Coastal sponge communities of the West Indian Ocean: morphological richness and diversity

Sponges are a reasonably ubiquitous, abundant and highly morphologically plastic taxon. They are very unusual in showing considerable morphological plasticity, not only within higher taxa but within species and at macro and micro scales. In this study we determined the prevalence of sponge morphologies at four coastal study regions in the west Indian Ocean. We show that tropical and subtropical assemblages (in the present study) can be separated on the basis of morphological composition alone (by > 4% arborescent forms or presence of palmate forms in the former). Inter‐tidal sponge assemblages can also be separated from those in the subtidal to a high degree of certainty, also on prevalence of morphologies (absence of tubular or branching forms in the former). The species diversity of many sponge assemblages has been quantified in various environments but, typically, only by specialists. Rarely have equivalent measures been made of morphological diversity. Values of morphological diversity (Shannon H′) were similar, though more variable, to those measured in temperate waters. Substratum nature had a major influence on morphological diversity and evenness, in contrast to geography and bathymetry. As coral reefs were the most diverse and caves and boulders the least, we suggest that substratum heterogeneity and water‐flow complexity are probably the chief determinant of sponge assemblage morphological diversity.     

A sponge diversity centre within a marine ‘island’

The exposed and gulf-stream warmed south-west coast of Ireland has a Lusitanean fauna composed of elements of the colder waters to the north and east, and others from the warmer Mediterranean Sea. Lough Hyne, a small marine body, is unusual on this coast in being very sheltered, but also in being characterised by many different niches within a small space (1 km²). Sponges are particularly abundant, morphologically varied and more than 100 species have been described. Species diversity was measured at 6 m intervals on vertical and inclined profiles (to a maximum of 30 m) at six sites, spanning a range of flow rate and sedimentation regimes. Diversity, richness, evenness and density varied significantly with both flow regime and depth, but was much lower on the surrounding Atlantic coast. Four different sponge communities were differentiated on the basis of sponge species assemblages which correlated with different environmental conditions. At sites of turbulent and fast flow conditions, sponge diversity and richness were lowest, with the highest values being found at the sites of moderate and high sedimentation. Significant differences were observed in all four ecological variables with respect to substratum angle with the exception of the site experiencing the most turbulent flow conditions. Lough Hyne was found to possess the second highest sponge species diversity (H=3.626) and richness (77 species) of all available figures from temperate, polar and tropical areas (of similar sized sampling area). The uniqueness, diversity and species composition of the sponge community at this location suggests Lough Hyne is, biologically, a marine island within the island of Ireland.     

Effect of disturbance on assemblages: an example using porifera

Extensive sponge assemblages are found in a number of habitats at Lough Hyne Marine Nature Reserve. These habitats are unusual in experiencing a range of environmental conditions, even though they are only separated by small geographic distances (1-500 m), reducing the possibility of confounding effects between study sites (e.g., silica concentrations and temperature). Sponge assemblages were examined on ephemeral (rocks), stable (cliffs), and artificial (slate panels) hard substrata from high- and low-energy environments that were used to represent two measures of disturbance (flow rate and habitat stability). Sponge assemblages varied considerably between habitat types such that only 26% (25 species) of species reported were common to both rock and cliff habitats. Seven species (of a total of 96 species) were found in the least-developed assemblages (slate panels) and were common to all habitats. Sponge assemblages on rocks and panels varied little between high- and low-energy environments, whereas assemblages inhabiting cliffs varied considerably. Assemblage composition was visualized using Bray-Curtis similarity analysis and Multi-Dimensional Scaling, which enabled differences and similarities between sponge assemblages to be visualized. Cliffs from high- and low-energy sites had different assemblage compositions compared to large rocks, small rocks, and panels, all of which had similar assemblages irrespective of environmental conditions. Differences in assemblages were partially attributed to sponge morphology (shape), as certain morphologies (e.g., arborescent species) were excluded from 2-D rock habitats. Other mechanisms were also considered responsible for the sponge assemblages associated with different habitats.     

Testing the suitability of a morphological monitoring approach for identifying temporal variability in a temperate sponge assemblage

Sponges are a dominant component of benthic assemblages in hard substratum environments across the world, but despite the importance of sponges, they are generally poorly represented in most marine monitoring programmes. There is considerable need to develop effective monitoring tools to monitor changes in sponge assemblages. Morphological monitoring has been proposed as a suitable method to monitor sponges and while morphological monitoring has already taken place at Skomer Marine Reserve (MNR), Wales, here we investigate whether species level and morphological level data sets are correlated with respect to temporal variation. Furthermore, we examine the environmental factors that correlate with the patterns of temporal variability. Both species and morphological data sets revealed significant seasonal changes and spatial variation in sponge assemblages; these data sets were highly correlated and explained by a number of environmental factors. We conclude that morphological monitoring of sponge assemblages may represents a cost-effective method for assessing temporal and spatial variation in sponges, where full species level monitoring is not possible, as patterns identified from morphological data were a suitable surrogate of species-level data.     

Substratum simplification reduces beta diversity of stream algal communities

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Response of the meiofaunal annelid Saccocirrus pussicus (Saccocirridae) to sandy beach morphodynamics

Australia is increasingly recognised as a global hotspot for sponge biodiversity, but there is a knowledge gap about sponge communities in northern Australia, including those in Commonwealth Marine Reserves. We aim to quantify sponge biodiversity of the eastern Joseph Bonaparte Gulf and adjacent Van Diemen Rise (VDR) and to examine spatial and environmental patterns in community structure. Sponges were collected with a benthic sled from 65 sites encompassing five geomorphic features (bank, terrace, ridge, plain, and valley), study area (as a proxy for distance offshore) and three environmental variables (depth, substrate hardness, and slope). A total of 283 species were collected, representing four classes, 53 families and at least 117 genera. Sponge richness and biomass were related to those of other taxa. Sponge diversity was generally highest further offshore and on raised geomorphic features, particularly banks. Sponge assemblages on the same bank were more similar than those from different banks, although full interpretation of patterns is limited by the relatively low sampling effort. The current study will help facilitate integrated marine management by providing a baseline species inventory, supporting the VDR’s carbonate banks as a key ecological feature, and highlighting the importance of sponges as habitat providers and potential biological surrogates for monitoring.     

Substratum‐dependent responses of ciliate assemblages to temperature: a natural experiment in Icelandic streams

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High assemblage persistence in heterogeneous habitats: an experimental test with stream benthic algae

1. The persistence of biological assemblages is positively affected by spatial heterogeneity. This influence may be indirect, through increased species richness. Another possibility is the increased availability of refuges from disturbances, which would prevent local loss. 2. We conducted a field experiment to test the hypothesis that greater roughness (a form of spatial heterogeneity) on the surface of substrata allows higher persistence of assemblages of stream benthic algae and that this relationship does not depend on species richness. Samples were taken on six occasions from smooth and rough artificial substrata used for algal colonisation. We calculated the persistence of assemblages using two analytical approaches: the mean distance to group centroid and the sum of the Euclidean distances between consecutive sampling occasions, both in a multivariate space. We also subsampled the data to take into account differences in species richness between treatments and thus to evaluate the effect of species richness on persistence. 3. Assemblages on rough substrata were more persistent than assemblages on smooth substrata. The effects detected were not due to the greater species richness on rough substrata, since a higher persistence of the assemblages on rough substrata remained after the subsampling procedures. 4. Our results indicate a strong positive relationship between substratum roughness and the persistence of stream benthic algal assemblages. We suggest that this is due to the presence of physical refuges in heterogeneous habitats.     

The Homoscleromorph sponge Oscarella lobularis,a promising sponge model in evolutionary and developmental biology

Sponges branch basally in the metazoan phylogenetic tree and are believed to be composed of four distinct lineages with still uncertain relationships. Indeed, some molecular studies propose that Homoscleromorpha may be a fourth Sponge lineage, distinct from Demospongiae in which they were traditionally classified. They harbour many features that distinguish them from other sponges and are more evocative of those of the eumetazoans. They are notably the only sponges to possess a basement membrane with collagen IV and specialized cell‐junctions, thus possessing true epithelia. Among Homoscleromorphs, we have chosen Oscarella lobularis as a model species. This common and easily accessible sponge is characterized by relatively simple histology and cell composition, absence of skeleton, and strongly pronounced epithelial structure. In this review, we explore the specific features that make O. lobularis a promising homoscleromorph sponge model for evolutionary and developmental researches.     

Modelling sponge species diversity using a morphological predictor: a tropical test of a temperate model

The contribution of sponges to marine surveys is often underestimated due to problems of identification, synonymous species and limited numbers of specialists in the field. Bell & Barnes (2001) illustrated how sponge morphological diversity (diversity of body forms) might be used as a predictor of sponge species diversity and richness. This study investigated these relationships at six tropical West Indian Ocean localities in a number of habitat types. These habitats included tropical coral reefs, soft substratum (seagrass, mangrove and sand), caves and boulders. Sampling was undertaken at three depth zones in coral reef habitats only (intertidal, 10–15 m and 20–25 m), with the other habitats sampled in less than 10m of water. Species diversity and richness were significantly correlated (P < 0.05) with morphological diversity at all localities and depths in coral reef and soft substratum habitats. However, no significant correlation was found between these variables in cave or boulder habitats. The slope of the linear regression found between morphological diversity and species diversity did not significantly differ between coral reef, soft substratum and temperate reef (data taken from Bell & Barnes 2001) habitats. Similarly coral reefs showed the same relationship between morphological diversity and species richness as temperate reefs, however the relationship between morphological diversity and species richness was significantly different at both habitats compared with soft substratum environments. Sponge morphological diversity therefore may be more useful as a predictor of sponge species diversity, rather than species richness, as the former relationship is common between more habitats than the latter.     

Aggregation dynamics and foraging behaviour of striped red mullet Mullus surmuletus in the western Mediterranean Sea

Habitat‐related heterogeneity of striped red mullet Mullus surmuletus heterospecific foraging assemblages was examined off the coast of Spain. Video‐based focal‐follows conducted on 122 M. surmuletus assemblages (446 total individuals) revealed an array of attendant species (n = 7) with composition linked to benthic habitat complexity; bare sandy substrata were characterized by homospecific groups of M. surmuletus, while habitats with rock and vegetation attracted a variety of scrounging labrids and sparids. Although the nature of the relationship between M. surmuletus and attendants requires further exploration, the present study indicates that substratum composition can be a driving factor explaining the dynamics of this heterospecific assemblage.     

Bryozoan assemblages on hard substrata: species abundance distribution and competition for space

Bryozoans are colonial invertebrates that often dominate epibenthic assemblages on the lower surfaces of hard substrata. Competition among neighbouring organisms is usually a critical process regulating biodiversity, and hard substrata have proved to be a suitable model habitat to analyse spatial interactions. We explored the relationships among abundance, species richness, diversity, competitive ability, coverage, available surface, depth and substratum size in an assemblage of bryozoans encrusting pebbles and cobbles in a bank off the eastern mouth of the Strait of Magellan. We also tested whether overgrowth competition can be regarded as hierarchical, and whether the species abundance distribution shows a mode of rare species and a decreasing frequency of increasingly abundant species. Abundance, species richness, diversity and overgrowth competition were highest on the largest substrata. Smaller pebbles tended to be encrusted by the commonest bryozoans, while the rarest species were mainly found on relatively larger clasts. A high proportion of the lower surfaces of most substrata was available for growth. Diversity values of relatively shallow stations were lower than expected under Caswell’s neutral model. Interspecific competition was hierarchical, but the abundance of colonies was not related to the competitive ability of each species. The species abundance distribution was bimodal, with a main mode of rare species and a second one partly composed of relatively abundant bryozoans with poor competitive abilities. This study shows that even in an encrusting assemblage where competition is hierarchical, the weakest competitors persist and the dominant species are far from being capable of monopolizing space.     

Space as a limiting resource in freshwater systems: competition between zebra mussels (Dreissena polymorpha) and freshwater sponges (Porifera)

Species interactions between two types of sessile benthic invertebrates, the zebra mussel (Dreissena polymorpha) and freshwater sponges (Porifera), were evaluated in Michigan City IN Harbor in southern Lake Michigan during 1996. The study objective was to define whether competition plays a role in structuring benthic communities using experimental techniques commonly employed in marine systems. Sponges were uninhibited by zebra mussel presence and overgrew zebra mussel shells on hard vertical substrata. In contrast, zebra mussels did not overgrow sponge colonies, but did show an ability to re-capture hard substrata if relinquished by the sponge. The negative affect of sponges on zebra mussels through overgrowth and recruitment suggests interactions that could eventually displace zebra mussels from these benthic communities. However, seasonal reduction of sponge biomass from autumn through winter appears to allow the zebra mussel a periodic respite from overgrowth, preventing exclusion of zebra mussels from the community and allowing these two taxa to co-exist.     

Low local but high beta diversity of tropical forest dung beetles in Madagascar

Aims We have compared local (alpha) and regional (beta) species diversities of dung beetles in wet forests in the main tropical regions including Madagascar. Madagascar is exceptional in lacking native large herbivorous mammals which produce the key resource for dung beetles elsewhere. Location Central and South America, mainland Africa, Madagascar and Southeast Asia. Methods Trapping data on dung beetles and data on mammalian faunas were obtained from published and unpublished studies. We used our original data for Madagascar. Results Species richness of dung beetles and that of large‐bodied (> 15 mm length) species in particular were highly significantly explained by the regional number of large‐bodied (> 10 kg) mammals (R² from 50 to 80%). For a given pairwise spatial distance between two communities, beta diversity was significantly higher in Madagascar than elsewhere, explaining the very high total species richness in Madagascar in spite of low local diversity. Main conclusion The presence and numbers of large herbivorous mammals greatly influence the species richness of dung beetles in tropical wet forests. The lack of native large herbivores rather than a limited species pool explains the low local diversity in Madagascar. Exceptionally high beta diversity in Madagascar suggests a pattern of old radiation involving extensive allopatric speciation.     

Gravel dredging alters diversity and structure of riverine fish assemblages

1. Human activities affect fish assemblages in a variety of ways. Large‐scale and long‐term disturbances such as in‐stream dredging and mining alter habitat and hydrodynamic characteristics within rivers which can, in turn, alter fish distribution. Habitat heterogeneity is decreased as the natural riffle–pool–run sequences are lost to continuous pools and, as a consequence, lotic species are displaced by lentic species, while generalist and invasive species displace native habitat specialists. Sediment and organic detritus accumulate in deep, dredged reaches and behind dams, disrupting nutrient flow and destroying critical habitat for habitat specialist species. 2. We used standard ecological metrics such as species richness and diversity, as well as stable isotope analysis of δ¹³C and δ¹⁵N, to quantify the differences in fish assemblages sampled by benthic trawls among dredged and undredged sites in the Allegheny River, Pennsylvania, U.S.A. 3. Using mixed‐effects models, we found that total catch, species richness and diversity were negatively correlated with depth (P < 0.05), while species richness, diversity and proportion of species in lithophilic (‘rock‐loving’) reproductive guilds were lower at dredged than at undredged sites (P < 0.05). 4. Principal components analysis and manova revealed that taxa such as darters in brood hider and substratum chooser reproductive guilds were predominantly associated with undredged sites along principal component axis 1 (PC1 and manova P < 0.05), while nest spawners such as catfish and open substratum spawners including suckers were more associated with dredged sites along PC2 (P < 0.05). 5. Stable isotope analysis of δ¹³C and δ¹⁵N revealed shifts from reliance on shallow water and benthic‐derived nutrients at undredged sites to reliance on phytoplankton and terrestrial detritus at deep‐water dredged sites. Relative trophic positions were also lower at dredged sites for many species; loss of benthic nutrient pathways associated with depth and dredging history is hypothesised. 6. The combination of ecological metrics and stable isotope analysis thus shows how anthropogenic habitat loss caused by gravel dredging can decrease benthic fish abundance and diversity, and that species in substratum‐specific reproductive guilds are at particular risk. The effects of dredging also manifest by altering resource use and nutrient pathways within food webs. Management and conservation decisions should therefore consider the protection of relatively shallow areas with suitable substratum for spawning for the protection of native fishes.     

Island biogeography of Caribbean coral reef fish

Aim The goal of our study was to test fundamental predictions of biogeographical theories in tropical reef fish assemblages, in particular relationships between fish species richness and island area, isolation and oceanographic variables (temperature and productivity) in the insular Caribbean. These analyses complement an analogous and more voluminous body of work from the tropical Indo‐Pacific. The Caribbean is more limited in area with smaller inter‐island distances than the Indo‐Pacific, providing a unique context to consider fundamental processes likely to affect richness patterns of reef fish. Location Caribbean Sea. Methods We compiled a set of data describing reef‐associated fish assemblages from 24 island nations across the Caribbean Sea, representing a wide range of isolation and varying in land area from 53 to 110,860 km². Regression‐based analyses compared the univariate and combined effects of island‐specific physical predictors on fish species richness. Results We found that diversity of reef‐associated fishes increases strongly with increasing island area and with decreasing isolation. Richness also increases with increasing nearshore productivity. Analyses of various subsets of the entire data set reveal the robustness of the richness data and biogeographical patterns. Main conclusions Within the relatively small and densely packed Caribbean basin, fish species richness fits the classical species–area relationship. Richness also was related negatively to isolation, suggesting direct effects of dispersal limitation in community assembly. Because oceanic productivity was correlated with isolation, however, the related effects of system‐wide productivity on richness cannot be disentangled. These results highlight fundamental mechanisms that underlie spatial patterns of biodiversity among Caribbean coral reefs, and which are probably also are functioning in the more widespread and heterogeneous reefs of the Indo‐Pacific.     

Flow velocity underpins microhabitat selection by gobies of the Australian Wet Tropics

1. Water flow is a critical driver of aquatic ecosystem health and function. Amid rising concerns over changing flow regimes, there is an urgent need to understand the functional mechanisms by which flow influences patterns of freshwater biodiversity. 2. We explored the functional link between flow velocity and microhabitat specialisation in a speciose group of freshwater gobies (comprising over half the total fish species richness) within insular streams of the Australian Wet Tropics under base flow conditions. We addressed two particular questions: (i) What is the relative selectivity of species towards streambed composition and water flow velocity? and (ii) Can patterns of microhabitat occupation be explained by differences in intrinsic flow performance among species? To answer these questions, we combined visual field observations of microhabitat use with flow tank assessments of flow speed performance. 3. Tropical freshwater gobies displayed strong specificity towards flow velocities, while being relatively non‐selective towards streambed composition. At opposite extremes of the spectrum, we found Sicyopterus lagocephalus occupying high‐flow (>1.0 ms^?1) microhabitats while Redigobius bikolanus selected slower‐flow (<0.05 ms^?1) areas. These patterns of microhabitat flow specificity were largely explained by the different abilities of species to swim and/or cling to the substratum under these different flow settings. 4. Our findings confirm suggestions that predictable base flows in tropical streams support habitat specialists, which include one species capable of occupying areas of extremely high flow that very few other fishes can withstand. 5. The functional link between flow and gobioid fish distribution patterns could occur throughout tropical streams of the Indo‐Pacific and Caribbean as a widespread phenomenon that may help inform stream flow management guidelines to maintain this substantial component of tropical freshwater biodiversity around the globe.     

THE SEASONAL DISTRIBUTION,ABUNDANCE AND DIVERSITY OF DESMIDS (CHLOROPHYTA) IN A SOFTWATER,NORTH TEMPERATE STREAM1

Suspended and benthic algal communities from a mildly acidic, third-order Rhode Island stream were examined to determine the seasonal distribution, abundance and diversity of the lotic desmids. Within a one-year sampling period, 148 species and 202 subspecific taxa of desmids were identified, representing 23 genera. Species of Cosmarium and Closterium accounted for approximately 70% of the desmids present, and were the most diverse and abundant taxa during all seasons except spring, when Hyalotheca dissiliens was the dominant desmid species. Average abundance and species richness generally were greatest during summer for both suspended and benthic desmids. Most desmids occurred in benthic habitats, and were randomly distributed among substrata. Average seasonal abundance was 7.4 × 10⁴ cells·g^?1 dry wt substratum, among 13 types of substrata. Highest desmid abundance was measured among substrata with intricate morphologies, such as Fontinalis spp., which was associated with 1.2 × 10⁶ desmid cells·g^?1 dry wt substratum, or 1.7 × 10³ cells·cm^?2 substratum. Cell division was observed for 70 desmid taxa, and average seasonal reproduction (based on cell numbers) among all substrata ranged from 4% in winter to 20% during summer. In addition, sexually produced zygospores were found occasionally for H. dissiliens. Desmids were distributed among most substrata examined in this stream, with abundance comparable to reported estimates from softwater lakes and acid bogs. In contrast to established dogma, lotic desmids are not incidental drift organisms, but rather comprise a viable and persistent component of the stream periphyton.     

Sponge–seaweed associations in species of Ptilophora (Gelidiaceae, Rhodophyta)

Sponge–seaweed associations in the seaweed genus Ptilophora are poorly understood; therefore, 94 specimens, representing all 17 species of Ptilophora, were examined to detail this phenomenon. All but 2 Ptilophora species were shown to produce surface proliferations, with 13 species found to have sponge associations. Evidence for facultative sponge epiphytism was found with species–specific interactions being unlikely. Results show that surface proliferations are not induced by sponge epiphytes, as they often occur in the absence of sponge epiphytes, and vice versa. The significant number of proliferate thalli found with sponge epiphytes suggests that there is a likely relationship between the presence of surface proliferations and sponge infestation. Sponge epiphytes and Ptilophora species appeared structurally related in that the sponge probably exploits a niche habitat provided by the alga, for which surface proliferations might aid the sponge in bonding to the alga.     

Responses of foliage‐living spider assemblage composition and traits to a climatic gradient in Themeda grasslands

For speciose, but poorly known groups, such as terrestrial arthropods, functional traits present a potential avenue to assist in predicting responses to environmental change. Species turnover is common along environmental gradients, but it is unclear how this is reflected in species traits. Community‐level change in arthropod traits, other than body size, has rarely been explored across spatial scales comparable to those examined here. We hypothesized that the composition and morphological traits of spider assemblages would differ across a gradient of climate and habitat structure. We examined foliage‐living spider assemblages associated with Themeda triandra grasslands along a 900 km climatic gradient in south‐eastern Australia. We used sweep‐netting to collect T. triandra‐associated spiders and counted juveniles and identified adults. We also measured morphological traits of adult spiders and noted their hunting mode. Associations with measures of habitat structure were less consistent than relationships with climate. Both juvenile and adult spiders were more abundant in warmer sites, although species richness was not affected by temperature. We found distinct turnover in species composition along the climatic gradient, with hunting spiders, particularly crab spiders (Thomisidae), making up a greater proportion of assemblages in warmer climates. A range of traits of spiders correlated with the climatic gradient. For example, larger spider species and species that were active hunters were more common in warmer climates. Changes in morphological traits across species, rather than within species drove the morphology‐climate relationship. Strong climate‐trait correlations suggest that it may be possible to predict changes in functional traits of assemblages in response to anthropogenic disturbances such as climate change.