CHITON (SYPHAROCHITON) PELLISERPENTIS AT MUDS'I'ONE BAY, KAIKOURA, NEW ZEALAND PETER L. HORN

Transcription

1 MAURI ORA, 1984, 11: tleachl<.ock E:R03IO:N DUE '1'0 l:!'eeding BY CHITON (SYPHAROCHITON) PELLISERPENTIS AT MUDS'I'ONE BAY, KAIKOURA, NEW ZEALAND PETER L. HORN Department of Zoology, University of Canterbury, Christchurch, New Zealand.* ABSTRACT Removal of mudstone rock during feeding by Chiton pe11iserpentis occurred at an estimated annual rate of 47,3 g/m 2 on the high shore and g/m" on the low shore, This was equivalent to about 2% of total erosion on the high shore and less than 5,5% on the low shore, Bioerosion attributable to grazing molluscs was not considered to be a major component of total erosion in Mudstone Bay, Kaikoura. KEYWORDS: intertidal mudstone erosion, Chiton pel1iserpentis, Kaikoura, INTRODUCTION Several authors have considered the effects of bioerosion, that is the removal of lithic substrate by the direct activities of organisms (Neumann,1966) f attributable to molluscs on rocky coastlines (review in Healy, 1968; Moe and Johannessen, 1980). Chi tons have a high proportion of magnetite in their radular teeth (Carefoot, 1965) and their consequent hardness has led several authors to postulate that they contribute significantly to rock erosion, particularly on some tropical limestone shores where chitons may be common (Lowenstram, 1962; Milliman, 1976; Taylor and Way, 1976). * Present address: Fisheries Management Division, Ministry of Agriculture & Fisheries, Private Bag, New Plymouth.

2 58 MAURI ORA, 1984, Vol. 11 New Zealand has a diverse chiton fauna with species occurring throughout the intertidal and subtidally on rocky shores (Powell, 1979). The most widespread species, Chiton (Sypharochiton) pelliserpentis (Quoy and Gaimard, 1835) is common on the Kaikoura Peninsula and occurs from the splash zone to the low intertidal (Rasmussen, 1965). As part of a study of the energetics of high- and low-shore groups of S. pelliserpentis in Mudstone Bay, Kaikoura (Horn, 1983), a quantitative estimate of erosion attributable to chitons was made and compared with measurements of total erosion occurring in Mudstone Bay made by Kirk (1977). METHODS Mudstone Bay lies on the southern coast of the Kaikoura Peninsula ( I E; ' S) and is open to the south and south-east. Its gently sloping glauconitic siltstone shores receive light wave action as the bay is sheltered by offshore rocks and reefs. Two groups of C. pelliserpentis were selected for study; "low-shore" chi tons were distributed between the Low Water Neap and Extreme Low Water Spring, and "high-shore" chi tons were found between Mean Sea Level and High Water Neap. The density of,chi tons in both groups was determined in January 1982 and January 1983 using randomly placed 1.0 m 2 quadrats 33 high-shore and 6 low-shore samples in 1982 and 50 and 15 in Population structure was monitored by measuring body lengths of 300 chitons selected at random each month throughout Faecal production by both groups was measured every two months in a tidal tank (see Ottawa.y, 1975) of flowing seawater at the Edward Percival Field Station, Kaikoura.' At least 80 chi tons were taken from the shore, sorted into groups of 4 to 9 individuals of about the same size, and allowed to adhere to a rock recently removed from their habitat (one group per rock). Each rock was placed in a 2 1 plastic container with a hole covered by a 0.25 mm mesh gauze in its base. The containers were covered with 1.5 mm mesh gauze and placed on rocks in a tidal aquarium (modified after Ottaway, 1975) at a height where the chitons experienced typical periods of immersion and emersion as determined from field observations. Air and water temperatures closely approximated those in the field. The gauze-covered hole in the container base ensured that water could flow in and out, but ensured that faeces were not lost. Each trial was run for two days; faeces were collected at each simulated low tide and dried to constant weight at 7S o C so that mean dry weight of faeces produced per animal per day could be calculated. Body lengths of all experimental animals were measured to obtain the mean length of animals in each container. Linear regression analysis was used to obtain a size specific rate of faeces production for each population at each time. Subsequently, collected faeces were ashed in a muffle furnace for 4 h at C to determine the proportion of inorganic material they contained. Annual production of faeces by the high- and low-shore chiton groups was calculated using a computer programme (Appendix

3 HORN - ROCK EROSION BY CHITONS E'20 c.2 U :J '0 ~ 10 c Body length (mm) Fig. 1. Regression lines relating chiton body length (nun) to daily faecal production (mg dry weight) measured six times during the year for the high-shore (broken lines) and low-shore (unbroken lines) chiton groups. For each line, ~ = 18, correlation coefficients ( ) given in parentheses (high shore, low shore). Ja = January (0.83, 0.77), Mr = March (0.72, 0.82), My = May (0.89, 0.84), Jy = July (0.69, 0.93), Se = September (0.65, 0.89), No = November (0.67, 0.88). 1 of Horn, 1983) that took into account monthly changes in population structure and defaecation rates. Density of mudstone from Mudstone Bay was found, by displacement, to be 2.40 g/cm' (s = 0.04, n = 5). RESULTS (x on Chiton densities ranged from 4 to 621m 2 on the 30.5, s = 19.4, area sampled = 21 m') and from the high shore (x = 8.9, s = 9.0, area = 83 m 2 ). low shore o to 251m 2 Mean body

4 60 MAURI ORA, 1984, Vol. 11 lengths during 1982 were 31.8 mm and 22.0 mm on the ~igh shores, respectively. and low The relationship between body length and daily faecal production was adequately described by a log-log regression (Fig. 1). Faecal production by low-shore chi tons was generally % greater than for similar sized high-shore animals at the same time of the year. The data exhibited seasonal trends which were similar for both groups. Greatest faecal production occurred in autumn (March and May), and least occurred in spring (November). The defaecation rate of a chiton in May was generally about twice the rate exhibited in November. The inorganic content of faeces did not appear to be related to season, and ranged between 88 and 96% (x 92.8%, n 18) on the high shore, and between 91 and 95.5% (x = 93.5%, n = 18) on the low shore. Annual faecal production by C. pelliserpentis on the high shore was estimated to be 50.9 g/m', and on the low shore, g/m'. Hence, the non-organic fractions of faeces were about 47 and 173 g/m'/year for the high- and low-shore chiton groups, respectively. As the mean density of rock from the study area was calculated to be 2.40 g/cm', the erosion attributable to the chitons was about mm/year on the high shore and mm/year on the low shore. It should be noted, however, that the diet of low-shore chitons included some encrusting coralline algae which contain about 75% inorganic matter (Paine and Vadas, 1976). Therefore, the erosion estimate for the low shore is likely to be an overestimate. DISCUSSION Kirk (1977) found that erosion of the shore in Mudstone Bay was occurring at mean annual rates of 1.31 mm on the low shore and 1.15 mm on the mid to high shore. The proportion attributable to radular rasping by C. pelliserpentis as indicated by the present study therefore would be 1.7% on the high shore and less than 5.5% on the low shore. These figures are comparable with those of % ( mm/year) of total measured erosion for the chiton Acanthopleura brevispinosa on Aldabra Atoll (Taylor and Way, 1976). While the results of some studies suggest that up to 50% of breachrock erosion can be attributable to biological action (e.g., North, 1954; Trudgill, 1972, cited in Taylor and Way, 1976), grazing molluscs (of which C. pelliserpentis is the most abundant) do not appear to be present in sufficient numbers to make a major contribution to total erosion in Mudstone Bay.

5 HORN - ROCK EROSION BY CHITONS 61 LITERATURE CITED Belle, R.~. van The systematic classification of the chitons (Mollusca: Po\yplacophora). Informations de la Societe Belge de Malacologie sere 11: Carefoot, T.H Magnetite in the radula of Polyplacophora. Proceedings, Malacological Society, London 36: Healy, T.R Bioerosion on shore platforms developed in the Waitemata Formation, Auckland. Horn, P.L Energetics of the chiton Sypharochiton pelliserpentis from a sheltered shore at Kaikoura. Unpublished M.Sc. thesis, University of Canterbury, Christchurch, New Zealand. Kirk, R.M Rates and forms of erosion on intertidal platforms at Kaikoura Peninsula, South Island, New Zealand. New Zealand Journal of Geology and Geophysics 20: Lowenstram, H.A Magnetite in denticle capping in recent chitons. Bulletin of the Geological Society of America 73: Milliman, J.D Marine Carbonates. Springer Verlag, Berlin. 375 pp. Moe, D. and Johannessen, P.J Formation of cavities in calcareous rocks in the littoral zone in northern Norway. Sarsia 65: Neumann, A.C Observations on coastal erosion in Bermuda, and measurements of the boring rate of the sponge Clinoa lampa. Limnology and Oceanography 11: North, W.J Size distribution, erosive activities, and gross metabolic efficiency of the marine intertidal snails, Littorina planaxis and L. scutulata. Biological Bulletin, Marine Biological Laboratory, Woods Hole 106: Ottaway, J.R Tidal tank system in operation at the Edward Percival Marine Laboratory, Kaikoura. Mauri Ora 3: Paine, R.T. and Vadas, R.L Calorific values of benthic marine algae and their postulated relation to invertebrate food preference. Marine Biology 4: Powell, A.W.B New Zealand mollusca: marine, land and freshwater shells. Collins, Auckland. 500 pp. Rasmussen, R.A The intertidal ecology of the rocky shores of the Kaikoura Peninsula. Unpublished Ph.D. thesis, University of Canterbury, Christchurch, New Zealand. 203 pp. Taylor, J.D. and Way, K Erosive activities of chitons at Aldabra Atoll. Journal of Sedimentary Petrology 46: