Habitat-specific Differences Research Articles

Adaptive brain size divergence in nine‐spined sticklebacks (Pungitius pungitius)?

Most studies seeking to provide evolutionary explanations for brain size variability have relied on interspecific comparisons, while intraspecific studies utilizing ecologically divergent populations to this effect are rare. We investigated the brain size and structure of first-generation laboratory-bred nine-spined sticklebacks (Pungitius pungitius) from four geographically and genetically isolated populations originating from markedly different habitats. We found that the relative size of bulbus olfactorius and telencephalon was significantly larger in marine than in pond populations. Significant, but habitat-independent population differences were also found in relative brain and cerebellum sizes. The consistent, habitat-specific differences in the relative size of bulbus olfactorius and telencephalon suggest their adaptive reduction in response to reduced (biotic and abiotic) habitat complexity in pond environments. In general, the results suggest that genetically based brain size and structure differences can evolve relatively rapidly and in repeatable fashion with respect to habitat structure.

Habitat type-based bioaccumulation and risk assessment of metal and As contamination in earthworms, beetles and woodlice

The present study investigated the contribution of environmental factors to the accumulation of As, Cd, Cu, Pb and Zn in earthworms, beetles and woodlice, and framed within an exposure assessment of the European hedgehog. Soil and invertebrate samples were collected in three distinct habitat types. Results showed habitat-specific differences in soil and invertebrate metal concentrations and bioaccumulation factors when normalized to soil metal concentration. Further multiple regression analysis showed residual variability (habitat differences) in bioaccumulation that could not be fully explained by differences in soil metal contamination, pH or organic carbon (OC). Therefore, the study demonstrated that in bioaccumulation studies involving terrestrial invertebrates or in risk assessment of metals, it is not sufficient to differentiate habitat types on general soil characteristics such as pH and/or OC alone. Furthermore, simple generic soil risk assessments for Cd and Cu showed that risk characterization was more accurate when performed in a habitat-specific way.

Inter- and intra-habitat bacterial diversity associated with cold-water corals

The discovery of large ecosystems of cold-water corals (CWC), stretching along continental margins in depths of hundreds to thousands of meters, has raised many questions regarding their ecology, biodiversity and relevance as deep-sea hard-ground habitat. This study represents the first investigation that explicitly targets bacterial diversity from distinct microbial habitats associated with the cosmopolitan reef-building coral Lophelia pertusa, and also compares natural (fjord) and controlled (aquarium) conditions. Coral skeleton surface, coral mucus, ambient seawater and reef sediments clearly showed habitat-specific differences in community structure and operational taxonomic unit (OTU) number. Especially in the natural environment, bacterial communities associated with coral-generated habitats were significantly more diverse than those present in the surrounding, non-coral habitats, or those in artificial coral living conditions (fjord vs aquarium). These findings strongly indicate characteristic coral-microbe associations and, furthermore, suggest that the variety of coral-generated habitats within reef systems promotes microbial diversity in the deep ocean.

Reproductive traits of an estuarine crab, <em>Neohelice</em> (= <em>Chasmagnathus</em>) <em>granulata</em> (Brachyura: Grapsoidea: Varunidae), in two contrasting habitats

Reproductive traits of an estuarine crab, Neohelice (= Chasmagnathus) granulata (Dana 1851), were compared between individuals living in contrasting habitats (mudflat and saltmarsh) of the same population in the brackish coastal lagoon of Mar Chiquita, Argentina. In both habitats, most measures of egg biomass decreased during embryogenesis, including total dry weight (DW) and organic matter (measured as ash-free dry weight, AFDW) per egg, the contents of ash, carbon, hydrogen and nitrogen (per egg and as percentage values of DW), the energy content (estimated from C; both per egg and per mg DW), and the C/N and C/H mass ratios. Egg size, wet weight (WW), and water content (in µg and % of WW), by contrast, increased significantly during the time of embryonic development. These parameters reached significantly higher final (near-to-hatching) values in mudflats than in saltmarshes (egg volume 0.0249 vs 0.0233 mm3; WW 36.5 vs 28.8 µg; water content 30.7 vs 23.2 µg per egg or 84.2 vs 80.2% of WW, respectively). Fecundity and reproductive effort did not differ significantly between habitats. Habitat-specific differences in the water content and size of crab eggs are discussed in relation to small-scale local variation in environmental conditions.

Habitat use and the effect of shrimp trawling on fish and invertebrate communities over the northern Gulf of Mexico continental shelf

Abstract Wells, R. J. D., James H. Cowan Jr, and William F. Patterson III. 2008. Habitat use and the effect of shrimp trawling on fish and invertebrate communities over the northern Gulf of Mexico continental shelf. – ICES Journal of Marine Science, 65: 1610–1619. The goals of this study were to characterize habitat-specific fish and invertebrate community structure over sand, shell-rubble, and natural reef substrata, and to assess the effects of trawling on the sand and shell-rubble habitats and their associated communities during quarterly trawl surveys over a 2-year period. Fish and invertebrate communities differed significantly among habitat types [analysis of similarities (ANOSIM); Global R = 0.436, p < 0.001), and with respect to trawling exposure (ANOSIM; Global R = 0.128, p < 0.001). Habitat characteristics were quantified from video transects sampled with a remotely operated vehicle, and included percentage coverage of tubeworms, bryozoans, anemones, corals, and algae, significantly affecting fish community structure. Diversity indices differed among habitats, with the highest Shannon diversity (H′) and Pielou's evenness (J′) over shell-rubble, specifically non-trawled shell-rubble. In addition, higher values of H′ and J′ were found over trawled sand relative to non-trawled sand habitats. Length frequency distributions of several abundant fish species showed truncated size distributions over trawled and non-trawled habitats and were both habitat- and species-specific. The study describes habitat-specific differences in community structure, highlighting the differences between trawled and non-trawled areas on the northern Gulf of Mexico continental shelf.

Feeding ecology of red snapper Lutjanus campechanus in the northern Gulf of Mexico

We used stable isotopes and stomach content analyses to describe diet of red snapper Lutjanus campechanus in the northern Gulf of Mexico (GOM). Approximately 1000 fish were collected over 2 yr to test possible effects of ontogeny, habitat, and a non-trawl artificial reef permit area on red snapper diets. Stable isotopes of carbon (δ 13 C), nitrogen (δ 15 N), and sulfur (δ 34 S) were measured in both red snapper and potential primary producers. Ontogenetic shifts in diet occurred with increasing red snapper size-at-age, resulting in higher δ 13 C and nitrogen δ 15 N and lower δ 34 S in larger fish. Stomach content results supported ontogenetic shifts in diet by showing a change in diet from zooplankton, mysid shrimp, and squid for juvenile red snapper (ages 0 to 1) to diets dominated by benthic crustaceans and fishes for adults (ages 2+). Habitat-specific differences in isotopes and stomach contents of similarly sized fish were identified; however, feeding differences appeared to reflect ontogeny more than habitat type. In addition, seasonal differences, both in δ 13 C and in prey identified from stomach contents, were detected, but were minimal. Red snapper from areas outside a single non-trawl reef permit area had higher δ 15 N and lower δ 34 S values than for conspecifics collected inside the non-trawl reef permit area. This study highlights the use of stable isotopes in detecting red snapper feeding differences inside and outside of an artificial reef permit area in the northern GOM, but additional studies are needed to verify if similar trends are present in other areas.

Habitat-specific differences in thermal plasticity in natural populations of a soil arthropod

When populations experience substantial variation in environmental conditions, they may evolve phenotypic plasticity in response to these varying selection pressures. Evolutionary theory predicts differentiation in the level of phenotypic plasticity among different habitats. We evaluated temperature-induced phenotypic responses in juvenile growth rate in natural populations of the springtail Orchesella cincta, inhabiting forest and heathland. These habitats typically co-occur but differ strongly with respect to, for example, thermal regime, relative humidity, and structure. Offspring of females from the two habitats were reared at different temperatures in climate rooms and the temperature response of juvenile growth rate and egg size was measured. We found a habitat-specific difference in plasticity of juvenile growth rate. The reaction norms of the forest populations were steeper than the reaction norms for heath populations at two replicated sampling sites. Egg weight itself was demonstrated to be a plastic trait with a higher egg weight at low temperatures, but the thermal response did not differ between habitats. We conclude that these populations have diverged due to strong local natural selection. Our results support the argument that the level of phenotypic plasticity itself can be under selection and that differentiation in reaction norms can occur even in neighbouring habitats with no barrier to gene flow. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 94, 265‐271.

Habitat use, growth, and mortality of post-settlement lane snapper ( Lutjanus synagris) on natural banks in the northwestern Gulf of Mexico

Three low-relief banks in the northwestern Gulf of Mexico were evaluated as nursery habitat of lane snapper ( Lutjanus synagris). Trawl surveys were conducted in three habitat types (inshore mud, shell ridge, offshore mud) to quantify lane snapper distribution and abundance. Heald Bank and Sabine Bank were trawled in 2003 while Freeport Rocks was trawled in 2000 (Freeport A) and 2004 (Freeport B). Density of lane snapper varied among banks and years sampled: Sabine Bank (20.8 ± 2.8 ind ha −1), Heald Bank (1.1 ± 0.4 ind ha −1), Freeport A (12.7 ± 2.3 ind ha −1), and Freeport B (3.0 ± 1.0 ind ha −1). Habitat-specific differences in density were observed, although patterns were not consistent among banks. Otolith microstructure analysis indicated that post-settlement lane snapper ranged in age from 21 to 66 d, with hatch dates from 1 May to 31 August. Growth rates varied from 0.90 mm d −1 at Heald Bank to 1.27 mm d −1 at Sabine Bank, and habitat-specific differences in growth were negligible. Mortality of post-settlement lane snapper ranged from 15.2% d −1 at Sabine Bank to 9.2% d −1 at Freeport A. Our findings indicate that Heald Bank, Sabine Bank, and Freeport Rocks all serve as settlement habitat of lane snapper, which appear to be capable of successful settlement across a variety of habitats.

Distribution, condition, and growth of newly settled southern flounder ( Paralichthys lethostigma) in the Galveston Bay Estuary, TX

Several flatfish species, including southern flounder ( Paralichthys lethostigma) recruit to estuaries during early life. Therefore, evaluation of estuarine sites and habitats that serve as nurseries is critical to conservation and management. The present study used density data in conjunction with biochemical condition and growth measurements to evaluate settlement sites used by southern flounder in the Galveston Bay Estuary (GBE). In 2005, beam-trawl collections were made in three major sections of the GBE (East Bay, Galveston Bay, West Bay). Three sites were sampled in each bay. Within each sampling site, replicate collections were taken from three habitats: 1) marsh edge (< 1 m depth), 2) intermediate zone (10–20 m from marsh interface; ∼ 1 m depth), and 3) bay zone (typically > 100 m from marsh interface; depth > 1 m). Average size of southern flounder collected was 12–19 mm standard length, and peak densities occurred in January and February. Catch data indicated that densities of southern flounder were significantly greater in East Bay (2.75 per 100 m 2) than in Galveston Bay (0.91 per 100 m 2) or in West Bay (0.45 per 100 m 2). Densities were statistically similar among habitats. Otolith-based estimates of age indicated that the majority of southern flounder collected were 35–45 days old and derived from early December to early January hatch-dates. Growth rates were similar among bays and among habitats, with the average growth rate being 0.40 mm day − 1 (range: 0.21–0.76 mm day − 1 ). RNA:DNA was above the established baseline value for nutritional stress, indicating that newly settled southern flounder in the GBE were in relatively high condition. Habitat-specific differences in RNA:DNA ratios were not observed; however, ratios were significantly lower in West Bay (average 8.0) than in East Bay (average 9.5) or in Galveston Bay (average 9.8), suggesting the condition of new recruits may vary spatially within the GBE. Findings from the current study suggest density and condition of newly settled southern flounder vary at the bay scale, suggesting that parts of GBE do not function equally as nurseries.

Epiphytic bacterial community composition on two common submerged macrophytes in brackish water and freshwater

BackgroundPlants and their heterotrophic bacterial biofilm communities possibly strongly interact, especially in aquatic systems. We aimed to ascertain whether different macrophytes or their habitats determine bacterial community composition. We compared the composition of epiphytic bacteria on two common aquatic macrophytes, the macroalga Chara aspera Willd. and the angiosperm Myriophyllum spicatum L., in two habitats, freshwater (Lake Constance) and brackish water (Schaproder Bodden), using fluorescence in situ hybridization. The bacterial community composition was analysed based on habitat, plant species, and plant part.ResultsThe bacterial abundance was higher on plants from brackish water [5.3 × 107 cells (g dry mass)-1] than on plants from freshwater [1.3 × 107 cells (g dry mass)-1], with older shoots having a higher abundance. The organic content of freshwater plants was lower than that of brackish water plants (35 vs. 58%), and lower in C. aspera than in M. spicatum (41 vs. 52%). The content of nutrients, chlorophyll, total phenolic compounds, and anthocyanin differed in the plants and habitats. Especially the content of total phenolic compounds and anthocyanin was higher in M. spicatum, and in general higher in the freshwater than in the brackish water habitat. Members of the Cytophaga-Flavobacteria-Bacteroidetes group were abundant in all samples (5–35% of the total cell counts) and were especially dominant in M. spicatum samples. Alphaproteobacteria were the second major group (3–17% of the total cell counts). Betaproteobacteria, gammaproteobacteria, and actinomycetes were present in all samples (5 or 10% of the total cell counts). Planctomycetes were almost absent on M. spicatum in freshwater, but present on C. aspera in freshwater and on both plants in brackish water.ConclusionBacterial biofilm communities on the surface of aquatic plants might be influenced by the host plant and environmental factors. Distinct plant species, plant part and habitat specific differences in total cell counts and two bacterial groups (CFB, planctomycetes) support the combined impact of substrate (plant) and habitat on epiphytic bacterial community composition. The presence of polyphenols might explain the distinct bacterial community on freshwater M. spicatum compared to that of M. spicatum in brackish water and of C. aspera in both habitats.

DO SOURCE–SINK DYNAMICS PROMOTE THE SPREAD OF AN INVASIVE GRASS INTO A NOVEL HABITAT?

Models of source-sink and other spatial patch dynamics have generated a number of ideas and predictions about species range expansion, the evolution of local adaptation, and the factors influencing population persistence, but relatively few empirical studies have applied these ideas due to the difficulty of measuring both patch-specific demography and movement rates. In this study, I used a combination of mark-recapture experiments, model fitting, and demographic approaches to ask how habitat-specific differences in population growth and dispersal affect spread of the invasive grass Aegilops triuncialis into serpentine environments. A. triuncialis germinated at lower rates but exhibited equivalent survival and greater growth in edge (extreme serpentine) than in core populations, even accounting for density differences between habitats. Estimated growth rates (lambda) for four of five edge subpopulations were strongly positive, ranging from lambda = 1.32 to 2.09 without propagule input from adjacent habitat. Local dispersal was best described by an exponential kernel, with a mean dispersal distance about twice as long on the edge (0.24-0.40 m) as in the core (0.18 m). Twenty-five percent of marked spikes in the edge were not relocated within the patch, suggesting greater rates of either seed predation or long-distance dispersal that reduced population growth. These results suggest that A. triuncialis can successfully spread into extreme serpentine habitats without sustained propagule input from adjacent populations. Further, asymmetric dispersal that may be both habitat- and density-dependent could slow growth rates on the edge. This pattern may also increase the importance of harsh edge patches as a source of long-distance dispersers.

Why are fruits colorful? The relative importance of achromatic and chromatic contrasts for detection by birds

The colors of fruits and flowers are traditionally viewed as an adaptation to increase the detectability of plant organs to animal vectors. The detectability of visual signals increases with increasing contrasts between target and background. Contrasts consist of a chromatic aspect (color) and an achromatic aspect (light intensity), which are perceived separately by animals. To evaluate the relative importance of fruits’ chromatic and achromatic contrasts for the detection by avian fruit consumers we conducted an experiment with artificial fruits of four different colors in a tropical forest. We displayed the fruits against two different backgrounds, an artificial background and a natural one, because they differed in achromatic properties. We found no effect of the type of background on fruit detection rates. Detection rates differed for the four fruit colors. The probability of detection was explained by the chromatic contrast between fruits and their background, not by the achromatic contrasts. We suggest that birds attend primarily to chromatic contrast probably because these are more reliably detected under variable light conditions. Consistent with this hypothesis, we found habitat-specific differences in the conspicuousness of natural fruit colors in the study area. Fruits of understory species that are subjected to the variable light conditions within a forest displayed higher chromatic contrasts than species growing in the open restinga forest with constant bright illumination. There was no such difference for achromatic contrasts. In sum, we suggest that fruit colors differ between habitats because fruit colors that have strong chromatic contrasts against background can increase plants’ reproductive success, particularly under variable light conditions.

Habitat heterogeneity as a consequence of substratum-orientation and kelp-canopy: Relating interdependent responses to common patterns

Much of our understanding of the relationship between canopy-forming species and benthic organisms is based on work done on horizontal surfaces, yet the orientation of substratum is known to have striking and widespread effects on benthos. An explicit quantitative understanding of the extent to which habitat related patterns on horizontal surfaces are representative of vertical surfaces is currently lacking and is needed before we can sensibly propose models that account for some of the major sources of subtidal heterogeneity. We tested the hypothesis that the influences of kelp forests ( i.e. canopy vs no canopy) on benthic organisms living on horizontal surfaces are different from those on vertical surfaces by testing for an interaction between the factors: habitat ( i.e. canopy vs no canopy) and orientation ( i.e. horizontal vs vertical). We found that for 5 of the 6 benthic taxa studied, differences between habitats that were detected for one orientation were not apparent for the alternate orientation. Our rejection of the null hypothesis and quantitative evidence of orientation and habitat specific differences ( i.e. each combination has unique differences) assisted us to recognize the conditions for which we may reliably anticipate the structure of benthic organisms, thereby improving the predictive power of models that account for widespread patterns in subtidal heterogeneity in this system. This opportunity provided us with a quantitative basis from which we propose an integrated understanding of processes and patterns that until now have been restricted to either habitat ( i.e. canopy vs no canopy) or orientation ( i.e. vertical vs horizontal substratum).

Interactions between habitat heterogeneity and food affect reproductive output in a top predator

1. Habitat heterogeneity has important repercussions for species abundance, demography and life-history patterns. While habitat effects have been more thoroughly studied in top-down situations (e.g. in association with predation), their role in bottom-up situations (e.g. in association with food abundance) has been less explored and the underlying mechanism(s) behind the ecological patterns have not commonly been identified. 2. With material from 1993 to 2003, we test the hypothesis that the reproduction of Finnish northern goshawks Accipiter gentilis (L.) is bottom-up limited by habitat composition, especially in situations where the density of their main prey (grouse) is low. Special emphasis was placed on identifying the mechanism(s) behind potential habitat effects. 3. While laying date and large-scale variation in the main prey density (but not habitat composition) were related to the number of eggs goshawks laid, small-scale differences in alternative prey density between different territories later influenced how many young were fledged via the mechanism of habitat-dependent partial-brood loss. As a result of this mechanism, a difference in nestling condition also arose between goshawk territories with differing habitat compositions. 4. As the relative proportions of different landscape elements in a given landscape is a function of large-scale differences in geomorphology and land use, this means that the reproductive performance of goshawks as averaged over larger scales can be understood correctly only in respect to the fact that habitat gradients differ across landscapes. 5. In addition to being one of the first papers identifying the mechanism of partial brood loss as being primarily responsible for the habitat-specific differences in the production of young, this study further illustrates the need to identify small-scale mechanisms to correctly understand the large-scale patterns of reproductive performance in territorial species. The repercussions of the observed habitat effect for local population development are discussed.

Seed size and seedling growth: differential response of Australian and British Fabaceae to nutrient limitation

Seed size is widely held to exert an important influence over plant establishment, but while large seeds are often assumed to be at an advantage in nutrient-limited conditions, there is in fact, little consistent evidence to support this hypothesis. Here, we examined the interspecific relationship between seedling growth and seed size for Australian and British Fabaceae species in nutrient solutions deficient in nitrogen, phosphorus, potassium or all nutrients combined (distilled water). The British species showed no consistent link between mean seed mass and seedling growth in nutrient-limited conditions. By contrast, all four nutrient-deficient treatments yielded a significant relationship for the Australian species. Linear regression showed that growth under balanced nutrient conditions was positively associated with growth without nutrients, although in fewer cases for the British species. We suggest that habitat-specific differences in regeneration conditions and/or evolutionary history may influence the role that seed size plays in dictating how seedlings of different species respond to nutrient shortage. We recommend caution in attempts to link traits like seed size to wider patterns of plant community ecology.

Turbidity and foraging rate in threespine sticklebacks: the importance of visual and chemical prey cues

AbstractIn aquatic habitats turbidity can affect the foraging efficiency of visual predators, directly influencing their capacity to detect prey. In a laboratory study we tested the effect of different loads of suspended sediment upon the foraging rates of threespine sticklebacks (Gasterosteus aculeatus). We compared the foraging rates of fish under a series of different turbidity treatments, testing fish originating from four habitats within a single drainage basin that differed in a number of environmental parameters including turbidity. Although we found habitat specific differences in foraging rates, these did not correspond to local turbidity levels. The findings of a follow up experiment revealed habitat-specific variation in boldness, which may be indirecly linked to the observed differences in foraging rate. The main finding of our study was that turbidity alone had no impact upon their prey capture rates, but that high turbidity in combination with saturation with prey odour extract caused prey capture rates to fall significantly. This suggests that olfactory cues can be more important than visual cues in determining foraging performance in this species, potentially influencing how they cope with naturally occurring periods of turbidity, and how they adapt to human-induced eutrophication.

Seed bank persistence of clonal weeds in contrasting habitats: implications for control

The ability of weeds to form a seed bank is important for their population dynamics and management because it provides a refuge enabling reinvasion after established target plants have died. However, knowledge of the differential seed behaviour of individual species over multiple years and varying environmental conditions is surprisingly rare but necessary for effective control of diverse weed populations. We established a seed burial experiment in alpine habitats differing in management regime (i.e., forest, hay meadow and pasture) to determine whether seeds of the unpalatable perennial weeds, Veratrum album (white hellebore) and Gentiana lutea (yellow gentian) were able to delay germination and remain viable over 3 years. Our study shows that both species formed a short-term persistent seed bank; in the third-year, the soil seed banks of both species were nearly depleted, having declined to <5% of their original size. Both species had strikingly different germination strategies: G. lutea seeds mainly germinated in their first-year, whilst the majority of V. album seeds germinated in their second-year. The fraction of dormant G. lutea seeds increased with seed age, indicating that seeds remained viable after forgoing germination in the previous year. Habitat-specific differences in seed germination increased with seed age, with germination fractions being lowest in moist hay meadows. This suggests that the negative effects of anoxic conditions became more pronounced as seeds aged in hay meadows. Conversely, seed dormancy was equal among habitats. The absence of a long-term persistent seed bank has important implications for the management of both nuisance and endangered-plant populations. In the case of V. album and G. lutea, re-colonization of habitats from the seed bank is unlikely after established plants have been removed.

Phenotypic homogeneity of two intertidal snails across a wave exposure gradient in South Australia

Dislodgement by the large drag forces imparted by breaking waves is an important cause of mortality for intertidal snails. The risk of drag-induced dislodgement can be reduced with: (1) a smaller shell of lower maximum projected surface area (MPSA); (2) a streamlined shell shape characterized by a squatter shell; and/or (3) greater adhesive strength attained through a larger foot area or increased foot tenacity. Snails on exposed coasts tend to express traits that increase dislodgement resistance. Such habitat-specific differences could result from direct selection against poorly adapted phenotypes on exposed shores but may reflect gastropod adaptation to high wave action achieved through phenotypic plasticity or genetic polymorphism. With this in mind, we examined the size, shape and adhesive strength of populations of two gastropod species, Austrocochlea constricta (Lamarck) and Nerita atramentosa (Reeve), from two adjacent shores representing extremes in wave exposure. Over a 5 day period, maximum wave forces were more than 10 times greater on the exposed than sheltered shore. Size–frequency distributions indicate that a predator consuming snails within the 1.3–1.8 cm length range regulates sheltered shore populations of both snail species. Although morphological scaling considerations suggest that drag forces should not place physical limits on the size of these gastropods, exposed shore populations of both snails were small relative to the maximum size documented for these species. Therefore, selective forces at the exposed site might favour smaller individuals with increased access to microhabitat refuges. Unexpectedly, however, neither snail species exhibited between-shore differences in shape, foot area or foot tenacity, which are likely to have adaptive explanations. Hence, it is possible that these snails are incapable of adaptive developmental responses to high wave action. Instead, the homogeneous and wave-exposed nature of Australia's southern coastline may have favoured the evolution of generalist strategies in these species.

Detecting growth under environmental extremes: Spatial and temporal patterns in nucleic acid ratios in two Antarctic bivalves

Growth of Antarctic benthic organisms is very slow due to temperature and food availability, and subtle differences in growth rate may be difficult to detect. Nucleic acid ratios (RNA/DNA, RNA/protein or total RNA concentration) are measures of protein synthesis potential and may be used to assess short-term growth rate in a range of marine organisms. We quantified nucleic acid ratios in the scallop Adamussium colbecki and the clam Laternula elliptica at five locations in the Ross Sea, Antarctica. We were able to detect species-specific, habitat-specific, and seasonal differences in nucleic acid ratios and related these to associated differences in primary productivity. By using nucleic acid ratios, future studies could relatively easily obtain a measure of growth rate from a multitude of locations with contrasting habitat characteristics, food availability and temperature regimes around the Antarctic continent. This would yield a unique understanding of spatial and temporal patterns in bivalve growth in this extreme environment.

Assessment of a shell bank and associated substrates as nursery habitat of postsettlement red snapper

Trawl surveys were conducted in 2000 and 2001 to examine patterns of distribution and abundance of postsettlement red snapper ( Lutjanus campechanus) on a shell bank, Freeport Rocks Bathymetric High (FRBH), in the northwestern Gulf of Mexico. In addition, otolith-based methods were used to determine age, hatch-date, growth and mortality of new recruits associated with FRBH. Date and region were significant factors affecting density of red snapper in 2000. Peak densities of red snapper were observed in July and August, and mean density among habitat types (shell bank, inshore mud, offshore mud) was similar (range: 50–52 ind hectare −1) in 2000. Alternatively, a habitat effect was detected during a limited survey conducted in 2001, with density higher on the shell bank than inshore or offshore mud habitat. Postsettlement red snapper were first detected at approximately 16 mm standard length, and individuals less than 20 mm were present in all habitats. Estimated ages of red snapper ranged from 26 to 121 d, with new settlers (≤20 mm) typically less than 28 d. Predicted hatch dates ranged from early April to mid August with a single peak occurring from late May to early June. Growth rate for the April–May cohort (0.817 mm d −1) was similar to the June–July cohort (0.830 mm d −1). Habitat-specific differences in growth were observed, and rates were highest for individuals from the inshore habitat (0.881 mm d −1). Mortality rates ( Z) during the early post-settlement period were approximated using catch curves, and early life mortality of red snapper was 12.1% d −1 ( Z=0.129). While the difference in mortality between cohorts was negligible, a habitat-specific difference in mortality was observed. Mortality rate of red snapper inhabiting the inshore mud habitat ( Z=0.045, 4.4% d −1) was lower than rates observed for individuals on the shell bank ( Z=0.120, 11.9% d −1) or offshore ( Z=0.099, 9.3% d −1) habitat. Individuals residing in the inshore habitat had significantly higher growth rates and significantly lower mortality rates, suggesting that recruitment potential was higher for these individuals.