Reproductive behavior of the Atlantic silverside,Menidia menidia (Pisces,Atherinidae)

Synopsis The spawning behavior of the Atlantic silverside,Menidia menidia, was studied at two sites on the North Edisto River estuary in South Carolina. Prespawning schools moved back and forth along the shoreline as the time of high tide approached. Spawning runs took place in the upper intertidal zone at high tide.Spawning fish deposited their eggs on three types of substrates: 1) the lower stems of cordgrass plants,Spartina alterniflora, 2) detrital mats, and 3) exposed cordgrass roots along erosional scraps. Spawning behavior during egg deposition and fertilization was similar for all three substrates. Females released eggs during a rapid fluttering motion of the posterior half of the body. A similar movement accompanied release of sperm by males. Behavior of fish just prior to spawning insured deposition of gametes at locations that provided protection from thermal and drying stress during development. Eggs were deposited at mean intertidal elevations of 1.8 and 1.5 meters above mean low water (MLW) at respective study sites. They were exposed to the atmosphere for approximately ten hours between successive high tides.During spawning runs in which eggs were deposited at the base of cordgrass plants, ambient dissolved oxygen concentrations of the water in the spawning area were sometimes reduced to < 1.0 mg. 1^–1. Spent fish, apparently incurring an oxygen debt while spawning, formed a nonschooling aggregation offshore from the spawning zone.The recurrent use of specific spawning substrates for egg deposition resulted in an uneven distribution of spawning runs along the shoreline at each study site.Contribution No. 409 to the Gulf Breeze Environmental Research Laboratory. Contribution No. 377 to the Belle W. Baruch Institute for Marine Biology and Coastal Research.     

Enzyme activity in intertidal sands and salt-marsh soils

Summary Arylsulphatase activity was detected and characterized in intertidal sands and salt-marsh soils. In sands, highest activity was found in the rhizosphere ofHippophae rhamnoides L; although low activity was also found near Ammophila roots and in sands lacking plant-cover. Highest arylsulphatase activities, however, occurred in salt-marsh soils underSalicornia sp. Properties of arylsulphatase were similar in nearly all respects, to those reported for soil and marine sediment arylsulphatase.Linearity between amount of sand or salt-marsh soil and cellulase and rhodanese activities were also demonstrated, but no attempt was made to further characterise these enzymes.     

Blue- and green-light signals for gamete release in the brown alga, Silvetia compressa

The intertidal brown alga Silvetia compressa releases gametes from receptacles (the reproductive tissue) rapidly upon a dark transfer (following a photosynthesis-dependent period in the light, termed potentiation). In this study, the wavelength-dependence of this process was investigated. During the potentiation period in white light (WL), gametes are not released. However, gametes were released during potentiation in blue light (BL), or in low red light/blue light (RL/BL) ratios, but not in RL alone, high RL/BL ratios, or in broadband blue-green light (B-GL) (presence of BL, but absence of RL). RL was as effective as WL for potentiation, i.e., both lead to gamete release following transfer to darkness. Rates of linear photosynthetic electron transport were similar in RL and BL. Gamete release in BL was inhibited by equal amounts of additional narrow-waveband light between the green and red regions of the spectrum, with light-induced gamete release restricted between <491 nm and 509 nm. Very little light-induced gamete release occurred between 530 nm and 650 nm. It is proposed that a BL-responsive photoreceptor is responsible for light-induced gamete release. Transfer of WL-potentiated receptacles to GL near 530 nm resulted in significant de-potentiation and reduced gamete release during a subsequent dark transfer. This effect was not seen at 509 nm or 560 nm and revealed the presence of a second photoreceptor system repressing or counteracting potentiation in the light. We propose that the restriction of gamete release to periods when irradiance is blue-shifted may constitute a depth-sensing mechanism for this intertidal alga, allowing controlled release of gametes at high tide and/or less turbid periods, thus minimizing gamete dilution, and promoting fertilization success.     

Cold tolerance of Littorinidae from southern Africa: intertidal snails are not constrained to freeze tolerance

All intertidal gastropods for which cold tolerance strategies have been assessed have been shown to be freeze tolerant. Thus, freeze tolerance is considered an adaptation to the intertidal environment. We investigated the cold tolerance strategies of three species of subtropical and temperate snails (Gastropoda: Littorinidae) to determine whether this group is phylogenetically constrained to freeze tolerance. We exposed dry acclimated and wet rehydrated snails to low temperatures to determine temperature of crystallisation (Tc), lower lethal temperature and LT₅₀ and to examine the relationship between ice formation and mortality. Tc was lowest in dry Afrolittorina knysnaensis (–13.6±0.4 °C), followed by dry Echinolittorina natalensis (–10.9±0.2 °C) and wet A. knysnaensis (–10.2±0.2 °C). The Tc of both A. knysnaensis and E. natalensis increased with rehydration, whereas Tc of dry and wet Afrolittorina africana did not differ (–9.6±0.2 and –9.0±0.2 °C respectively). Wet snails of all species exhibited no or low survival of inoculative freezing, whereas dry individuals of A. knysnaensis could survive subzero temperatures above –8 °C when freezing was inoculated with ice. In the absence of external ice, Afrolittorina knysnaensis employs a freeze-avoidance strategy of cold tolerance, the first time this has been reported for an intertidal snail, indicating that there is no family-level phylogenetic constraint to freeze tolerance. Echinolittorina natalensis and A. africana both showed pre-freeze mortality and survival of some internal ice formation, but were not cold hardy in any strict sense.     

A Test of the Higher-Taxon Approach in the Identification of Candidate Sites for Marine Reserves

Alternatives to species-level identification have been advocated as one solution to the problem of selecting marine reserves with limited information on the distribution of marine biodiversity.This study evaluated the effects on selection of candidate sites for marine reserves from using the higher-taxon approach as a surrogate for species-level identification of intertidal molluscs and rocky reef fishes. These effects were evaluated by determining the percentage of species included in candidate reserves identified from genus-, family- and order-level data by a complementarity-based reserve selection algorithm, and by testing for correlations between the irreplaceability values of locations. Candidate reserves identified from genus- and family-level data of intertidal molluscs included a similar percentage of all species as the reserves identified from species-level data. Candidate reserves selected from genus- and family-level data of rocky reef fishes included, respectively, 3–7% and 14–23% fewer species than reserves selected from species-level data. When the reserve identification process was constrained by a practical planning limit (a maximum of 20% locations able to be reserved) the reserves selected from genus- and family-level data of intertidal molluscs, and genus-level data of rocky reef fishes, included a similar percentage of species as the reserves identified from species-level data. Irreplaceability values of locations for species, genera and families of intertidal molluscs were highly correlated, and irreplaceability values of locations for species and genera of rocky reef fishes were highly correlated. This study suggests that genus- and family-level data for intertidal molluscs, and genus-level data for rocky reef fishes, are suitable surrogates for species in the identification of candidate sites for marine reserves.     

Where Turbellaria? Concerning knowledge and ignorance of marine turbellarian ecology

Present knowledge of marine free-living turbellarians is reviewed in the light of a standard ecological text. Little is known of their ecological adaptations or fitness but they are clearly successful animals. Diversity within communities can be considerable, possibly relating to the small size of individuals and temporal and spatial resource gradients. Little is known about genetic diversity, environmental tolerances and demography. Local distribution patterns are better known but rarely proven with appropriate statistics. New data from work in Northern Ireland on behaviour and habitat choice of Procerodes littoralis and on species richness, population aggregation and microdistribution of meiofaunal turbellarians is presented.Nothing appears known about intra-specific competition but more about inter-specific relationships including responses to biogenic structures. Dietary information is sparse and mostly observational with little known about feeding efficiency. Studies at Sylt have revealed mainly univoltine or polyvoltine annual life cycles partly related to habitat preference and possibly giving some evidence of r and K selection. Factors controlling local abundance are largely uninvestigated and the reasons for the apparent rarity of many species not understood.Lists and counts of turbellarians within particular habitats provide some knowledge of community structure but very few numerical analyses, e.g. of diversity and similarity, exist. Contribution to energy flux and to structuring communities has not been determined in spite of the turbellarians' assumed major role as meiofaunal predators. Nevertheless marine turbellarians appear particularly fitted to studies such as habitat heterogeneity, community stability and biogeography and are surely suitable subjects for field and laboratory experiments. There is no lack of direction for studies to proceed. Impulsion is required.     

The effects of seaweed harvesting on fishes: a response

References Rangeley (1994) has argued that sampling biases, errors in experimental design and low statistical power make the conclusion by Black & Miller (1991), that there was no large impact of an experimental harvest of the seaweedAscophyllum nodosum on fishes, unmerited and misleading. Our study was conducted mainly at two sites close to Lower Argyle, southern Nova Scotia, Canada where harvesting ofAscophyllum nodosum has been carried out continuously for 30 years along-side intense and productive shellfish and finfish fisheries. In southern Nova Scotia, on-going removal ofAscophyllum nodosum with mechanical harvesters is about 17% (Sharpe & Semple 1991); planned annual removal in New Brunswick where Rangeley has worked will be only about 5% (Anon. 1993). The context of our investigation was a small-scale (400 m²) but extreme, experimental removal (100%) of the seaweed. Our focus was on fishes that moved into the intertidal zone as the tide rose in the evening and early morning and were caught in trammel nets, and on the fishes that were retained in a fixed seine that was set at high tide in early morning and the fishes removed at the next low tide. Thus, subject to the constraint on the size of the fishes imposed by the nature of our fishing gear with its stretch mesh of 25 mm, we were concerned with the identity and abundance of fishes that moved in and out of the intertidal zone and their behavioural response to the 400 m² patches of habitat that formed our experimental units. The most striking result was the unexpected, low abundance of fishes caught in our gear.     

A re-evaluation of marine nematode productivity

Nematodes are the most abundant multicellular animals in marine sediments but their role in the benthos has not been properly quantified yet. In nearly all energy-flow budgets of marine systems their annual production P is given as about nine times their mean biomass B and their part in the total energy-flow is consequently estimated as anywhere between 3 and 30% of the total (carbon) input in the benthic system. Our laboratory experiments demonstrate that nematode productivity is much higher than P/B 9 per year and may reach values of over 60 for bacterial grazers. To obtain more reliable estimates for field populations we propose a regression equation relating egg-to-egg development time T_min to temperature (t) and adult female weight (W in µg wet weight):log T_min = 2.202–0.0461 t + 0.627 log W. When multiplied by the constant biomass turnover per generation (P/B)_gen = 3, development rate 1/T_min is a good predictor of daily P/B. This method was applied to two series of field data. A rather stable community from a sublittoral mud in the North Sea had an annual P/B = 20. A less stable Aufwuchs community from Sargassum in Japan had an annual P/B = 58.     

Mobilifilum chasei: Morphology and ecology of a spirochete from an intertidal stratified microbial mat community

Spirochetes were found in the lower anoxiphototrophic layer of a stratified microbial mat (North Pond, Laguna Figueroa, Baja California, Mexico). Ultrastructural analysis of thin sections of field samples revealed spirochetes approximately 0.25 m in diameter with 10 or more periplasmic flagella, leading to the interpretation that these spirochetes bear 10 flagellar insertions on each end. Morphometric study showed these free-living spirochetes greatly resemble certain symbiotic ones, i.e., Borrelia and certain termite spirochetes, the transverse sections of which are presented here. The ultrastructure of this spirochete also resembles Hollandina and Diplocalyx (spirochetes symbiotic in arthropods) more than it does Spirochaeta, the well known genus of mud-dwelling spirochetes. The new spirochete was detected in mat material cellected both in 1985 and in 1987. Unique morphology (i.e., conspicuous outer coat of inner membrane, large number of periplasmic flagella) and ecology prompt us to name a new free-living spirochete.This article is dedicated to the memory of our dear friend and colleague David G. Chase (1935–1987)     

Distribution and abundance of the intertidal saltmarsh beetle, Bledius spectabilis

ABSTRACT.

• 1 The burrowing intertidal beetle Bledius spectabilis Kraatz (Coleoptera: Staphylinidae) occurs on all but the very youngest marshes on Scolt Head Island, Norfolk, U.K., and may reach densities of up to 4000 m^?2 (combined adult and larval densities).
• 2 The beetles are restricted to a narrow band about 1m wide along the edges of creeks draining the low marsh areas of the island, and the vertical range is restricted to a zone between 0.8 and 1.55m above Ordnance Datum. The majority of the aggregations and the highest densities are between 1.2 and 1.3m O.D., that is between about 24 and 40cm below MHWN (mean high water at neap tides).
• 3 Poor drainage, in particular the presence of long-standing pools of water over the soil surface, is important in restricting the beetles to well-drained edge regions.
• 4 Transplant experiments suggest that displacement by saltmarsh vegetation, in particular Halimione portulacoides (L.) Aell., sets the upper limit of the beetles’distribution.
• 5 The zonation of B.spectablis fits well into the general scheme that has been proposed for the zonation of marine animals from rocky shores.
• 6 The beetles produce large (3–6 mm diameter), rapidly draining, abundant (up to 9% of soil volume) and persistent burrows. We suggest that these beetle aggregations may play an important role in the development of the saltmarsh.