Aspects of the ecology and biology of the periwinkle Melaraphe neritoides (L.) (Gastropoda: Prosobranchia) on a Maltese rocky shore

Abstract

Population, growth and migration studies were made on Melaraphe neritoides (l.) (Gastropoda : Prosobranchia) at Xghajra - a moderately exposed micro-tidal shore on the east coast of Malta. This snail constitutes the bulk of the biomass in the rocky supralittoral of the Maltese Islands and occurs at the highest shore levels, notwithstanding its mode of reproduction which entails the broadcasting of planktonic egg capsules. The population within a fixed shore area was sampled every month for 14 consecutive months to investigate seasonal changes in density, sizestructure, and reproductive biology of the snails. A gross decline in density of snails was recorded from September (21,411 individuals/m2) to October 1993 (9,420 individuals/m2). During this period, autumn storms activated the snails which had been dormant during the dry season. Loss of snails was probably due to lateral migration out of the study area, dislodgement by waves, or post-reproductive mortality. The population remained more or less stable later on but increased slightly in May 1994 (10,937 individuals/m2) and in July 1994 (10,893 individuals/m2) due to peak recruitment of juveniles during this period. Adult females slightly outnumbered adult males (1.12:1) throughout, except in July 1994 when males constituted ~53% of the adult snail population. This was due to post-reproductive mortality of females during this month. Adult snails were distributed on the shore from ~1 m to a vertical distance of 24m from mean sea-level. This high-shore distribution is typical of exposed rocky shores in the Mediterranean and in Europe. Juveniles occurred from mean sea-level to a variable distance upshore which ranged from 4m to 10m. The mean vertical shore height of adult snails was highest in August 1993 (8.4m) and lowest in January 1994 (5.7m) and May 1994 (5.1m). A downshore migration of adult snails in January 1994 was correlated with reproduction, when snails move closer to the sea to increase the chances of successful spawning. Smaller snails (<4mm) slightly shifted their distribution upshore from April to July and December 1994. Presumably, with growth they become more able to withstand the drier conditions upshore. Monthly size-frequency histograms were virtually monomodal : the 5.2 6.0mm shell width size-class dominated throughout, except during the period May to July 1994 when a sub-population of juveniles was recruited. The mean shell width of the population (5.17 .±0.53mm) was high with respect to other populations of M.nerltoides studied in Malta and abroad. Female snails were significantly larger than males with mean shell widths of 5.26±0.57mm as compared to 5.07±0.48mm for males. There was a quasi-balanced distribution of the adults along the whole vertical range of the population and a size-gradient was only evident during peak recruitment of juveniles when the latter invaded the lower shore. The breeding season of M.nerltoides was from November 1993 to July 1994. Males had their peak sexual activity from November 1993 to February 1994 while that of females lasted longer, to May 1994. It appears that there may be variability in the time of reproduction locally and along other European shores. This is expected since all phases of the reproductive cycle of M.neritoides depend on the physical conditions of the shore. Sexual activity was correlated with the low sea temperatures and a decrease in sea temperature is apparently an essential trigger for reproduction. Capture-mark-recapture experiments revealed a high growth rate with a shell height of 7mm being attained in less than 3.5 years. Snails may reach sexual maturity (~3mm shell height) in less than one year. This observed high growth rate exceeds that found by other authors in Plymouth, N.Wales, Ireland and in the Balearic Islands. This explains the uniform size distribution at Xghajra and is most probably related to the presence of a sewage outfall in the vicinity and the resulting high productivity along this coast. Migration experiments were carried out in 1993, 1994 and 1995. Individually marked snails, released in 1 m2 plots at 2m and 8m from mean sea-level, were tracked weekly for several weeks, after which the remaining snails at each plot were sexed. These experiments emphasised the relationship between the activity of these snails and the condition of the sea. Snails were most active during stormy periods but they became inactive again when the shore dried out during calm weeks, even in winter. Generally, snails in the lower shore levels were more active than those in the higher levels since the former had better chances of being wetted by splash from low waves. There was an indication that, at both shore levels, males showed more activity than females. Orientation of the snails was not strongly directional in the short term. Extent of lateral displacements shown by the snails was considerable. Snails in both plots showed a general migration pattern: they crawled downshore during the activation phase from August to October-November and later sought higher shore levels up to April-May. Probably, in autumn the snails were seeking damper conditions and food downshore in areas wetted for the first time after the long dry summer. The later upshore movement is mainly due to immersion-avoidance behaviour which is typical of the Littorinidae. There was a slight correlation of migration with reproduction. Migration patterns are affected by many factors, which include: the complexity of the topography, shore-level, weather conditions, food gradients and reproduction as well as others which may act or interact differently from time to time and place to place.