Foraging movements and homing in the limpet Patella vulgata L

In two populations of Patella vulgata in the Isle of Man foraging activity is restricted to daytime high waters, with about 75 per cent of the population foraging on any one day. Movement commences as the tide covers the limpets, and they travel an average of 0·4 m from their home sites to which they return at least an hour before the falling tide exposes them again. The mean speed on the outward and return journey is around 0·60 cm min^?, and during the intervening browsing period about 0·08 cm min^?. This behaviour contrasts with that of the same species in Alderney, where foraging is limited to nocturnal low waters: there is possibly a geographical trend in behaviour. P. vulgata, especially in the Isle of Man, is one of the more mobile limpets.     

Monitoring rocky-shore communities: a critical look at spatial and temporal variation

Increasingly programmes are being set up to monitor rocky-shore communities in order to provide baseline data which will indicate changes resulting from subsequent pollution. However, these efforts are complicated by several factors. Firstly, there are overall changes in the composition of communities both within and between years. Secondly, there is variation within certain communities due to a mosaic distribution of components, the mosaic format changing continuously with a cycle of several years. This paper reports on studies of a medium-exposed rocky shore in the Isle of Man (U. K.). It describes patterns of spatial and temporal variation, and looks at certain implications for monitoring programmes: (a) the frequency of sampling, and the duration of the sampling programme, in the light of seasonal and long-term variation; (b) the efficiency, in terms of the minimisation of variability, of sampling the same area by different strategies — belt transects, small random quadrats, single large quadrat; (c) the effect of the distribution patterns of some commoner species on the variation between samples.     

An investigation of the movement of the scallop,Pecten maximus

Summary 1. In the experimental area specimens of the scallop,Pecten maximus, occur at a density of one every 1 1/2 m². Over 90% are recessed in the bottom.2. Unrecessed specimens remain in the same position on an average for 6 days. Recessed specimens remain on an average 27 days.3. The experimental area has virtually unidirectional tidal flow. The scallops tend to orientate themselves on the bottom so that this flow complements their ciliary currents.

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Eine Untersuchung über die Fortbewegung der KammuschelPecten maximus

Kurzfassung ObgleichPecten maximus (L.) normalerweise im Meeresboden ruht, kann dieses Mollusk recht aktiv schwimmen. Frühere Untersuchungen über die Fortbewegung werden durch Wiederfang markierter Individuen mit Hilfe einer Dredsche durchgeführt; eine derartige Methode kann lediglich etwas über weiträumigere Ortsveränderungen aussagen. In der vorliegenden Studie wurde nun ein kleines Areal mit fixierten Bezugspunkten versehen und — unter Anwendung der Methode des wissenschaftlichen Schwimmtauchens — in bestimmten Zeitabständen 8 Wochen lang überwacht. Auf diese Weise konnte sowohl die Häufigkeit der Ortsveränderungen als auch die Orientierung erfaßt werden. Durchschnittlich verblieben nicht eingegrabene Individuen nur 6 Tage am gleichen Ort, eingegrabene Tiere dagegen auf Grund von Beobachtungswerten 17 Tage (Berechnungen deuten jedoch auf einen korrigierten Endwert von 27 Tagen hin). Es wird angenommen, daß spontanes Schwimmen im wesentlichen nur im Anschluß an Störungen durch andere Tiere erfolgt. Die Orientierung am Boden richtete sich vorwiegend nach dem am Untersuchungsort praktisch nur in einer Richtung laufenden Wasserstrom.P. maximus orientierte sich dabei im allgemeinen derart, daß die Wasserströmung ihre Freßströmungen unterstützt.

     

Reproductive Investment in Brachyura

The costs of reproduction in brachyurans are reviewed. In both sexes a number of aspects of reproductive behaviour and physiology can result in higher mortality and/or slower growth. Mortality can be increased by the greater exposure or the reduced mobility of the crabs: these are illustrated by selected examples, but there is a dearth of quantitative data. Slower growth can result from reduced feeding, or the diversion of resources to reproduction. These have been better quantified. The investment by females in egg production is examined in detail. In free living crabs investment per brood in dry weight terms ranges from 3–21%, with a mean of about 11%. This investment is constrained by body morphology. However, in commensal/parasitic crabs the value rises to 60–90%: reasons for this are discussed. Many crabs produce more than one brood per year, and annual investment rises with brood number, though with some trade off. For temperate free living species annual investment ranges from 6–64%, with a mean of 22%. However tropical species produce more broods, and annual investment can reach 250%. In both temperate and tropical regimes there is a higher annual investment by commensal/parasitic species, of up to 470%. Priorities for future research are outlined.     

The establishment of fucoid zonation on algal-dominated rocky shores: hypotheses derived from a simulation model

1. A model was developed for the growth of intertidal algae with photosynthesis simulated both in air and in a tidal water column. Morphological data on dry mass per unit area and length–area relationships were used to separate the growth of different fucoid species. The relative growth rate of fronds at any height on the shore depended on a trade-off between net photosynthetic performance and tolerance to desiccation.
2. The simulated zonation patterns and growth rates were consistent with those observed previously for Fucus spp. and Pelvetia canaliculata .
3. The simulated growth of Ascophyllum nodosum was always slower than for the other species. This species did not form its characteristic distribution zones in simulations without including further processes in the model. However, Ascophyllum collected from the field could be separated into upper and lower shore morphologies which formed separate zones when they were simulated in competition with each other.
4. Several hypotheses were proposed concerning the relative locations and sharpness of interspecies boundaries on the shore. Zonation patterns were relatively insensitive to changes in most model parameters except the desiccation rate.     

Food supply, grazing activity and growth rate in the limpet Patella vulgata L.: a comparison between exposed and sheltered shores

The limpet Patella vulgata L. is an important microphagous grazer on intertidal rocky shores of north-west Europe, occurring across the wave exposure gradient. Groups of P. vulgata were selected at mid-tide level of two exposed shores and two sheltered, fucoid dominated shores on the Isle of Man, British Isles, and manipulated to equivalent densities and population structure. The level of grazing activity and growth rate were determined over a 1-year period. At the same time, the abundance of epilithic microalgae, measured as the concentration of chlorophyll a, was determined as an estimate of food supply. Microagal abundance showed a seasonal pattern in both exposed and sheltered conditions, with higher levels in winter compared to summer. In both seasons, the microalgal resource was more abundant on the sheltered shore studied. The level of grazing activity in P. vulgata showed a seasonal pattern on the exposed but not the sheltered shores. Averaged over the year, grazing activity on the exposed shores was over double that on sheltered shores. Thus, in sheltered conditions, food supply for limpets was high and grazing activity low; in exposed conditions, food supply was low and grazing activity high. The growth rate of P. vulgata, measured as increase in shell length, showed no significant difference between exposed and sheltered shores. Growth rate was also determined in P. vulgata at natural densities. Although the overall density declined with decreasing exposure to wave action, the density per unit area of grazeable substance was higher in shelter. In these populations, the mean growth rate was over twice as high on exposed compared to sheltered shores.     

Megalopae and early postlarval stages of East African Percnon (Decapoda: Brachyura: Grapsidae)

The megalopae and first crab instars are described for three of the four Indo-west Pacific species of the grapsid crab Percnon—P. abbreviatum, P. guinotae and P. planissimum . Keys are provided to these three species and for the Atlantic/East Pacific P. gibbesi . The validity of separating P. planissimum and P. gibbesi is discussed, as are the implications of the unusually large megalopa in Percnon .     

The winter and summer partitioning of dissolved nitrogen and phosphorus. Observations across the Irish Sea during 1997 and 1998

Winter and summer water column profiles from a transect co-inciding with the 54° N parallel across the Irish Sea were examined for relationships between the fractions of total dissolved nitrogen and phosphorus. Inorganic nitrogen and phosphorus gave the expected seasonal variation. As inorganic nitrogen concentrations decreased during the summer, dissolved organic nitrogen concentrations increased. Dissolved organic phosphorus showed little seasonal variation and formed a smaller fraction of the total dissolved pool compared to nitrogen. Winter inorganic nitrogen and phosphorus salinity relationships were conservative: winter organic nitrogen and phosphorus salinity relationships were not conservative. In the winter, dissolved organic nitrogen and phosphorus formed a significant fraction of the total dissolved pool.     

Aspects of the biology of the deep-sea spider crab, Dorhynchus thomsoni (Crustacea: Brachyura)

Pelagic first crab stages of the deep-sea spider crab, Dorhynchus thomsoni , are reported from mid-water samples taken at a variety of localities in the Porcupine Seabight and the Rockall Trough. Some of these samples were from hauls made near the surface over depths of 3000 m or more.
An analysis of these pelagic samples, and of benthic catches of D. thomsoni held by the Institute of Oceanographic Sciences and the British Museum (Natural History), indicate that most females and at least some males undergo the puberty prior to the third crab instar, these individuals therefore probably pass through a total of only three larval and three post-laval instars. There is at least a possibility that some individuals moult after the moult of puberty, unlike any other spider crabs.