Plasticity in the temporal organization of behaviour in the limpet <Emphasis Type="Italic">Cellana grata</Emphasis>

The behaviour of intertidal consumers is often tightly constrained to tidal movements, although activity patterns can vary within these constraints. Spatio-temporal variability in behaviour of a limpet, Cellana grata, was analysed over different tidal conditions (spring and neap tides) and during different times of the year (one summer and one winter) at sites in Hong Kong. Activity was generally dictated by tidal movements, being concentrated when animals were awash. Plasticity in behaviour was observed, with some limpets anticipating activity during the summer period and delaying activity during winter time. Limpets were active for a time equal, or slightly less, than time awash. As the time awash exceeded ~14–16 h, however, activity duration decreased. Within this general pattern, tidal variation as well as variation among times of the year was noted, with the lowest dependence on time awash being recorded during winter neap tides. Limpets showed a slight preference for being active during nighttime, which was particularly evident when animals were emersed during the summer period. Although the basic activity in C. grata is constrained to a specific temporal window, this limpet is able to modulate its foraging strategies and resting height, according to local, daily changes in environmental conditions.     

An assessment of variation in molluscan grazing pressure on Hong Kong rocky shores

Several methods of assessment have been used to document variation in grazing pressure on temperate rocky shores, although often these methods are applied without consideration of local conditions or species. In this study, a comparison was made between abundance counts of inactive molluscan grazers at low tide, direct observations of grazer activity and distribution throughout day and night tidal cycles, and records of grazing marks on wax discs, for the mid-shore of Hong Kong. The abundance of grazers found during low-tide counts varied among dates, sites and species. This method, however, did not record all grazer species that day/night observations showed to migrate from the low shore with the rising tide. Low-tide counts, therefore, underestimate grazing pressure (number of active grazers per unit area) and grazer guild (number of species). Grazing marks on wax discs also recorded a greater number of species than the low-tide counts of inactive grazers, and included grazers that were seen to migrate up shore during day/night observations. Certain limpet species, however, avoided the wax and did not leave grazing marks, showing this method to also underestimate grazing pressure. All methods showed grazing pressure to be variable at spatial scales of tens of metres or less and also temporally variable between sampling dates. The sole use of either low-tide counts or wax discs is likely to underestimate grazing pressure, due to variation in shore topography and grazer foraging behaviour, especially on shores with a narrow tidal range such as in Hong Kong. To gain a more accurate assessment of total grazing pressure, it is suggested that recording of grazing marks on wax discs should be used in conjunction with direct day/night observations.     

Diurnal and lunar patterns of larval recruitment of Brachyura into a mangrove estuary system in Ranong Province, Thailand

The present study followed the temporal recruitment pattern of brachyuran larvae in a mangrove tidal creek on the Andaman Sea coast of Ranong Province, Thailand, based on the assumption that the processes governing recruitment are important for the overall population dynamics of mangrove brachyuran crabs. Plankton net samples were taken on five occasions: on two new moon spring tides, one waxing moon neap tide, one full moon spring tide and one waning moon neap tide during October and November 1997. In addition collectors for larval crab megalopae were employed every 3 days through one dry season and one wet season (March–October 1998). Both the plankton net samples and collector samples revealed four major brachyuran groups in three families: Ocypodidae, Grapsidae and Portunidae. The grapsid group was further separated into two morphotypes which were identified as Metaplax and sesarmid species. Identified group mean numbers per cubic metre were ocypodids 3.0, sesarmids 0.8 and Metaplax 0.5, while portunid megalopae were very scarce (≪0.1 m⁻³). Further analysis of plankton net samples showed that when considering the parameters date, depth, current direction and the diel cycle, Metaplax and ocypodids distribute differently in the tidal and lunar cycle. Metaplax recruitment dominates on flood tides and on bottom layers, followed by middle and surface layers. Conversely, ocypodid abundance varied significantly with date only. Notably recruitment was not dependent on the diel cycle for either group. The collector samples of megalopae showed that recruitment of ocypodids, Metaplax and sesarmids occurred on full and new moon spring tides, while portunid megalopae preferred to settle on full moon spring tides. Since tidal currents were related to the lunar cycle megalopa groups are also cross-correlated with tidal amplitude, except for the portunid group. It is concluded that megalopae recruit in a similar manner to what has been found in other regions of the world, except that the abundance of ocypodids and Metaplax is not influenced by the diel cycle. Received: 14 February 2000 / Accepted: 24 November 2000     

Abundance of estuarine crab larvae is associated with tidal hydrologic variables

Abundances of brachyuran megalopae and juveniles were measured throughout consecutive tidal cycles during six 2 to 3 d sampling periods in summer 1992, and associated with rates of change of tidal hydrologic variables in the Newport River Estuary. Current speeds and rates of pressure change fitted sinusoidal (tidal) models well; however, rates of salinity and temperature change did not. Analysis of plankton samples taken during spring and neap tides showed peak abundances during nighttime rising tides for all taxonomic groups: Callinectes sapidus, Uca spp., Xanthidae, and Pinnixa spp. megalopae, and Pinnotheres spp. juveniles. Megalopal and juvenile abundances from time-intensive sampling were related to rates of changes in the hydrologic variables using stepwise logistic regression. No hydrologic variable accounted well for the presence of Uca spp. megalopae. Megalopal presence was best predicted by current speed for Pinnixa spp. megalopae, and rates of changes in pressure for xanthid megalopae and Pinnotheres spp. juveniles, and salinity for C. sapidus megalopae. These variables might act as cues causing megalopae to ascend into the water column at a particular point in the flooding tide, and subsequently descend to or near the bottom prior to ebb flow. In this way, larvae which develop on the continental shelf or lower estuary undergo transport up the estuary by behaviorally altering their swimming activity and depth concurrent with tidal changes.     

Temporal and spatial variation in the density of mobile epifauna and grazing damage on the seaweed<Emphasis Type="Italic"> Ascophyllum nodosum</Emphasis>

The temporal and multiple-scale spatial variation in the density of mobile epifauna and grazing damage was investigated in populations of the brown seaweed Ascophyllum nodosum (Fucales: Phaeophyta). The relationship between density of grazers and grazing marks was also analyzed. The study was carried out in two locations of the northeastern Atlantic: one at the Isle of Man, in the Irish Sea, and the other at Tjärnö, on the Swedish west coast. Furthermore, the effect of grazing marks on the probability of frond breakage was evaluated in a 1-year experiment carried out on the Isle of Man. High temporal and small-scale spatial variability was recorded in the density of mobile epifauna. A high percentage of species were herbivores, i.e. mesograzers. Adult plants of A. nodosum have several primary vegetative fronds with apical growth. Grazing marks were commonly found along the fronds. Similar levels of grazing damage were detected at the two locations, despite the observed qualitative differences in the grazing assemblages. Small-scale spatial variability in grazing marks was important, with differences in the density of grazing wounds among plants, fronds, and even within fronds. A significant correlation between the proportion of grazed apices and the density of the isopod Idotea spp. and grazing amphipods considered together was detected. This probably reflects the simultaneous feeding activity of these two groups of grazers and suggests the presence of feeding facilitation mechanisms between them. Grazing marks were concentrated in the apical part of the shoots. The results of this study also showed that grazing damage increased the probability of breakage, especially of shorter fronds. This suggests that grazing damage affects the demography of shoots and plants, increasing the transition probabilities to smaller sizes. The presence of interactive effects between grazing and plant density in these A. nodosum populations of northeastern Atlantic shores was also discussed on the basis of the results of this and a previous study.     

Effects of brackish water incursions and diel phasing of tides on vertical excursions of the keystone predator Pisaster ochraceus

The physical factors that constrain the vertical foraging excursions of the keystone predator, the sea star Pisaster ochraceus, hold considerable interest because they indirectly shape the vivid patterns of zonation of rocky shore communities by impeding or enhancing the ability of P. ochraceus to traverse the intertidal zone. In this paper, we describe a study conducted in the Pacific Northwest of North America in which we examined, in the field and laboratory, the abiotic factors that can affect vertical excursions by P. ochraceus. Our field observations revealed that the extreme upward reach and average shore level height reached by P. ochraceus were significantly lower for daylight high tides than nocturnal high tides. Based on diver observations following a severe storm, it would also appear that these diurnal movements can be impeded by freshwater incursions into the intertidal zone; a regularly occurring event in the Pacific Northwest. As part of an experimental investigation into this phenomenon, we observed that sea stars maintained in tall cylindrical aquaria, without tidal flux, remained near the bottom during daylight and moved to the top of the column at night, suggesting that photoperiod alone can influence the cycle of vertical movement. Adding a freshwater layer to the aquaria restricted these vertical excursions. Our results suggest that on rocky coastlines susceptible to fresh water incursions, the suppression of foraging may be an important factor in the spatial and temporal variation in the intensity of predation. Furthermore, given the relative increase in frequency and intensity of freshwater incursions in the Pacific Northwest and the intolerance of P. ochraceus to lowered salinity, there is the long-term potential to significantly alter patterns of species zonation in this essential marine habitat.     

Mechanisms of habitat segregation between an invasive (Mytilus galloprovincialis) and an indigenous (Perna perna) mussel: adult growth and mortality

The invasive mussel Mytilus galloprovincialis and the indigenous mussel Perna perna coexist intertidally on the south coast of South Africa through partial vertical habitat segregation: M. galloprovincialis dominates the upper shore and P. perna the lower shore. Recruitment patterns can explain the zonation of P. perna, but not the invasive species. We examined the role of post-recruitment interactions by measuring spatial and temporal differences in adult growth and mortality rates of the two species. Specifically, we tested the hypothesis that interspecific differences in growth and mortality reflect adult distribution patterns. The two study locations, Plettenberg Bay and Tsitsikamma, are 70 km apart with two sites (separated by 300–400 m) per location, each divided into three vertical zones. Growth was measured seasonally using different marking methods in 2001 and 2003. Cumulative adult mortality was measured through summer in 2003/2004. Both species generally grew more slowly upshore, but they showed different effects of season. For P. perna, growth was significantly reduced in winter in the low zone, but unaffected by season in the high zone. For M. galloprovincialis, growth was either unaffected by season or increased in winter, even in the high zone. Thus, growth of P. perna and M. galloprovincialis was reduced under cool winter and warm summer temperatures, respectively; and while growth was more similar between species in summer, M. galloprovincialis grew much faster than P. perna in winter. Mortality of P. perna increased upshore. For M. galloprovincialis, mortality was not zone-dependent and was significantly greater than for P. perna on the low-shore and (generally) across the shore in Tsitsikamma. Both species had higher growth and mortality rates in Plettenberg Bay than in Tsitsikamma. Thus, P. perna seems able to maintain spatial dominance on the low-shore and at certain sites because of higher mortality of M. galloprovincialis. We conclude that seasonality in growth of the two species reflects their biogeographic affinities and that coexistence is possible through pre-recruitment effects that limit the vertical distribution of P. perna and post-recruitment effects that limit M. galloprovincialis.     

Spatial and temporal strategies in the surface activity of some sandy beach arthropods living along the French Atlantic Coast

During a field study on the eco-ethology of sandy beach arthropods conducted along the French Atlantic Coast it emerged thatTylos europaeus, Phaleria cadaverina andLabidura riparia were among the most abundant species that lived in the eulittoral. Tetradirectional pitfall traps, placed from the base of the dune to the tidal limits, were used. The traps intercepted surface-active arthropods, and the number of traps varied according to the levels of the tides. Data were recorded hourly during two periods corresponding to a spring tide and the following neap tide. At the saine time the most important environmental parameters were registered each hour. The results indicate that all three species show prevalently nocturnal surface activities. ForT. europaeus andPh. cadaverina an influence of the synodic and tidal phases was found on the zonation of the surface activity and on the extent of movement. The latter was due to the greater or smaller distances that the isopods and tenebrionids had to move from the diurnal burrowing sites to the foraging zones. ForL. riparia no variations in the total mean zonations were found between the two synodic phases, and only at neap tide were seaward, followed by landward movements found. Multiple regression analysis showed that the environmental parameters registered in this period were never limiting, as threshold values in this case probably were not reached.     

Rhythmicity of larval release in three species of intertidal brachyuran crabs (Crustacea: Brachyura) from Inhaca Island (Mozambique)

 A study of rhythmicity of larval release in three species of intertidal brachyuran crabs, based on laboratory and field experiments, was undertaken at Inhaca Island, southern Mozambique, using Leptodius exaratus and Macrophthalmus grandidieri from December 1994 to January 1995, and Arcotheres palaensis from April to July 1995. L. exaratus and M. grandidieri showed a semi-lunar cycle in larval release. The release of larvae for L. exaratus, a species having conspicuous larvae, occurred in the first half of the night, after the post-crepuscular high tide, which suggests maximisation of protection of larvae from visual predation. The larval release activity matched the late spring and early neap tides. Results from the field were similar to those from the laboratory. M. grandidieri, having inconspicuous larvae, did not show a pattern related to the light–dark cycle and hatched during spring tides (around full and new moons) to maximise larval dispersion. A. palaensis, living inside the host mussel which inhabits the lowest section of the intertidal zone, did not show a relation with moon phase, tidal or light–dark cycles. Received: 16 February 1999 / Accepted: 8 December 1999     

Movement patterns of the limpet Cellana grata (Gould) observed over a continuous period through a changing tidal regime

Time-lapse videography of limpets mounted with light-emitting diodes was used to monitor the movements of a population of the non-homing Cellana grata on a vertical gully wall in Hong Kong. Animals were monitored for >7 days to examine spatial and temporal variation in their behaviour as the tides transited from a semi-diurnal to an almost diurnal pattern. Movement was synchronised with the tides, irrespective of the day–night cycles. Limpets rested low on the shore and were stimulated to move by the rising tide. Individuals moved up shore with the flooding tide, maintaining themselves in the awash zone, and then down shore on the ebbing tide, until they reached a resting height when the tide then fell beneath them, exhibiting ‘zonal shuttling’. A tight coupling of limpet position to tide height persisted through the changing tidal pattern, and almost all animals displayed the same organisation of activity over all tides. Initiation of activity and maximum height reached were probably controlled by the tides, but the cessation of activity may have been controlled by an internal clock. The pattern observed is consistent with the threat of attack from aquatic predators coupled with the need to minimise physical stresses while exposed. It is also consistent with the avoidance of grazing lower on the shore where interspecific competition may be more intense. Limpets showed two peaks of activity per tide, corresponding to when the rate of change of tide height was the greatest, except when tides became much reduced during the transit to a diurnal pattern. Movement was triphasic: an initial rapid phase, followed by a slower phase in the high shore and then a rapid phase before the limpets stopped moving. This structure is common in limpets and in this case is likely to be a consequence of animals maintaining themselves within the awash zone. Tide height appears to determine foraging activity, but with modifications in the behaviour in response to factors operating at more local temporal and spatial scales.     

Estuarine epifauna recruit despite periodic hypoxia stress

In some estuaries, the recruitment of epifaunal benthic invertebrates coincides with a significant environmental stress, low water-column dissolved oxygen, termed hypoxia (̀ mg O₂ l^-1). Recruitment of epifaunal species was measured in the lower York River, a subestuary of the Chesapeake Bay, USA, which experiences predictable, periodic hypoxia associated with neap/spring tidal cycling during summer. Recruitment substrata were exposed during 48-h deployments in two areas with differing levels of hypoxia, and epifauna were allowed to recruit during periods of low oxygen (neap tides) and high oxygen (spring tides) in 1996 and 1997. Recruitment was often high during neap tides, even when severe oxygen depletion (<0.5 mg O₂ l^-1, <0.71 ml O₂ l^-1) occurred during deployments; indeed, peak recruitment episodes of several dominant epifaunal taxa, and of total epifauna, coincided with hypoxic events during both summers. Increased recruitment during neap tides suggests that factors besides hypoxia influenced recruitment in the York River; these factors may have included changes in larval availability and lower current speeds. This study illustrates how the relationship between recruitment and large-scale stresses, such as hypoxia, may be difficult to predict, since large-scale stresses are often correlated with numerous other factors. Short-term hypoxia appears to have little effect on recruitment in the field for many epifaunal species in this ecosystem, which may explain, in part, why substrata exposed for longer durations (1 month) in this system showed few effects of hypoxia on community composition or diversity. High larval tolerance of hypoxic stress may allow communities to persist even though the summer hypoxia season coincides with the recruitment of many epifaunal species. Electronic Supplementary Material is available if you access this article at http://dx.doi.org/10.1007/s00227-002-0930-6. On that page (frame on the left side), a link takes you directly to the supplementary material.     

Rhythmic activity responses of the fiddler crab Uca crenulata to artificial tides and artificial light

The fiddler crab Uca crenulata, from California (USA), was exposed to artificial tides in order to differentiate between the influence of a tidal cycle from that of a light-dark cycle on its locomotor activity. Most crabs could be well synchronized by tides, but the activity patterns of other crabs was merely exogenously reinforced. Under constant conditions, after tidal treatment, crabs showed bimodal or unimodal activity patterns. When a 24 h light-dark cycle and a 12.35 h tidal cycle acted as concurrent stimuli, their effectiveness in synchronizing the crabs' activity was variable, depending on the undividual. It is likely that crabs which tend to exhibit a unimodal activity pattern are more sensitive to a light stimulus, whereas crabs with a bimodal activity pattern respond preferably to the tides. Thus, responses of endogenous activity of U. crenulata to tides and light-dark cycles are similar, suggesting that the basic oscillarory mechanism for circadian and tidal activity may be the same.     

Chlorophyll blooms and the spring/neap tidal cycle: observations at two stations on the coast of Connemara,Ireland

Water temperature, chlorophyll a and phaeophytin were recorded every 2 to 4 d, in order to describe short-term variability in phytoplankton blooms. Data were collected from May to September 1984 and March to September 1985 at shore stations on the coast of Connemara, western Ireland. Comparisons with offshore data suggest that the shore data is representative of coastal water. Chlorophyll a was highest at neap tides in spring, and at spring tides in early summer. In late summer, transient chlorophyll blooms occurred at neap tides, possibly due to a localized accumulation of flagellates in stable conditions.     

Foraging and homing behaviour in the high-shore, crevice-dwelling limpet Helcion pectunculus (Prosobranchia: Patellidae)

The foraging activity of the high-shore, crevice-dwelling limpet Helcion pectunculus (Gmelin, 1791) (Prosobranchia: Patellidae) from an exposed shore on the eastern Cape coast of South Africa was monitored. Activity was compared during spring and neap tides and between spring, summer, autumn and winter. Rhythms of activity in this limpet varied depending upon microhabitat; individuals inhabiting both east- and west-facing rock surfaces were active during nocturnal low tides, but limpets on west-facing rock surfaces were also active during daytime low tides, whilst in the shade. Individuals travelled further during foraging excursions in winter (xˉ=85.53 cm) than in either spring (xˉ=55.7 cm) or summer (xˉ=48.8 cm) and also during spring low tides (xˉ=89.8 cm) compared with neaps (xˉ=40.9 cm). H. pectunculus exhibited rigid homing to a fixed scar within a crevice, and feeding excursions were found to consist of three distinct phases: a relatively rapid outward phase, a much slower foraging phase and a rapid homeward phase. Whether or not these limpets graze throughout an excursion is not known. Foraging was always highly directional, with a mean vector which took limpets onto an area of the rock face with the highest microalgal biomass. The fact that H. pectunculus took advantage of optimal feeding areas and memorized their location to enable return visits suggests a learning component in herbivorous gastropod foraging behaviour. Received: 29 December 1997 / Accepted: 8 May 1998     

Nocturnal feeding migrations of Nerita plicata,N. undata and N. textilis (Prosobranchia: Neritacea) on the rocky shores at Mkomani,Mombasa, Kenya

The vertical zonation of the three common rocky shore neritids at Mkomani, Mombasa, Kenya, Nerita plicata Linnaeus, N. undata Linnaeus, and N. textilis Dillwyn, as a function of feeding migrations and of size, was studied from 28 February to 24 March 1983. These snails perform feeding migrations at night starting at around mid-ebb tide and return to their resting positions with the flood tide. They remain in their resting positions throughout the day until the next nocturnal ebb tide. The direction of migration is sizerelated, with the larger snails of each species moving in the opposite vertical direction to the smaller ones, so that the populations as a whole exhibit no statistically significant net vertical displacement. The larger individuals of two of the species, N. plicata and N. undata, invariably move downwards to their feeding levels, while the smaller individuals move upwards; the larger individuals of N. textilis display a different pattern of migration, moving downwards on and around spring-tide days and upwards on and around neap tide days, while the smaller individuals move in the opposite directions. N. textilis rest above their feeding level around spring tides, and below that level around neap tides. It is demonstrated how these nocturnal migratory feeding rhythms are integrated into the spring-neap and seasonal cycles of the snails' daytime resting positions. The adaptive significance of these migrations is also discussed.     

The role of colonization in determining spatial patterns of Proscoloplos bondi sp. nov. (Orbiniidae: Annelida) in coralline algal turf

We describe Proscoloplos bondi sp. nov., a new species of orbinid polychaete found in coralline algal turf on rocky intertidal shores near Sydney, Australia. We used field sampling to describe P. bondi's patterns of spatial and temporal variation on a number of shores. P. bondi was significantly more abundant in low- than in mid-shore areas (ANOVA, P<0.05), but local populations were extremely variable in space and time. Despite extensive sampling, we did not find evidence of sexual reproduction in P. bondi, but commonly observed worms with regenerating heads and tails, which indicated the importance of asexual reproduction. We, therefore, tested the hypothesis that adult colonization contributes to its patterns of abundance. First, patches of artificial turf were placed near to and away from natural coralline turf and sampled after 2, 4 and 12 months. Generally, populations established themselves more quickly in patches of new habitat directly adjacent to coralline turf than in patches that were isolated. Second, patches of turf were deployed for 2-week periods in low- and mid-shore areas (repeated four times). Proscoloplos bondi were found significantly more often in patches in low-shore areas than in mid-shore areas and at some sites more than others (ANOVA, P<0.05), demonstrating the importance of adult colonization in determining patterns of abundance. Overall, the limited dispersal from spatially and temporally ephemeral populations of P. bondi combined with the patchiness of coralline turfs suggest that metapopulation dynamics may be important in the persistence of P. bondi on rocky shores.Communicated by G.F. Humphrey, Sydney     

Factors affecting variability of foraging excursions in a population of the limpet Patella vulgata (Mollusca,Gastropoda)

Paths moved by the limpet Patella vulgata L. were monitored on a sheltered vertical rocky shore in North Wales using time-lapse photography throughout nocturnal low tides in April and November 1992, roughly corresponding to periods of minimum and maximum gonad ripeness, respectively. Various motion parameters, including total duration, total length, and maximum distance reached from home were computed from 124 complete foraging routes obtained from 18 higher-zoned and 18 lower-zoned limpets. P. vulgata typically performed a single loop per night (average total length, ca. 70 cm), moving from the individual home scar to graze the surrounding algal grounds (average maximum distance, ca. 25 cm), then following its own trail back home. On the average, limpets moved from the home scar during three of four available nocturnal low tides and exploited about half the emersion time. Evidence for size-related variability in behaviour was found, as a positive correlation was assessed between both total duration and legth of the excursions, and shell length. In addition, zonation was proved to affect the limpets' foraging behaviour, since low-shore limpets moved faster and covered greater distances than high-shore ones. Moreover, consistent seasonal variation in foraging behaviour emerged, since in April the excursions were longer and longer-lasting than in November, but limpets exploited a larger fraction of potential activity phases in November than in April. The within-population variability in the temporal and spatial characteristics of the foraging excursions is discussed in relation to the available data on zonal and seasonal variation of food resources and in relation to physiological changes due to reproductive cycle.     

Field observations on the rhythmic behaviour of Nerita textilis (Gastropoda: Prosobranchia)

Field observations on Nerita textilis Dillwyn inhabiting rocky cliffs at Sar Uanle (Southern Somalia) revealed that during the resting phase some of the snails are scattered on the upper cliff and some are clustered at lower levels. During feeding they all migrate downward, but to different extents. Maximum activity of both groups occurs during low water (LW) between midday and midnight. During spring tides (ST), the scattered snails reside higher up and make smaller feeding excursions than during neap tides (NT). During ST, some clustered snails join the belt of scattered individuals, while the trend reverses during NT. A diurnal and a tidal component appear in their periodicity.Publication of the Centro di Studio per la Faunistica ed Ecologia Tropicali del C.N.R. Researches on the coast of Somalia. The shore and the dune of Sar Uanle, No. 14.     

Tidal influences on the photoperiodic induction of tetrasporogenesis in Bonnemaisonia hamifera (Rhodophyta)

Photoperiodic control of tetrasporangium formation was studied in two low intertidal populations of Bonnemaisonia hamifera (Trailliella-phase) in Galway Bay, Ireland between September 1985 and February 1986. Photoperiodic induction was affected by tides, principally by the time of day at which high and low water occurred through the spring/neap cycle. High water of spring tides falling at the beginning and end of the day reduced the effective daylength for plants, and caused an early onset of reproduction. The threshold irradiance signalling light-dark transition was estimated to lie between (1-) 5 and 10 mol photons m^-2 s^-1 (400 to 700 nm). Low water of spring tides falling in the middle of the day exposed plants to high air temperatures during periods when seawater temperatures were below the lower limit for sporangium induction. This caused reproduction to be resumed. Quantitative differences between the two populations were caused by microclimatic effects (temperature, light reduction during tidal flooding) resulting from their different vertical heights on the shore.     

Size-assortative mating and non-reciprocal copulation in a hermaphroditic intertidal limpet: test of the mate availability hypothesis

Mating behaviour and mating patterns are affected to a large extent by body size in both hermaphrodites and gonochorists. Detailed research on mating patterns, mate choice, pre- and post-copulatory sexual selection in marine pulmonates is wanting, thus warranting more attention as a study system in the future. The simultaneous hermaphroditic limpet Siphonaria capensis (Pulmonata: Basommatophora) shows size-dependent fecundity, and acts as a suitable organism to test the effects of body size on mating patterns, mating success, gender expression and reciprocity in hermaphroditic marine gastropods. We mainly used bootstrap resampling techniques to estimate the effects of different factors on mating patterns. In the populations studied, a strong size-assortative mating pattern was observed, where small-scale spatial distribution of potential mating partners (the mate availability hypothesis) could explain 65% of this pattern, while mate choice and mating constraints explaining the rest. No significant difference in mating success between limpets with different body size was found, even though in one population, the sperm recipients were larger than the non-copulating limpets. Interestingly, we found that intromission was non-reciprocal during copulations. This may mean that this species does not conform to the common rule of reciprocity predicted for hermaphrodites, unless there is sex-role alternation between individuals in a mating pair. The mating partners consisted of similar sized, acting males and females, thus without any indication of the body size determining the gender expression. The matings took place in early mornings only at spring tides and the animals were observed laying egg masses only during neap tides. Since siphonariid limpets possess both a spermatheca and a seminal vesicle, which may have either a sperm storing and/or digestive function, post-copulatory sexual selection (e.g., sperm competition) cannot be dismissed.