Population Structure and Recruitment of the Intertidal Gastropod Melarhaphe neritoides (L.)on an Exposed Rocky Shore on the South Coast of IrelandAuthor(s): David McGrathSource: Biology and Environment: Proceedings of the Royal Irish Academy, Vol. 97B, No. 1(Jul., 1997), pp. 75-79Published by: Royal Irish AcademyStable URL: http://www.jstor.org/stable/20499985 .

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POPULATION STRUCTURE AND

RECRUITMENT OF THE INTERTIDAL

GASTROPOD MELARIHAPHE NERITOIDES

(L.) ON AN EXPOSED ROCKY SHORE ON

THE SOUTH COAST OF IRELAND

David McGrath

ABSTRACT

The population structure and recruitment of the marine intertidal gastropod Melarhaphe nenitoides (L.) were examined at a series of vertical stations on the exposed rocky shore at Carnsore Point,

Co. Wexford, during summer. The data show that a settlement occurred during May/June, and suggest that it extended throughout most of the eulittoral zone. No recruitment was observed on open rock in either the Verrucaria or jellow lichen zones. Growth of the recent recruits appeared to be faster lower on the shore. Mean adult size was largest among the snails located highest on the shore and decreased downshore to the top of the eulittoral zone. Mean size then increased further downshore.

D. McGrath,

Department of Life

Sciences, Regional Technical College, Galway, Ireland, and

Department of Zoology and Animal

Ecology, University College Cork, Ireland.

Received 25 November 1996. Read 22 May 1997.

Published 31 July 1997.

INTRODUCTION

The marine intertidal gastropod Melarhaphe neritoides (L.) occurs from the coast of Norway southwards along the Atlantic coast of Europe, around the shores of the Mediterranean to Mo

rocco, and sporadically in the Azores, Cape Ca naria and the Cape Verde Islands (Rosewater 1981). It is a common species on the more ex posed rocky shores around the Irish coast. While usually described as an upper shore and splash zone species (e.g. Hayward et al. 1995), it has a consid erable range in Irish waters, occurring down to

mid-tide level (Crapp 1973) or to low-water neap tide level, and below (Ryland and Nelson-Smith 1975).

M. neritoides spawns planktonic eggs dunrng the period September to April at Plymouth, England, with egg capsules being found in the plankton from October to April (Fretter and Graham 1980). Settlement, while observed in all months except January, was maximal from March to April, and again from September to October (Fretter and Graham 1980). Minimum size of the shell at settle ment in this area was 0.4mm in diameter and 0.37mm in height, and the maximum recorded density was 405 m-2 (Fretter and Manley 1977). The settlement site at Plymouth was on a vertical rock face above mean high water neap (MHWN).

Growth rates among adults have been studied in populations from Bnrttany (Daguzan 1976), Ply

mouth (Lysaght 1941) and north Wales (Hughes and Roberts 1980), and there appear to be differ ences in growth charactenrstics at the different sites (Myers and McGrath 1993). Fretter and Manley (1977) give data on the growth of newly settled snails.

Myers and McGrath (1993) described the set tlement and growth of M. neritoides from two locations, Tragumna and Camsore Point, on the south coast of Ireland. Their study sites were located immediately above the Lichina pygmaea zone (Tragunma) and on open rock half-way be tween mean high water spring (MHWS) and

MHWN (Camsore Point), and were therefore limited in vertical extent relative to the intertidal range of Irish populations.

The present account describes the population structure, recruitment and subsequent growth of

M. neritoides at a series of vertical heights on the exposed shore at Camsore Point during a three

month penrod in summer. It addresses the question of how the extensive vertical range of M. neritoides in Irish waters is maintained. Does the species settle throughout its intertidal range or is settle

ment limited to a specific zone with later migra tion establishing the observed zonation?

MATERIALS AND METHODS

Melarhaphe neritoides were collected from the exposed rocky shore at Carmsore Point, Co. Wex

BIOLOGY AND ENVIRONMENT PROCEEDINGS OF THE ROYAL IRISH ACADEMY, VOL 97B, No, 1, 75-79 (1997) 0 ROYAL IRISH ACADEMY 75

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BIOLOGY AND ENVIRONMENT

ford (Irish National Grid Reference T120035), at a series of six vertical stations along a transect during the months May to July 1978 inclusive. Collec tions were made in the yellow lichen and Verrucaria zones, from open rock about half-way between

MHWS and MHWN, and from among barnacles at about MHWN (upper barnacles), mid-tide level (MTL) (mid-barnacles) and mean low water neap (MLWN) (lower barnacles). Samples were taken from haphazardly positioned quadrats (Krebs 1989), and in the case of the lichen zones also from crevices, on 23 May, 24 June and 21 July. The shell height of individuals was measured to the nearest 0.05mm using an ocular micrometer. Sam ple sizes collected are shown in Fig. 1. Growth rates were determined as increase in the mean height of the cohort.

The shore at Carnsore Point where the study was carried out is a granite shore facing approxi mately south. Overall, the fauna was typical of Irish Atlantic exposed rocky shores dominated by limpets, barnacles and mussels, with an exposure rating of 3 on the Ballantine (1961) scale. The spring tidal range is approximately 4m, and the shore is gently sloping with a width of c. 75m (Healy and McGrath 1986).

RESULTS

The shell height frequency distributions for the samples of M. neritoides collected from each station during the three-month study are shown in Fig. 1. The results show the June settlement event previ ously described in Myers and McGrath 1993 for the open rock site. The absence of the smallest size classes, c. 0.5mm in height, shows that no recent settlement had occurred on any part of the shore prior to June, and inJune recently settled snails were found on the shore in the three bamacle zones. There is no evidence from the height frequency data of recent settlement in the high shore in either of the lichen zones. A size increase is evident in the recently settled cohort in July (Fig. 1). This size increase, measured as change in mean shell height

of the new cohort from June to July, was 0.09mm in the open rock site and was progressively larger

downshore through the three barnacle zones at 0.21mm, 0.32mm and 0.33mm respectively.

The largest adult animals were to be found in the highest (yellow lichen) zone exarmined at about the upper limit of the snail's distribution on the shore, with size decreasing downshore to the open rock site. An increase in size downshore from this latter site is obvious in July (Fig. 1).

DISCUSSION

Intertidal gastropod population structure has been classified into two types by Vermeij (1972).

76

In type 1, characteristic of littoral fringe species, shell size tends to increase in an upshore direction; by contrast, in type 2, typical of lower shore species, shell size often tends to decrease in an upshore direction.

M. neritoides at Carnsore Point shows a pattern at variance with the above types in that snail size declines down to about half-way through its range and then shows an increase to the lowest part of the range. This pattern is similar to that observed by Branch and Branch (1981) and Chapman (1994) for the Australian species Nodilittorina uni

fasciata Gray.

Size gradients in intertidal winkles have been shown to be associated with environmental factors, including temperature, desiccation, wave action, crevice size and availability, competition, preda tion, migration and settling behaviour (Chen and

Richardson 1987).

The most parsimonious explanation for the pattern in population structure seen in the present data set at Camsore Point is as follows: settlement occurs throughout the eulittoral zone and is fol lowed by more rapid growth downshore, with the population of larger snails of the high shore lichen zones .being older and maintalned either by spo radic settlement in pools or by an upshore migra tion from lower shore zones.

At Carnsore Point, recently settled M. neri toides appeared on the open rock site, about half

way between MHWS and MHWN, and among barnacles downshore to about MLWN level dur ing the period June to July. Recently settled snails

were not seen in the Verrucaria or yellow lichen zones at that time. However, recently settled indi viduals were seen in pools in the Verrucaria zone at Camsore Point in the previous December (Myers and McGrath 1993). Such settlement, even if spo radic, would allow for direct recruitment into the upper shore lichen zones without an upshore mi gration of recently settled juveniles as suggested by Fretter and Graham (1980).

Settlement refers to the irreversible loss of larval pelagic motility and is essentially an instanta neous occurrence (Roegner 1991). The M. nen toides found in this study are recruits, settled individuals which have survived some post-settle

ment time until recorded by an ecologist (Keough and Downes 1982; Pawlik 1992). In this case, the smallest recruits may have settled on the previous tide and others may have been on the shore for up to a month. Settlement is often inferred from recruitment data measured days or even weeks after settlement has taken place (Seed and Suchanek 1992). While settlement, per se, was not observed during this study, the data suggest that M. neritoides settles virtually throughout its entire verti cal range at Carnsore Point, with the exception of the highest levels in the yellow lichen zones.

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IF)OPULATION STRUCTURAE ANI) RECRUITMENT OF .V1I.`uR iiHA1Ivi

MAY 1978 JUNE 1978 JULY 1978

NO SAMPLE 1 5 Yellow lichen 1 5 Yelbw lichen

10 n-88 10 n=144

5 5

201 verrucaria

20 verrucaria

20 verrucaria

n=97i n=137 n-102

1 0 10 1 0

0 1 Lpenoc 2l0 0*IuO-2 IItu. Open rock 20 Open rock 20 Open rock

I 5 n=91 n-77 n=1 28

? 201 I Upper barnacles 20 1 Upe banacles 20 Upper barnacles

20 | Mid-barnacles 20 Mid-barnacles Mid-barnacles

| g ~n=199 J

n.n145 15nn148 10. 10 1 0

20 Lower barnacles 20 Lower barnacles Lower barnacles | | n.79 l n-=48 n-104

I oL0 1 ol 11 0xIl L L j 0. 2. 3. 0. 20 5 0. 20 35 10 10 10

SHELL HEIGHT (mm) Fig. 1-The population structure of Melarhaphe neritoides from six heights on the shore at Carnsore Point during the months May to July 1978. No sample was taken in the yellow lichen zone in May.

Branich anid Birainch (1981) reported the pres enice of simall recruits in intertidal pools over the elntire raitge of N\(Odilittorina uimifasciata, anid Cheni

anid Richardson (1 987), workinig onl the same

species, noted that larval settlenment appeared to occur throughout its vertical range.

The larger size of recenitly settled M'. iieritoides oni the lower shore at Cartnsore IPoint in JLly,

77

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BIOLOGY ANI) ENVIRONMENT

when compared with the mid-range population, would appear to derive from faster growth rates there. The growth rates calculated for the first

month may be depressed by the continued settle ment of snails. Thus the growth rate of 0.09mm estimated for the open rock site is lower than the

monthly rate of 0.14mm calculated over the grow ing season for this site by Myers and McGrath (1993). The growth rates at the open rock site at

Carnsore Point and at Trugumna are lower than those calculated for north Wales (Hughes and

Roberts 1990) or Brittany (Daguzan 1976) (Myers and McGrath 1993). However, the growth rates calculated at Carnsore Point from the lower shore sites are considerably higher than at the open rock site. Had these growth rates been maintained throughout the growing season, growth to a shell height of 2mm would have been achieved in five

months. This is a higher growth rate than that estimated for north Wales populations (Hughes and Roberts 1990), a minimum of nine months to 2mm shell height, but still substantially lower than those from Brittany, where a height of 3mm was reached after seven months' growth (Daguzan 1976). The growth rates calculated in the present study for recent recruits, as shell height increase, cannot be compared with those reported by Fretter and Manley (1977), who measured shell diameter.

Whether the higher growth rates estimated for the lower shore snails were due to better feeding related to longer immersion times or reduced den sity-dependent competition (Branch and Branch 1981) is not known. The strong correlation be tween density and mean size suggests that Nodilit torina unifasciata shows density-dependent growth, and that the vertical size gradient seen in this species may simply reflect a gradient of density and

subsequent growth in habitats where food is limit ing at high densities (Chapman 1994). Density data are not available for the Melarhaphe examined dur ing the present study.

The largest snails were found on the high shore. According to the data of Myers and Mc

Grath (1993), these are old animals with low growth rates. The scarcity of larger, older animals in the eulittoral may be due to a lack of suitable crevices (Raffaelli and Hughes 1978) or to an

upshore migration with age and growth (Chen and Richardson 1987). Alternatively, the size gradients seen may reflect differential tolerances of different size classes to physical factors such as temperature, desiccation or wave action (Chen and Richardson 1987). Chen and Richardson (1987) have sug gested that migration could have been a significant cause of the size gradients they observed in Nodilit torina unffasciata. They calculated the migration rates of marked snails and concluded that a snail could cover a distance of 30m in a month. Their

marked animals had a minimum size of 2mm, however. The distance between the upper and

78

lower barnacle sites at Carnsore Point is of the order of 20m. The possibility that downshore

migration might have a role in the size gradients seen at Carnsore Point cannot be excluded.

ACKNOWLEDGEMENTS

I wish to thank Dr Brenda Healy, Dr Noel Mulligan and Donncha McGrath for technical sup port, and Professor A.A. Myers and Dr David Reid for their helpful comments on the manuscript. Financial support for the Carnsore Point study was provided by the Irish Electricity Supply Board.

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POPULATION STRUCTURE AND RECRUITMENT OF MELARHAPHE

The subvention granted by

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