Canopy gaps formed by mangrove trimming: an experimental test of impact on litter fall and standing litter stock in Southwest Florida (USA)

Mangroves in Florida (USA) are subject to horticultural pruning that may increase the size of canopy gaps and alter rates of litter production and accumulation. Mangrove canopy gap formation is a common phenomenon, known to alter abiotic conditions near the forest floor. Using a series of field experiments in Rookery Bay, Florida, the effects of mangrove trimming on canopy density, mangrove litter production, standing litter stocks, and the decomposition rate of Rhizophora mangle leaves on the forest floor were assessed. Litter trap collections over the year following mangrove trimming indicated that pruned mangrove stands (canopy coverage: 42.8±0.9%; mean±S.E.) produced approximately one-half of the litter of mangrove stands with relatively complete canopies (canopy coverage: 72.1±0.5%). However, there was no significant difference between the mass of standing litter on the forest floor beneath reduced canopy and intact canopy mangroves. Also, R. mangle leaves held on the forest floor in fiberglass litter bags at both reduced canopy and control sites did not decompose at different rates over 28 days. These results indicate that while system-wide mangrove litter production should be reduced by the formation of these gaps in mangrove forests, postproduction influences may obscure any site-specific declines in standing litter stocks.     

Feeding clusters and olfaction in the mangrove snail Terebralia palustris (Linnaeus) (Potamididae: Gastropoda)

Large numbers of the snail Terebralia palustris (Linnaeus) (Potamididae; Gastropoda) are frequently observed feeding in a cluster on a single fallen mangrove leaf, yet none are present on leaves nearby. Consequently, we investigated the food-finding ability of T. palustris in a Kenyan mangrove forest using field experiments. We estimated the attractive effect of different cues and analysed the paths (video-recorded) of snails when approaching a food-related odour. This intertidal snail can potentially use both air-borne and water-borne odours to locate food. T. palustris is attracted to conspecifics feeding on leaves, while intact leaves as well as non-feeding snails are not attractive. Moreover, the guiding stimulus appears to be compounds released when the leaves are damaged.T. palustris also seems able to discriminate between different food items; it is more strongly attracted to green mangrove leaves than senescent or fallen ones or mangrove propagules, probably because green leaves release a greater amount of attractive cues.Feeding snails thus recruit more snails to feed on the same leaf. The ecological implications of this behaviour are discussed: a large number of snails on the same leaf counteracts the ability of crabs to remove the leaf being eaten by the snails.     

Size structure and distribution of Ovassiminea brevicula (Gastropoda) in a Thai mangrove swamp

Ovassiminea brevicula (Pfeiffer) (Assimineidae: Gastropoda) was oneof the most abundant and representative benthic invertebrates in the SamutSongkhram mangrove swamp, Thailand. It lived mainly in the mangrove forest with high average densities ranging from 44 to 340 snails·m^-2and abundant in the sites where much litter, dead branches and seedlingswere present. Average shell length in the mature forest ranged from 4.73to 5.74 mm (range: 2.26–8.30 mm) and size structures showeddifferent patterns among the stations. In the seaward mudflat, O. breviculaappeared immediately after the saplings of Avicennia alba Blume1826 were planted. Thereafter, its densities increased with the growth of A. alba trees, probably due to provision of favourable microhabitats underthe canopies. In the mudflat with only small saplings and outside the canopyof the young forest 2 years after planting, mean shell lengths were 3.45 and3.19 mm respectively. Under the canopy of the young forest, however,mean shell length was 4.34 mm with a few larger snails. Three cohortswere separated for snail populations in the mature forest and larval recruitmentseemed to occur from February to May.     

Habitat and zonation patterns of Littoraria species (Gastropoda: Littorinidae) in Indo-Pacific mangrove forests

The zonation of 15 species of Littoraria in the scabra group is described, using a transect method, at 13 mangrove localities in Australia, Thailand, Malaysia and Hawaii. At these localities the littorinids occurred only on trees, and showed both vertical and horizontal patterns of zonation, with broad overlaps between co-occurring species. It is suggested that vertical distribution is determined mainly by behavioural responses, while the landward limits of horizontal distribution may be set by physiological tolerance. Several species occurred predominantly on either leaf or bark substrates, but no consistent associations with tree species were found. On the basis of local distribution patterns, Littoraria species can be classified as continental or oceanic in character. Oceanic species may be less tolerant of sediment in the water or on the substrate.     

Biomass and standing stock on sublittoral hard substrates in the Oosterschelde estuary (SW Netherlands)

From 1979 to 1991 the species composition of communities living on hard substrata (hardsub) in the Oosterschelde has been studied — in both the littoral and sublittoral zones. From 1984 onwards, biomass was also measured. This paper deals mainly with the distribution and the development of biomass on sublittoral hardsub in the Oosterschelde. Analysis has shown that the most important abiotic factors regulating the flora and fauna are: quantity and nature of the substrate; sedimentation; exposure to water movement (mainly currents); and light. The construction of the storm-surge barrier has influenced those factors. The main consequences for the flora and fauna on sublittoral hard substrata have been through the increased amount of available hard substratum by about 10% until 1984 and a further 20% from 1984 to 1987, the main barrier construction period). Within the same period (until 1987) the biomass per square metre also increased. This caused a net increase of hardsub biomass — in the sublittoral — of about 35%.After the barrier was completed sedimentation increased; in some parts of the basin hardsub organisms were covered by sediment and have not recovered; the total quantity of available hard substratum decreased by an amount yet to be established. For the purpose of this paper it is tentatively estimated at 20%, but the process is still going on.Tidal current velocities are smaller in the post-barrier situation, which caused a shift from more passive suspension feeders to more actively filtering species. The relative importance of suspension feeders on hard substrata has decreased by about 20% after the building of the storm-surge barrier. In 1990 and 1991 it increased again.Overall water transparency increased, but the lower limit of macroalgal growth has not gone deeper, as nearshore turbulence and turbidity did not change significantly.Effects on hardsub were small in the beginning. During the construction period (1985–1987) no clear effects were registered. After the completion of the barrier total species diversity increased at first, followed by a decrease from the second half of 1988 onwards. Biomass increased rather sharply, at first, but decreased very sharply in 1989. In 1990 a recovery in biomass became apparent. Developments in biomass and species composition differed per sampling location. An attempt is made to explain some of those developments, in relation to the abiotic changes brought about by the storm-surge barrier. This appeared difficult, because climatic influences obscured the effects of the barrier. The most explicit of those masking effects was brought about by a temporary, huge increase of the brittlestar (Ophiothrix fragilis). This animal covered the substratum in relatively thick layers (up to 5 cm) and more or less suffocated the other fauna. It was therefore difficult to quantify the effect of increased sedimentation on the fauna. The increase of Ophiothrix is probably not caused by the storm-surge barrier, but by a succession of several mild winters.It is clear that a new equilibrium in the basin is still to be reached. Total effects in terms of species richness and of biomass will continue to be monitored, and the results used to advise the water authorities as to management and nature friendly dike building methods.     

Fluctuations in the reproductive condition of Cassidula auris-felis, a Malayan mangrove ellobiid snail (Pulmonata: Gastropoda)

Cassidula auris-felis Brug, occurs abundantly on flat, mangrove mud on the west coast of Malaya at a level within three feet of the highest spring tides. Ovotestesof only very few snails lacked developing male gametes but oocytes were often very uncommon, becoming abundant at irregular intervals. Abundant oocytes were found in large proportions of the population after periods of more than eight days without tidal wetting. Abundant oocytes were not found after periods when the mud was left exposed for periods of seven days or less. Statistically, proportions of predominantly female snails are shown to be correlated with the number of days without tidal wetting in a complete period of continuous exposure prior to collection. In addition, rather more snails bore abundant oocytes in the lunar weeks following new and full moon than in weeks following half moons. Conditions leading to abundant oocytes also appear to promote copulation in adults and commencement of gametogenesis in young snails.     

Natural selection for apostasy and crypsis acting on the shell colour polymorphism of a mangrove snail, Littoraria filosa (Sowerby) (Gastropoda: Littorinidae)

Littoraria filosa (Sowerby) is a member of the L. scabra group, found amongst the foliage of mangrove trees in northern Australia. The colour of the shell is polymorphic, showing two discrete ground colours, either yellow or orange-pink, with a variable degree of superimposed brown patterning. At a site on Magnetic Island, northern Queensland, colour frequencies of small snails were similar on different backgrounds. Amongst larger shells yellows were more frequent on Avicennia trees with abundant foliage, and browns on relatively bare trees, suggesting that visual selection for crypsis occurred. There was no evidence of substrate selection by the morphs. Yellow shells were cooler than brown shells, but differences in colour frequencies on sunny and shaded trees, and at different seasons, did not suggest climatic selection. By manipulating the colour frequencies of subpopulations of small snails isolated on individual trees, it was shown that the disappearance of yellow and brown shells was frequency-dependent. This result is consistent with hypotheses of mimicry of background elements by the morphs and of apostatic selection by unknown predators. Only the latter can account for the persistence of the highly conspicuous pink morph at a low frequency.     

Management of natural beds and standing stock evaluation of Hypnea musciformis (Gigartinales, Rhodophyta) in south-eastern Brazil

Hypnea musciformis, a source of-carrageenan, is abundant along the coast ofBrazil. Although the species has been exploitedsporadically on the north-east coast, this has beendone in the absence of data on seasonal fluctuationsin stock and on management of the natural beds. Thispaper evaluates the stock of a population from theUbatuba Bay, S-E Brazilian Coast, and how thepopulation recovers from monthly harvests. Dry monthlyyield ranged from 4.9 to 201.2 g m^-2 and showedno obvious seasonal pattern. The mean recovery rateafter harvesting is estimated as 35 days, and thestanding stock in the Bay (1.5 km) is approximately3.68 t y^-1 dry wt.     

Differences in morphology in relation to microhabitat in littorinid species from a mangrove in Papua New Guinea (Mollusca: Gastropoda)

Field studies on the north coast of Papua New Guinea show that there are three species in the Littorina scabra complex on mangroves. Two are restricted to root structures, while the third occurs on the leaves. The species differ in shell shape, ornamentation and colour, in penis shape, and in radula morphology. The shells of the leaf species are much weaker than those from the roots. Many of the differences observed may be adaptive on the preferred substratum.     

Size variation in Trebralia palustris (Gastropoda: Potamididae) of Iriomote Island,southern Japan,and its effect on some population characteristics

The growth stages of Trebralia palustris can easily bedistinguished by the presence of a varix on the last whorl and the shape ofthe outer lip. In a T. palustris population in the mangal of IriomoteIsland, southern Japan, juveniles are the most abundant and sub-adults arethe least abundant. The mean size of sub-adults is not different from that ofadults in sites with similar environmental conditions, but the mean sizesdiffer in different life environments. Sub-adult and adult snails in awell-developed mangrove stand of Rhizophora stylosa and Bruguiera gymnorrhiza with rich food resources are larger than those in arather open stand of short Avicennia marina trees with comparativelypoor food resources. The size of snails positively affected some population traitssuch as fecundity and the grazing rate of mangrove litter.     

Competition between Cellana tramoserica (Sowerby) (gastropoda) and Patiriella exigua (Lamarck) (asteroidea), and their influence on algal standing stocks

Caging experiments were undertaken in the field to test if competition occurs within and between the limpet Cellana tramoserica (Sowerby) and the starfish Patiriella exigua (Lamarck). Both species suffer from intraspecific competition, even at natural densities, and increased density reduces body weight and survival in Cellana tramoserica and growth rate in Patiriella exigua. Cellana tramoserica depresses the growth of Patiriella exigua, while the starfish unexpectedly seems beneficial to Cellana tramoserica, increasing its body weight. The difference in these effects may be due to the divergent methods of feeding used by the two species. C. tramoserica digs into the substratum and removes most available microflora, while Patiriella exigua everts its stomach onto the rock face and can only remove superficial or loosely attached microflora and detritus, and leaves a mucous web which may supplement the diet of Cellana tramoserica. Even at low densities C. tramoserica can completely prevent macroalgae from developing, while Patiriella exigua is unable to do so, although it shows the rate of development. Macroalgae grew in all the cages which contained only P. exigua, but in spite of this the starfish still suffered from intraspecific competition, probably because they cannot feed on macroalgae. Cellana tramoserica also competes by interference, and on contact with Patiriella exigua it extends its mantle and pallial tentacles, both of which may release mucus. P. exigua retreats from Cellana tramoserica, and those parts of its body that have been touched by the limpet may temporarily be immobilised and become wrinkled up. It is suggested that this interference behaviour by C. tramoserica is derived from a similar but more intense reaction that it has to invertebrate predators. Patiriella exigua lacks a pelagic larva and does not have a refuge outside the range of Cellana tramoserica. Other factors must thus be sought to explain the continued coexistence of Patiriella exigua with Cellana tramoserica. It is suggested that although C. tramoserica has a competitive effect on Patiriella exigua, it (and other grazers) may be necessary for P. exigua, preventing macroalgal growth and maintaining a surface suitable for P. exigua to feed on.     

Ultrastructure of spermatozoa in Orthalicidae (Mollusca,Gastropoda, Stylommatophora) and its systematic implications

Sperm morphology of orthalicid gastropods Clessinia pagoda, Spixia tucumanensis, Plagiodontes daedaleus (Odontostominae) and Drymaeus hygrohylaeus, D. poecilus, Bostryx stelzneri (Bulimulinae) are examined and described for the first time using transmission electron microscopy. Spermatozoa show the general characteristic of Pulmonata: an acrosomal vesicle, sperm nucleus helical, mitochondrial derivative forming a continuous sheath with paracrystalline material and coarse fibers associated with axonemal doublets. Features in the acrosomal complex and shape of the nucleus distinguish orthalicid sperms from other stylommatophoran. The acrosomal pedestal is traversed by fine striations in all species examined except in S. tucumanensis. The structure and thickness of the perinuclear sheath with a single or double layer of electron-dense material ensheathing the nuclear apex is characteristic of the group. The presence of a subnuclear ring in Drymaeus, Bostryx and Clessinia species is also reported. A data matrix of eleven species per 34 characters (16 sperm plus 18 anatomical and shell characters) from orthalicids plus other stylommatophoran and systellommatophoran representative species was constructed. Three cladistic analyses (sperm-based, anatomical-based and a combined sperm + anatomical-based) were performed to test the phylogenetic potential of sperm ultrastructure in orthalicid systematics and understand how sperm characters affect the topology and resolution of the obtained trees. Stylommatophora resulted in a monophyletic clade in the sperm-based and in the combined-character analysis. Orthalicidae is monophyletic only in the combined-character cladogram. Within Orthalicidae, Odontostominae is recovered as a monophyletic clade in all analyses, while Bulimulinae is paraphyletic in all trees except in the combined phylogeny. The present study and cladistic analyses performed support the hypothesis that characters on sperm ultrastructure are informative for stylommatophoran systematic and phylogenetic approaches, providing synapomorphies at familiar, subfamiliar and generic level.     

Biomass, carbonate standing stock and production of the mediterranean coralCladocora caespitosa (L.)

Summary The Mediterranean coralCladocora caespitosa often occurs in large beds, i.e. populations of hemispherical clonies with stock densities varying between 1.9 and 4 coloneis ·m⁻². Laboratory measurements of volume, skeleton weight, surface and number of corallites per colony, coupled with mean annual growth rates evaluated through sclerochronology, allowed for the estimation of biomass, skeleton bulk density, calcimass (carbonate standing stock) and secondary production (both organic and inorganic) of twoC. caespitosa beds at 4 and 9 m depth. The mean colony biomass varied between 0.73 and 0.99 kg dw ·m⁻², corresponding to a calcimass between 2 and 5 kg CaCO₃·m⁻². Organic secondary production was 215.5–305.4 g dw of polyps ·m⁻²·y⁻¹, while the potential (mineral) production was 1.1–1.7 kg CaCO₃·m⁻²·y⁻¹, for the year 1996–1997. These values show thatC. caespitosa is one of the major carbonate producers within the Mediterranean and one of the major epibenthic species originating stable carbonate frameworks both in recent and past times.     

Meta-analysis of standing crop reduction byRhinanthus spp. and its effect on vegetation structure

We performed a quantitative literature review on the effect of the root hemiparasiteRhinanthus on vegetation standing crop. (1) Across all available experimental studies in mixed vegetation and in pots, above-ground biomass of co-occurring species is generally reduced, with on average 40% and 60% of the value in the controls respectively. Total above-ground biomass, as the sum of parasite biomass and biomass of co-occurring species, decreases in most cases. For field experiments this reduction amounts, on average, to 26% of the control value. This implies that there is no compensation by the parasites’ biomass for the loss of biomass of co-occurring species due to parasite infection. This can be attributed to the low resource-use efficiency of hemiparasites. Meta-analysis confirmed these trends. (2) In pot experiments, the negative effect of the parasite on the above-ground biomass of the host increases with the number ofRhinanthus plants. In field experiments, we found no relationship between biomass reduction andRhinanthus density. (3) Total above-ground biomass reduction in field experiments increases with standing crop of the vegetation. However, reduction in above-ground biomass of co-occurring species seems to decrease with standing crop. Functional and species diversity buffer the community against negative effects ofRhinanthus. (4) In field experiments, functional groups are affected differently byRhinanthus spp. Grasses and legumes are mostly strongly reduced by the hemiparasites. Non-leguminous dicots mostly benefit from the presence ofRhinanthus. (5) In one out of four weeding experiments,Rhinanthus spp. has a significant (positive) effect on species number. However, the response of plant diversity to invasion of parasitic plants requires further research.     

Concealment in epipelagic pterotracheid heteropods (Gastropoda) and cranchiid squids (Cephalopoda)

Epipelagic pterotracheid heteropods and young cranchiid squids rely primarily on transparency for concealment; yet they have opaque structures, the eyes and visceral organs that compose the visceral nuclei, which can only be camouflaged in other ways. These two groups have achieved convergent solutions to this problem. The visceral nucleus has a narrow and conical shape and a covering layer of iridophores that lies parallel to the surface of the organ. The eyes also have iridophore layers and tapered shapes. A minimal ventral silhouette results when the long axes of the visceral nucleus and eyes are oriented vertically, with the narrowest ends directed downward. In pterotracheids, this is actively achieved by tilting the nucleus and eyes and flexing the body and proboscis. In cranchiids, tilting of the organs alone suffices and adjustments are accomplished much more rapidly than in the pterotracheids.     

Population bottlenecks: influence on mitochondrial DNA diversity and its effect in coregonine stock discrimination

This study was designed to test the hypothesis that anadromous populations of North American whitefish, Coregonus clupeaformis (Mitchill), believed to have suffered more intense population bottlenecks during past glaciation events, should reveal less mitochondrial DNA (mtDNA) variability and population-genetic structuring than anadromous populations of European whitefish, C. lavaretus (L.). C. clupeaformis exhibited extremely low levels of diversity and population structuring, in terms of number and frequency of clonal lines as well as sequence divergence estimates, as compared with populations of C. lavaretus. These results support the hypothesis that the severity of population bottlenecks related to Pleistocene glaciation events may be largely responsible for the level of mtDNA variability observed. This in turn influences the sampling strategy required to maximize the usefulness of mtDNA analysis in stock discrimination.