Habitat partitioning by whiting species (Sillaginidae) in coastal waters

Synopsis The ways in which the distributions of six species of whiting (Sillaginidae) in the coastal marine waters of south-western Australia are related to the type of substrate (bare sand vs. seagrass), degree of exposure of habitat, water depth and body size have been investigated. Whiting in near shore waters (< 1.5 m) were sampled using a fine-meshed seine net, while those in shallow (5–15 m) and deep (20–35 m) waters of the inner continental shelf were sampled with a trawl net. Shallow nearshore waters are shown to provide nursery habitats for five of the six whiting species. In these waters, Sillaginodes punctata, Sillago burrus, Sillago schomburgkii and Sillago vittata mainly occur in protected areas, while Sillago bassensis predominantly occupies areas that are more exposed to wave and swell activity. The first three of these species also use estuaries as nursery areas. In nearshore waters, whiting were captured almost exclusively over bare sand, rather than in interpersed beds of the seagrass Posidonia spp., presumably reflecting the fact that the dense canopies produced by the wide blades of Posidonia spp. must inhibit penetration by the benthic whiting species. As 0 + S. punctata increase in size, they tend to move offshore during the day and inshore at night. Many mature representatives of S. schomburgkii are present in nearshore areas, whereas the other four species move offshore into inner-shelf waters as they increase in length. Sillago burrus and S. vittata remain in shallow inner-shelf waters, whereas the larger S. bassensis subsequently migrate into deeper inner-shelf waters. Large Sillago bassensis thus co-occurs with Sillago robusta, which is mainly found in those deeper waters, but does not reach as large a size. The larger S. punctata occupy areas near reefs which could not be sampled by trawl netting. There are thus interspecific differences in (i) the times of recruitment of the 0 + age class into nearshore areas, (ii) the types of habitat occupied during juvenile and adult life, and (iii) the degree to which fish move into more offshore waters as they increase in length, and one species is restricted to deeper waters. The resultant partial segregation among habitats of the coastal waters of south-western Australia by different size groups of these relatively abundant whiting species presumably reduces the potential for intra- and interspecific competition amongst these species.     

Extreme temperatures,foundation species,and abrupt ecosystem change: an example from an iconic seagrass ecosystem

Extreme climatic events can trigger abrupt and often lasting change in ecosystems via the reduction or elimination of foundation (i.e., habitat‐forming) species. However, while the frequency/intensity of extreme events is predicted to increase under climate change, the impact of these events on many foundation species and the ecosystems they support remains poorly understood. Here, we use the iconic seagrass meadows of Shark Bay, Western Australia – a relatively pristine subtropical embayment whose dominant, canopy‐forming seagrass, Amphibolis antarctica, is a temperate species growing near its low‐latitude range limit – as a model system to investigate the impacts of extreme temperatures on ecosystems supported by thermally sensitive foundation species in a changing climate. Following an unprecedented marine heat wave in late summer 2010/11, A. antarctica experienced catastrophic (>90%) dieback in several regions of Shark Bay. Animal‐borne video footage taken from the perspective of resident, seagrass‐associated megafauna (sea turtles) revealed severe habitat degradation after the event compared with a decade earlier. This reduction in habitat quality corresponded with a decline in the health status of largely herbivorous green turtles (Chelonia mydas) in the 2 years following the heat wave, providing evidence of long‐term, community‐level impacts of the event. Based on these findings, and similar examples from diverse ecosystems, we argue that a generalized framework for assessing the vulnerability of ecosystems to abrupt change associated with the loss of foundation species is needed to accurately predict ecosystem trajectories in a changing climate. This includes seagrass meadows, which have received relatively little attention in this context. Novel research and monitoring methods, such as the analysis of habitat and environmental data from animal‐borne video and data‐logging systems, can make an important contribution to this framework.     

The genetic structure of a marine teleost, <Emphasis Type="Italic">Chrysophrys auratus,</Emphasis> in a large,heterogeneous marine embayment

There is considerable interest in the processes that generate genetic divergence in marine species and the spatial and temporal scales over which these processes operate. Shark Bay, a large embayment (~13,000 km²) on the arid west coast of Australia, has been described as a focal point for genetic divergence in marine species due to its heterogeneous environment. This study represents the first DNA-based analysis of the genetic structure of a marine species with pelagic early life stages (ELS) in this region. Twelve microsatellite loci were used to compare the genetic composition of the teleost Chrysophrys auratus from five areas within Shark Bay, and from near-by shelf waters approximately 250 km to the south. The results suggest that the microsatellite composition of C. auratus is homogeneous across most of Shark Bay and near-by shelf waters. This genetic homogeneity is probably maintained via small amounts of contemporary gene flow, which may reduce the potential for C. auratus to locally adapt to the Shark Bay environment. A weakly differentiated assemblage in Freycinet Estuary in the Western Gulf of Shark Bay provided an important exception to the otherwise homogeneous microsatellite composition of C. auratus in the study area. This weak differentiation probably reflects restrictions to gene flow into and/or out of this site due to the temporal isolation of breeding adults, selective mortality of immigrants and/or the presence of hydrological barriers to larval transport. Each of these factors can be linked to environmental heterogeneity in Shark Bay.     

Marine introductions in the Shark Bay World Heritage Property, Western Australia: a preliminary assessment

The presence and impacts of non‐indigenous species (NIS) in marine areas of high conservation or World Heritage significance have rarely been examined. Case studies worldwide suggest that the potential exists for the introduction of NIS to significantly impact conservation values in regions conserved for the uniqueness and diversity of native assemblages. In this study, a preliminary investigation was conducted to provide information essential for managing marine introductions in the Shark Bay World Heritage Property. A focused fouling plate survey sampled a total of 112 encrusting taxa, of which 10 (11.2%) were classified as introduced and 10 others as cryptogenic. Eight introduced bryozoans: Aetea anguina (Linnaeus, 1758), Bugula neritina (Linnaeus, 1758), Bugula stolonifera Ryland, 1960, Conopeum seurati (Canu, 1928), Savignyella lafontii (Audouin, 1826), Schizoporella errata (Waters, 1878), Watersipora subtorquata (d’Orbigny, 1842) and Zoobotryon verticellatum della Chiaje, 1828; one tunicate, Styela plicata Lesueur, 1823; and an introduced hydroid, Obelia dichotoma (Linnaeus, 1758) were frequent, and in some cases dominant, components of encrusting communities. Of the 20 most frequently occurring species detected in the Bay, four were introduced and of the 20 species with highest average percent cover per plate, six were introduced. At one site, space occupation by NIS averaged 71.6% ± 7.4 of plate live cover. Space occupation by an individual NIS was as high as 62.4% of plate area (mean 7.82% ± 1.8). NIS were detected at sites lacking commercial traffic and ballast water discharge and isolated by distance and physical environment, suggesting that hull fouling of recreational craft may be the most important vector in the region. Seventy‐five percent of NIS detected in Shark Bay are established in Australian ports to the south of Shark Bay, while 33% are established to the north, tentatively implicating temperate affinity NIS and the movement of vessels from Australian ports south of Shark Bay as a greater risk to the region.     

Diets of reef‐dwelling labrids (Choerodon species) vary with body size,season and habitat: influence of foraging ability,specialization and opportunism

Contemporary multivariate statistics were used to test the hypotheses that the dietary compositions of three populations of labrids on the west Australian coast are related to body size and undergo seasonal changes and to elucidate the relative extents and basis for any dietary differences within and between those populations. Gut content analyses determined the dietary compositions of Choerodon rubescens in marine waters of the outer reefs in the World Heritage Area of Shark Bay (26° S; 114° E) and of Choerodon schoenleinii in inner protected reefs of that large embayment. The dietary compositions of C. rubescens and C. schoenleinii differed significantly among length classes, progressed serially with increasing body size, both overall and almost invariably in each season and were more closely related to body size than season, whose effect was at best minimal. The size‐related dietary change in C. rubescens involved, in particular, a shift from crustaceans and non‐mytilid bivalves to mytilid bivalves and echinoid echinoderms. Although the diet of C. schoenleinii followed similar size‐related changes, it contained a greater volume of gastropods when the fish were small and mytilids when large and only a small volume of echinoids. The dietary composition of C. rubescens in the Abrolhos Islands, 300 km to the south of Shark Bay, was related both to length class and season and differed from that of this labrid in Shark Bay with the ingestion of lesser volumes of mytilids and greater volumes of echinoids. The size‐related changes in diet imply that these species shift from foraging over soft substrata to over reefs as their very well‐developed jaws become sufficiently strong to remove attached and larger prey. The dietary compositions of C. rubescens and C. schoenleinii in Shark Bay and of C. rubescens at the Abrolhos Islands were related far more to habitat–locational differences than to length class and season. The above intraspecific and interspecific differences in diet are consistent with qualitative accounts of the relative abundances of the main prey in their respective environments, supporting the view that, despite specializations in their feeding apparatus, these labrids can feed opportunistically to a certain extent and could thus potentially respond to moderate changes in the composition of their prey caused by climate change and other anthropogenic effects.     

Reproductive Biology and Protandrous Hermaphroditism in Acanthopagrus latus

Detailed macroscopic and histological studies of the gonads of a full size and age range of Acanthopagrus latus from each season in Shark Bay, Western Australia, demonstrate that this species is a protandrous hermaphrodite in this large subtropical embayment. Although our scheme for the changes that occur in the ovotestes of A. latus during life is not consistent with some of the conclusions drawn for this species elsewhere, it is similar to that of Pollock (1985 J. Fish. Biol. 26: 301–311) for the congeneric Acanthopagrus australis. The ovotestes of males develop from gonads which contain substantial amounts of both testicular and ovarian tissue. The testicular component of the ovotestes of all males regresses markedly after spawning. During the next spawning season, the ovotestes either become gonads in which the testicular zone again predominates and contains spermatids and spermatozoa (functional males), or gonads in which the ovarian zone now predominates and contains mature oocytes (functional females). Once a fish has become a functional female, it remains a female throughout the rest of its life. In Shark Bay, A. latustypically spawns on a limited number of occasions during a short period in late winter and early spring and has determinate fecundity. The mean potential annual fecundity was ca. 2000000. The total length of 245mm, at which, during the spawning period, 50% of A. latus become identifiable as males, is very similar to the current minimum legal length (MLL) of 250mm, which corresponds to an age of ca. 2.5 years less than that at which 50% of males become females. Thus, although the spawning potential ratio suggests that the present fishing pressure is sustainable, the current MLL should be reviewed if recreational fishing pressure continues to increase.     

Juveniles of two sillaginids,Sillago aeolus andS. sihama,occurring in a surf zone in the Philippines

Sillaginid juveniles collected from the surf zone at Tigbauan, Iloilo, Philippines, between May 1986 and September 1987 were identified asSillago aeolus (n=702, 8.9–26.0 mm SL) andS. sihama (n=3414, 8.6–22.9 mm SL), based on the numbers of dorsal and anal soft fin rays and vertebrae. The two species were easily distinguishable by the pattern of melanophores distributed on the caudal fin base,S. aeolus having a triangular-shaped cluster, whereas the melanophores formed a vertical line inS. sihama. The ratios of pre-anal and caudal peduncle lengths to SL also differed between the species, both being higher inS. aeolus. The occurrence ofS. aeolus was limited to the dry season, from January to March. On the other hand,S. sihama occurred year-round, although a peak was observed in the dry season, from November to April.     

Length–weight and length–length relationships for three Sillago species (Sillaginidae) from the Persian Gulf

This study investigates length–length and length–weight relationships of three Sillago species [Sillago arabica McKay & McCarthy (Memoirs of the Queensland Museum, 27, 1989, 551), Sillago attenuata McKay, 1985 and Sillago sihama (Forsskål, 1775)] captured in the Persian Gulf, Iran. A collection of 220 specimens was captured from August 2015 to January 2016 by cast net (mesh size: 10 mm) and beach seine (mesh size: 15 mm). The relationships of total length (TL) and standard length (SL) and the relationships between total length and body weight are given. This study presents the first reference on LWRs for two of the species (S. arabica and S. attenuata) in their range of distribution.     

Reappearance of Eumarcia fumigata (G.B. Sowerby II, 1853) (Bivalvia: Veneridae) into the Swan River,Western Australia

ABSTRACT

The venerid bivalve Eumarcia fumigata was a common species in Western Australia (WA) during the Pleistocene, where it was distributed as far north as Shark Bay. It became extinct in WA as the climate changed several thousand years ago but remains common in eastern Australia from southern Queensland to South Australia and Tasmania. The species has recently reappeared in the Swan River, probably due to shipping movements. Of the > 60 marine and estuarine species introduced into WA it is only the third confirmed introduction from eastern Australia, and the first that is a reappearance in the Swan River of a species present in the geological past. The present reappearance of E. fumigata, and the introduction of other species, has been made possible by the removal of a rocky bar at the mouth of the estuary and the creation of more marine conditions in the lower Swan estuary.     

Comparisons between the age structures, growth and reproductive biology of two co-occurring sillaginids, Sillago robusta and S. bassensis, in temperate coastal waters of Australia

Sillago robusta and S. bassensis occupy the open sandy areas of the deeper waters (20–35 m) of the inner continental shelf of the lower west coast of Australia. Comparisons were made of their age and size compositions, growth rates, ages and sizes at first maturity, and spawning times. Most S. robusta were less than 3 years old (maximum 7 years), while S. bassensis often reached 7 years of age (maximum 10 years). The maximum length of S. robusta (200 mm) was far shorter than S. bassensis (328 mm), whereas the reverse pertained for the von Bertalanffy growth coeffcient (K), i.e. c. 1.0 v. c. 0.3. Differences in K reflect the fact that c. 80% of the asymptotic length is achieved by S. robusta after 2 years, but not until 6 years of age by S. bassensis. Sexual maturity was reached by up to 50% of S. robusta at the end of their first year of life, and by almost all fish at the end of their second year of life. Most S. bassensis did not reach maturity until the end of their third year of life. In both species, those individuals that reached maturity early, were significantly longer than those that did not reach maturity at the same age. The gonadosomatic indices and proportions of mature gonads, and the numbers of vitellogenic and hydrated oocytes and post-ovulatory follicles, were relatively high in ovaries of both species between December and March. Although both species spawn in these 4 months, some S. bassensis also spawn between September and November and in March and April. In the middle of the spawning period, the ovaries of S. robusta and S. bassensis frequently contained oocytes that ranged widely in size and development, together with post-ovulatory follicles, suggesting that both species are multiple spawners. Juveniles of S. robusta remain in the deeper waters of the inner continental shelf, whereas those of S. bassensis migrate inshore to their nursery areas in surf zones. As S. robusta reaches sexual maturity at an earlier age and smaller size than S. bassensis, it is probably more advantageous for the juveniles of this species to remain in deeper water, and thereby conserve energy for gonadal maturation, rather than migrating into shallow waters for only a very few months.     

Grazing rates of Elysia tomentosa on native and introduced Caulerpa taxifolia

The marine alga Caulerpa taxifolia Vahl (C. Agardh), recognized globally as one of the most prolific non-native species introductions, has been introduced to several temperate locations from where it has since rapidly expanded. C. taxifolia is protected by a toxin (terpenoid) in its tissues that limits grazing by native herbivores. Sacoglossan molluscs of the genus Elysia are among the few organisms that graze C. taxifolia; however, little is known about their feeding ecology. In the current study, we quantified the grazing rates of Elysia tomentosa on native C. taxifolia (Moreton Bay, Queensland) and introduced C. taxifolia (Botany Bay and Lake Conjola, New South Wales). Grazing rates were similar at Moreton Bay sites and Botany Bay; however, they were significantly lower in Lake Conjola. At the maximum observed grazing rate, slugs ate their body weight in C. taxifolia (dry weight) every 18–24 h. Differences in grazing rates between locations may be explained by differences in C. taxifolia morphology rather than native or introduced origin. Handling editor: J. Padisak     

有害甲藻Stoeckeria algicida在辽东湾的时空分布

Stoeckeria algicida为甲藻纲胸甲球藻科,有侵噬鱼类细胞杀鱼的能力,可导致鱼类成群死亡,同时也会杀死其他海洋微藻。由于该藻个体微小、形态学鉴定困难,研究较为迟缓,我国海域几乎没有该藻的研究报道。近几年,高通量测序技术的发展极大地推动了微型/微微型浮游植物的鉴定研究,为了解我国辽东湾海域是否存在Stoeckeria algicida及其分布情况,以18S rD NA V4区作为目标基因,结合高通量测序技术,专门设计了微型/微微型浮游植物鉴定引物对V4(F/R),随后对辽东湾2014年四季海水中微型和微微型浮游植物多样性进行了检测。结果发现,Stoeckeria algicida除了春季未检出外,其他季节均有检出,温度是影响该藻繁殖的主要因素。虽然Stoeckeria algicida在整个环境样品中优势度不太明显,但其夏季密度较高(最高达2.753×10~3个/L),高值区主要分布在辽东湾东西两岸,致灾风险较高,应引起有关方面足够重视。Stoeckeria algicida在我国海域首次报道,其危害后果严峻,必须加强监测监管。     

Development of eggs, larvae and juveniles of laboratory-reared blue whiting,Sillago parvisquamis (Percoidei: Sillaginidae)

The embryonic, larval and juvenile development of blue whiting,Sillago parvisquamis Gill, are described from a series of laboratory-reared specimens. Mean egg diameter and mean total length (TL) of newly-hatched larvae were 0.71 mm and 1.58 mm, respectively. The eggs were non-adhesive, buoyant and spherical with an oil globule (mean diameter 0.18 mm). Hatching occurred about 20 hours after fertilization at a temperature of 24.0–25.0°C, newly-hatched larvae having 38–40 myomeres. The yolk and oil globule were completely absorbed 3 days after hatching at 2.8–3.2 (mean 3.0) mm TL. Notochord flexion was completed by 7.2–8.2 (7.7) mm TL, and pectoral and caudal fin rays fully developed by approximately 10 mm and 8.5 mm TL, respectively. Completion of fin development occurred in the following sequence: caudal, pectoral, anal and second dorsal, first dorsal and pelvic, the last-mentioned by approximately 11 mm TL. The larvae ofS. parvisquamis andS. japonica, which closely resemble each other in general morphology and pigmentation, could be distinguished as follows. Newly-hatchedS. parvisquamis larvae had more myomeres thanS. japonica (38–40 vs. 32–34) and more melanophores on the dorsal surface of the body (19–28 vs. about 40).Sillago japonica had a vertical band of melanophores on the caudal peduncle, which was lacking in postflexionS. parvisquamis larvae. In addition, juveniles ofS. parvisquamis (larger than 23 mm TL) had melanophores on the body extending anteriorly to below the lateral line to form a midlateral band, whereas no obvious band occurred on similarly-sizedS. japonica juveniles.     

Population structure of the South American Estuarine crab, Chasmagnathus granulatus (Brachyura: Varunidae) near the southern limit of its geographical distribution: comparison with northern populations

Crabs are among the most conspicuous and ecologically important invertebrates of the large intertidal zones that characterize estuarine and protected coastal areas in temperate regions. The habitat, population structure and breeding cycle of Chasmagnathus granulatus (Brachyura: Varunidae), a semiterrestrial burrowing crab endemic to the warm temperate coasts of the Southwestern Atlantic, were studied in San Antonio Bay (Argentina), near the southern limit of its range. San Antonio Bay has no freshwater input, winter is relatively colder, and summer warmer, than northern habitats of this species. Crabs lived both in vegetated and unvegetated zones, but density and sex ratio varied among dates and zones. The maximum observed density was 136 crabs/m² , the maximum carapace width (CW) was 32 mm (males) and 29.8 mm (females), ovigerous females were found only in November and January, and the smallest ovigerous female measured 17 mm CW. The population structure, spatial distribution, and recruitment pattern of C. granulatusdid not differ between San Antonio Bay and northern habitats. The higher density, smaller maximum size and shorter reproductive cycle observed in San Antonio cannot be atributed to changes associated with a latitudinal cline and other factors, such as thermal amplitude and food availability, need to be studied.     

Replacement of large copepods by small ones with eutrophication of embayments: cause and consequence

Species and size compositions of the planktonic copepod community were compared between two eutrophic embayments, Tokyo Bay and Osaka Bay. Within these bays, the median body weight of the community was lowest at the innermost station and increased gradually offshore. In Tokyo Bay, which is more eutrophic than Osaka Bay, the dominant species was Oithona davisae and the median carbon weight of the community was ca 0.1 µg. In Osaka Bay, Paracalanus sp., Calanus sinicus and Corycaeus spp. were dominant and the median weight was 1–2 order of magnitude higher (2–78 µg) than in Tokyo Bay. Some 40 years ago, when eutrophication was less prominent, Acartia omorii, Paracalanus sp. and Microsetella norvegica comprised a considerable portion of the community in Tokyo Bay. The proliferation of O. davisae might have been caused with the recent change in food particle composition and/or dissociated life cycle of this species from the benthic anoxic layer. Decrease in the size composition of the copepod community may make the embayment ecosystem unfavorable for planktivorous fish, but not for jellyfish, e.g. Aurelia and Bolinopsis. This may also result in an acceleration of the flux of biogenic materials from the embayment to the outer ocean.     

Age, growth and reproduction of Sillago schomburgkii in south-western Australian, nearshore waters and comparisons of life history styles of a suite of Sillago species

Samples from sheltered nearshore waters in south-western Australia, in which Sillago schomburgkii spends its entire life cycle, have been used to determine the age structure, growth rate, age and length at first sexual maturity, and spawning period of this whiting species. Several S. schomburgkii reached four to seven years in age and one 12+ fish was caught. The respective maximum and asymptotic lengths (L) were 350 and 333 mm for females and 348 and 325 mm for males, while the growth coefficients (K) for females and males were 0.53 and 0.49, respectively. Sexual maturity was attained by both sexes of S. schomburgkii at ca. 200 mm, a length reached at the end of the second year of life. Monthly trends exhibited by gonadosomatic indices, the proportions of mature gonads and the prevalence of advanced oocytes and post-ovulatory follicles demonstrate that S. schomburgkii spawns predominantly from December to February. The presence of yolk vesicle and yolk granule oocytes and post-ovulatory follicles in the same ovaries during the spawning period, indicate that S. schomburgkii is a multiple spawner. The patterns of growth of the five Sillago species, that occur in south-western Australian marine waters, fall into two categories. The first, which consists of S. burrus and S. robusta, has a small L, i.e. < 190, and a high growth coefficient (K), i.e. 1.0, whereas the second, which comprises S. schomburgkii, S. vittata and S. bassensis, attain a larger size, i.e. L > 00 mm, and has a low K, i.e. 0.5. The lengths and ages at maturity of S. schomburgkii, S. bassensis, S. burrus and S. robusta, as well as of S. analis and S. flindersi found elsewhere in Australia, are linearly related to their asymptotic lengths and maximum ages, respectively. The two smallest species, S. burrus and S. robusta, attain maturity at ca. 130 mm. However, the former species, whose juveniles occupy productive nearshore waters, grows rapidly and reaches this length by the end of the first year of life, whereas the latter species, which is restricted to deeper waters, grows more slowly and thus does not attain this length until a year later. Sillagoflindersi, which is slightly larger than S. burrus and S. robusta, migrates out into deeper waters and attains maturity at ca. 170 mm and two years of age. Although S. schomburgkii, S. analis and S. bassensis attain maturity at ca. 200 mm and reach similar lengths, the first two of these species, which remain in nearshore waters and display more rapid growth, reach maturity one year earlier than the last species, which migrates out into deeper and presumably less productive waters. While S. vittata reaches a similar size and likewise migrates out into deep waters, it reaches maturity earlier, i.e. at the end of its first year of life.     

Notes on the biology of some psammophile Turbellaria of the Black Sea

The biology of two marine turbellarians, Cercyra hastata (Tricladida) and Pseudomonocelis ophiocephala (Proseriata) has been studied over a period of five years. They are the main components of the biocoenosis of the saccocirrus sand of Sevastopol Bay. These species have a significant role in the processes of secondary production and transformation of the organic matter in the coastal zone.     

Fear factor: do dugongs (<Emphasis Type="Italic">Dugong dugon</Emphasis>) trade food for safety from tiger sharks (<Emphasis Type="Italic">Galeocerdo cuvier</Emphasis>)?

Predators can influence plants indirectly by altering spatial patterns of herbivory, so studies assessing the relationship between perceived predation risk and habitat use by herbivores may improve our understanding of community organization. In marine systems, the effects of predation danger on space use by large herbivores have received little attention, despite the possibility that predator-mediated alterations in patterns of grazing by these animals influence benthic community structure. We evaluated the relationship between habitat use by foraging dugongs (Dugong dugon) and the threat of tiger shark predation in an Australian embayment (Shark Bay) between 1997 and 2004. Dugong densities were quantified in shallow (putatively dangerous) and deep (putatively safe) habitats (seven survey zones allocated to each habitat), and predation hazard was indexed using catch rates of tiger sharks (Galeocerdo cuvier); seagrass volume provided a measure of food biomass within each zone. Overall, dugongs selected shallow habitats, where their food is concentrated. Foragers used shallow and deep habitats in proportion to food availability (input matching) when large tiger sharks were scarce and overused deep habitats when sharks were common. Furthermore, strong synchrony existed between daily measures of shark abundance and the extent to which deep habitats were overused. Thus, dugongs appear to adaptively manage their risk of death by allocating time to safe but impoverished foraging patches in proportion to the likelihood of encountering predators in profitable but more dangerous areas. This apparent food-safety trade-off has important implications for seagrass community structure in Shark Bay, as it may result in marked temporal variability in grazing pressure. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Seascape genomics reveals fine‐scale patterns of dispersal for a reef fish along the ecologically divergent coast of Northwestern Australia

Understanding the drivers of dispersal among populations is a central topic in marine ecology and fundamental for spatially explicit management of marine resources. The extensive coast of Northwestern Australia provides an emerging frontier for implementing new genomic tools to comparatively identify patterns of dispersal across diverse and extreme environmental conditions. Here, we focused on the stripey snapper (Lutjanus carponotatus), which is important to recreational, charter‐based and customary fishers throughout the Indo‐West Pacific. We collected 1,016 L. carponotatus samples at 51 locations in the coastal waters of Northwestern Australia ranging from the Northern Territory to Shark Bay and adopted a genotype‐by‐sequencing approach to test whether realized connectivity (via larval dispersal) was related to extreme gradients in coastal hydrodynamics. Hydrodynamic simulations using CONNIE and a more detailed treatment in the Kimberley Bioregion provided null models for comparison. Based on 4,402 polymorphic single nucleotide polymorphism loci shared across all individuals, we demonstrated significant genetic subdivision between the Shark Bay Bioregion in the south and all locations within the remaining, more northern bioregions. More importantly, we identified a zone of admixture spanning a distance of 180 km at the border of the Kimberley and Canning bioregions, including the Buccaneer Archipelago and adjacent waters, which collectively experiences the largest tropical tidal range and some of the fastest tidal currents in the world. Further testing of the generality of this admixture zone in other shallow water species across broader geographic ranges will be critical for our understanding of the population dynamics and genetic structure of marine taxa in our tropical oceans.