ndian Journal of Marine Sciences Vol. 28, September 1999, pp. 320-324 Aspects on clustering and movements of the intertidal gastropod, Planaxis sulcatus (GastropodaJPlanaxidae) in the Suez Canal Saad Zakaria Mohammed Marine Science Department, Faculty of Science, Suez Canal University, smailia, Egypt Recieved 1 December 1997, revised 3 May 1999 Clustering behavior of the gastropod Planaxis sulcatus was investigated seasonally and in different shore levels on the Suez Canal rocky shores. The numbers of clusters 1m2 and individual abundance cluster in hot summer were signilicantly greater than in winter particularly in high and midshores. Clusters were more frequent in crevices than over boulders. There is a decline in snail size in summer due to the appearance of new recruits. Results of movement experiments revealed that the spreading of snails is faster in winter than in summer which showed high mortalities. Aggregations and assemblages in molluscs and other benthic organisms are common phenomena in rocky shores. n intertidal gastropods, aggregation and clustering are caused by attraction to common resources in order of avoidance of physical stressesl.2. Structure and development of clusters formed by the littorinoid gastropods were investigated in relation to shore zones 3.4 and to physical and biological hazards s. 6. Excessive numbers of the gastropod Planaxis sulcatus formed distinct clusters along the Suez Canal rocky shores, but little is known about clustering behaviour and factors affecting its abundance and distribution. The present study deals with the zonation of Planaxis clusters, its abundance on boulders and crevices and the movements of these snails along the intertidal flats in the Suez Canal. Materials and Methods The present study was carried out on the rocky shore of the western side of Suez Canal (lat 30 25 N, long 32 19 E) during winter (January - February) and summer (July-August) seasons, 1997. The description of study area is given in Mohammed 7. Three replicates of five transects (20 m length and 1 m width) parallel to the shore line were made at three vertical shore levels. They were high shore ( m above sea level), midshore (at sea level) and downshore (J m below sea bvel). n each transect, the numbers of Pianaxis clusters were counted fortnightly at mid day during neap tide and their means were calculated per square metre. The individual numbers in each cluster and its means were estimated per cluster. Shell heights (from apex to the base of the operculum) were measured by Vernier caliper and means were calculated. The frequent occurrence of Planaxis clusters in crevices or above the boulders were also recorded at the three shore levels and their mean frequencies were estimated. Water temperatures in crevices and above boulder (in case of downshore and midshore) and in crevices and water pools above rock surface (in case of highshore), were measured at noon every day and means were calculated. To investigate the movement distances by snails on rock surfaces, large numbers (> 1000 individual 1m2) of Planaxis were painted and divided into three groups. These groups were located away from each other (0.5 km distance) on rock surface at the highest level of spring tide on the first of January and June 1997 respectively. Painted snails were counted fortnightly in original points and in the intervals of 0-1, 1-2, 2-3, 3-4 and> 4 m away from the original points (in all directions) and their mean numbers were calculated for each group and at each interval. Dead painted snails were collected during the investigation, counted and its mean percentages were estimated. Results and Discussion Variations in water temperatures were considerably pronounced between seasons and among shore levels. n winter, its mean value above boulders was 20.] OC±1.1 S.D. without significant diffe rence between shore levels (one-way NOV A, P > O.!). n summer, highshore temperature (33.0 C± 1.5) was significantly greater than midshorc (29.6 DC ± 1.3,
MOHAMMED: CLUSTERNG & MOVEMENTS OF PLANAXS SULCATUS 321 P < 0.05) and downshore one (28.9 C ± 2.1, P < 0.001). Crevice temperatures in mid and down shore were similar to that on boulders in both seasons. Highshore crevice temperatures were slightly lower (20.9 C ± 0.61) than those over boulders surfaces in winter and summer respectively. Planaxis individuals often aggregate in clusters along the intertidal flats in the Suez Canal. Cluster numbers fluctuated widely either seasonally or with shore level (two-way ANOVA, P < 0.01). n highshore and midshore, the number of clusters in summer (Fig. la) were significantly greater than in winter (P < 0.05 in case of highshore and P < 0.01 in case of midshore). Midshore counts were higher than upshore ones (P < 0.05 in winter and P < 0.001 in summer). Downshore clusters were scarce without significant difference between the two seasons. The clusters were found either in crevices or over boulders. ts frequency occurrence in these habitats are shown in Fig. lb. More than 71 % and 78% of these clusters were recorded in crevices in winter and summer respectively. n winter, the frequencies of the midshore crevice clusters were significantly greater than in high shores (P < 0.05). Such difference, however, did not appear in summer, which had more or less similar values in both shore levels. Downshore clusters were found in similar percentages in both habitats (crevices and boulders) (Fig. B). The number of individuals per cluster (Fig. C) in summer were 1.5-2 times those of winter at all shore Winter 15 A Summer 10 s 100 ~ B ~~ J ~ [ ~~~:~:: 40 1 [l 20 J a 1.. - 1- -+- 1 C 15 ]10 n n D.D-j j:. LLlLl H.S M.S D.S Shore area 12 J. 10 : 8. 6.; 4.j i 2 -; a f H.S M.S D.S Fig. -Number of clusters (A), frequency occurrence on boulders and crevices (8 ) number of individuals per cluslcr ( e ) and sizes (0) of Planaxis sulcatus on Suez Canal shores. (H.S. = highshore, M.S. = midshore and D.S. = uuwnshorc).
322 NDAN 1. MAR. SC., VOL. 28, SEPTEMBER 1999 levels suggesting the active clustering in hot season. n both seasons, the midshore counts were approximately 2 and 3-5 times that at highshore and downshore respectively. The mean animal sizes (Fig. 1D) in summer were generally lower than in winter (P < 0.05 at highshore and downshore and P < 0.001 at midshore). This partially attributed to the appearance of recruits in summer, which could affect the size distribution. Regarding the shore elevation (Fig. D), midshore animal sizes in winter were significantly greater than highshores (P < 0.05) and downshores (P > 0.001) ones. n summer, the midshore values were however, more or less similar to highshores ones and highly significant than those in down shores (P < 0.00 ) reflecting the preference of young animals to cluster downshore. Although no trails were conducted to investigate the size distribution in different habitats (crevices and boulders), however, it is observed that small size animals frequently occurred in crevices than bigger sizes. Results of movement experiments (Fig. 2 A, B) revealed that snails spread in winter somewhat faster than in summer. More than 7% of snails had moved further than 4 m in winter while less than 2 % migrated to similar distance in summer. n winter, 7/1J997(Winter) n. 1/6/1997(Summer) 100 80 60 1 ~ +- + ' 40 2g ~~hl211l 30.. ~~. -L=---' 4/2 1/7. 1517 ~~Uo 20 ~, +D-t-..r::Li-- -1 ) s 5/3 r- e _ N 6 '<t Distance (m) ~: t ~]18 10 t. o ~.n+-.~ +-- - - i 0.. e s SSE o N < '<t X Fig. 2-Percentage frequ encies of moved snails at different distances from original point (O.P.)
MOHAMMED: CLUSTERNG & MOVEMENTS OF PLANAXS SULCATUS 323 2S 20 S...-. ~ 10 C.- 5 ~ EO - 7-Jan ~ 35 T J ~l :: L,.,, -_/ O. Winter.,- -' / -' -'-l------r---f--- ----j 21-Jan 4-Feb 19-Feb 5-Mar Summer /,,, - -- ~ -:--+1 - l!-- _-----l -Jun S-Jun -Jul S-Jul -Aug Fig. 3-Seasonal variations of cumulative mortalities (%) of Planaxis sulcatus. high percentage (73%) of snails moved from its original points within the first two weeks, (Fig. 2A), most of them within short distance (1-2 m). With time progression, the percentages of moved snails increased to become 97% at the end of experiment. About 25% of these snails spread 2-4 m away from original points within six weeks revealing the active migration in winter. n summer, most movements (66%) restricted within the first 2m (Fig. 2B). Less than 10% moved further and only few individuals succeeded to move extra distance (> 4 m). The percentage of dead individuals in winter (Fig. 3A) increased from 3.6% in the first two weeks to 23.7% at the end of the experiment. n summer, it increased from 2.5 to 35% for similar period (Fig. 3B) which is significantly (P < 0.01) greater than winter. The present results reflected the pronunciation of clustering ij1 P. sulcatus during hot summer as a response to dryness and heat stress. Such behaviour seemed to be functioning in reducing desiccation and minimizing stress resulting from physical hazards 8 9. Extensive observations on Jittorinoids, reveaied, however extreme variations in clustering in relation to physical conditions. t occurs during warm as well as cold weather. Magnus & Haecker 8 and Hulings'O observed cyclical clustering in P. sulcatus following submersion and emersion by tides. They attributed that to the breaking of waves and postulated the sperm transfer occurred with clustering during the flooding tide. n Suez Canal shores, emerged clusters (in midshore and upshore) were more pronounced than submerged (down shores) ones and seemed unlikely not confined to the reproduction period. The frequent occurrence of clusters in crevices were extremely greater than on boulder surfaces. n terms of physical extremes, boulders surfaces are more severe microhabitat than crevices. Emson & Faller-Fritsch showed that crevices may be limiting resources for winkles on exposed shores. The addition of artificial crevices resulted in an eight fold increase in abundance. Similar effects of artificial crevices have been reported by Raffaelli & Hughes '2 and Hughes & Roberts 13 and strongly suggested that exposed populations are generally crevice - limited. t is assumed that winkles search randomly for crevices without homing behaviour. From reported findings and from the present results, it is clear that crevice abundance could limit the abundance of Planaxis in density-dependent manner and influencing local pattern of population structure. The chance of snail encountering a suitable crevice will decrease as the density increases. n the midshore and upshore the individual number per cluster and the animal sizes were remarkably higher than downshore. The reasons for these figures are complex and likely to include both biological and ecological interactions. Physical hazards in the downshore (as desiccation, thermal stress) are not extreme as in the upshore, so the animals don't have to aggregate in clusters or seeking for crevices to protect themselves from high temperature and prolonged desiccation. Raffaelli & Hughes ' 2 attributed partially the morphological and size clines in ittorinoids to its exposure to physical extremes and to the predation by other animals. Also, Hulings 'O found an obvious difference in mean length and size distribution between submerged and exposed populations and revealed it to the variations in physical extremes between the two habitats. Results of movement experiments revealed generally short-distance diffusions of Planaxis particularly in summer. Similar results were recorded in Australian sh o res'~ and artributec! the little mixing in Planaxis populations to the lack of long-distance migration which is characteristic to other gastropods.
324 NDAN 1. MAR. SC., VOL. 28, SEPTEMBER 1999 The assumed reasons for such behaviour could be its clustering phenomenon or its attachment to the substratum by dried mucus which is mostly occurs during warm period of the year to avoid thermal stress. Faller-Fritsch & Emson 27 recorded good attachment and less diffusion in littorinoids under physical stresses. Also, the flourishing of food (algae) in mild winter could enhance its movements for grazing. The high mortalities in summer agree with the previous results in that heat stress and desiccation have destructive effects on Planaxis populations l5 Vermeij 16 recorded death from desiccation on shelterd boulder shore, and found great mortalities among littorinoids in dry summer and the preference of small snails to hide between small pebbles to avoid prolonged desiccation. Faller-Fritsch & Emson 2 and Shepherd & Breen17 in discussing the physical causes of mortalities in winkles and abalones reported that desiccation, heat, and dislodgment as interacting factors account for majority of mortalities. The importance of these factors is clear from several adaptation which have evolved to minimize their effects, including shell shape, crevice seeking behaviour (in winkles), activity patterns in relation to environmental factors and well developed tolerance of high temperature and prolonged desiccation 8 19 n Suez Canal, waves are very weak and there is no dislodgment for Planaxis, hence desiccation and heat seemed likely the interacting factors responsible for its mortalities. References Levinton J S, Marine ecology (New Jersey Press, Engle Wood Cliffs) 1982, 526. 2 Faller- Fritsch R 1 & Emson R H, in Th e ecology oj rocky coasts, edited by P G Moore & R Seed, (Hodder & Stronghton Press) 1985, 144. 3 lones D A, A field guide to the seashore of Ku wait and the Arabian Gulf, (University of Kuwait Press) 1986, pp. 192. 4 Hawkins S 1 & Hartnall R G, Mar Environ Res, 9 ( 1983) 131. 5 Stephenson T A & Stephenson A, LiJe between tide marks on rocky shores, (W. H. Freeman & Co. San Francisco) 1972, pp.425. 6 Fish 1 D & Sharp L, in The ecology oj rocky coasts, edited by P G Moore & R Seed (Hodder & Stronghton Press ) 1985, 132. 7 Mohammed S. Z, ndian J Mar Sci, 28 ( 1999) 325. 8 Magnus DB & HaeekerU,Sarsia, 34 ( 1Y68) 137. 9 Daguzan 1, C Bioi Mar, 17 (1976) 21 3. 10 Hulings N C, Dirasat, 14 (1987) 155. Emson R H & Faller-Fritsch R J, J Exp Mar Bioi Ecol, 23 (1976) 285. 12 Raffaelli D G & Hughes R N, l Anim Ecol, 47 ( 1978) 71. 13 Hughes R N & Roberts D 1, Oecologia, 47 (1980) 130. 14 Rhode K, Oecologia, 49 (1981) 344. 15 Hart A & Begon M, Oecologia, 52 ( 1982) 37. 16 Vermeij G, Biogeography and adaptation: Patterns oj marine life, (Harvard University Press) 1978, pp. 51 3. 17 Shepherd S A & Breen P A, in Abalolle oj the world : Biology, fisheries and culture, edited by S A Shepherd, M. J Tegner& S. D Guzman del proo, (Fishing News Books Press, London) 1992, 27. 18 Connell J H, Annu Rev Ecol Syst, 3 (1972) 169. 19 Hines A, Anderson S & Brisbin M, Veliger, 23( 1980) 11 3. )