouch feeding on nurseeggs. Risbec (1935) also reported partial ovovivipar-

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ity in a New Caledonia population, where embryosreached the brood pouch at the morula stage andhatched as veligers. Houbrick (1987) reported fromNorth Queensland, Australia, lecithotrophic develop-ment and a brief planktonic phase in P. sulcatus. Heobserved a sigeral notch and protoconch sculpturing,indicating the presence of a free swimming larval stage,and considered the possibility that the Indian Oceanand Pacific Ocean populations of P. sulcatus maybelong to two cryptic species, the Indian Ocean speciesshowing the direct and the Pacific Ocean species, indi-rect development.

The differences in reproductive modes adopted bydifferent populations of P. sulcatus of the Indo Pacif-ic may reflect differences in the tidal heights whichthey occupy. Earlier Robertson (1974) had pointedout that P. sulcatus is a species which exhibits sev-eral reproductive modes according to environment andgeographical locations. Houbrick (1987) reported thatthe North Queensland population inhabits low to midtidal zones of rocky shores which allow the snail toremain submerged for comparatively longer time peri-ods, thus facilitating the development of a planktoniclarval phase. On the contrary, the Arabian Sea popula-tion of P. sulcatus occurring in Pakistan occurs at about3 meter tidal height. This population remains exposedfor prolonged periods. Due to longer exposure timethis population of P. sulcatus may have advantage incomplete ovoviviparity and direct development.

Acknowledgments

This work was undertaken for the research project Liv-ing Marine Resources of Pakistan sponsored by theOffice of Naval Research, USA. The authors thankMiss Zarrien Ayub, Ph.D. student of the Centre ofExcellence in Marine Biology for her help in the col-lection of the research material.

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