Responses of intertidal invertebrates to rising sea surface temperatures in the southeastern Indian Ocean

Introduction: The west coast of Western Australia (WA) is a global hotspot for increasing sea surface temperatures and marine heatwaves. Methods: We used visual survey transects to compare mollusc and echinoderm populations on three coastal intertidal platform reefs on the Perth shoreline with two intertidal platforms at the west end of Rottnest Island (32°S) which are under the influence of the southward flowing Leeuwin Current. Results: In 1983, temperate species dominated Perth coastal molluscan diversity, but the tropical mussel Brachidontes sculptus dominated density. Species richness on coastal platforms remained constant in 2007 and 2021, but total densities were lower in 2007 as B. sculptus declined; partial recovery occurred on coastal platforms in 2021. Tropical species were a significant component of mollusc diversity and density at Rottnest Island in 1982 and 2007. Total mollusc density declined by 98% at the island sites of Radar Reef and 86% at Cape Vlamingh and total echinoderm density by 52% and 88% respectively from 2007 to 2021; species diversity also declined sharply. Discussion: Tropical species have moved southward in WA subtidal environments, but tropical, temperate and WA endemic species all suffered losses of biodiversity and catastrophic declines in density of 90% or more on the two Rottnest Island intertidal platforms. Data presented here provide a sound basis for exploring the possible causes of the catastrophic mortality at the west end of Rottnest Island and monitoring for recovery

Marine benthic flora and fauna of Gourdon Bay and the Dampier Peninsula in the Kimberley region of North-Western Australia

Surveys undertaken to characterise the marine benthic habitats along the Dampier Peninsula and further south at Gourdon Bay in the Kimberley region of Western Australia were augmented with epibenthic sled sampling of soft and hard bottom habitats. This paper describes the species collected, their biomass and relative abundance for the main groups of marine macrophytes and invertebrates. Five localities were surveyed; Gourdon Bay, Quondong Point to Coulomb Point, Carnot Bay to Beagle Bay, Perpendicular Head and Packer Island. Sampling was limited to fifteen epibenthic dredge operations from a range of habitat types and was designed to target the most common habitat types and to obtain species identifications of the most important species and those which typified different habitat types. Surveys covered a total of 1,350 m 2 of seabed in depths between 11 and 23m. We identified 415 taxa comprising: 1 seagrass, 43 algae, 52 sponges, 30 ascidians, 10 hydroids, 14 scleractinian corals, 52 other cnidarians, 69 crustaceans, 73 molluscs and 71 echinoderms. Despite the limited nature of the sampling, a significant number of new species, range extensions and new records for Western Australia and Australia were recorded. Within the algae, one range extension (Halimeda cf. cuneata f. digitata not previously recorded in Western Australia) and one possible new species of Areschougia were recorded. Two range extensions were present in the ascidians; the solitary ascidian Polycarpa cf. intonata has previously only been recorded in Queensland and Cnemidocarpa cf. radicosa only in temperate Australian waters. There were several range extensions for the crustacea, for example, the sponge crab, Tumidodromia dormia, has only been recorded in Queensland. One species of holothurian of the genus Phyllophorus could not be identified from the literature available and may represent a new species. Similarly, a small species of the echinoid Gymnechinus could possibly be a new species. The collections of hydroids, hard corals, crinoids and molluscs contained no new species or range extensions. There was difficulty in identification of some groups to species level due to the status of the current taxonomic literature (e.g. Cnidaria, Porifera and ascidians) and there may be a number of new species among the material collected. Among the anthozoa, there is at least one new species of Chromonephthea and potentially 10 range extensions to Western Australia. Sinularia cf. acuta and Chromonephthea curvata are both new records for Australia with both previously recorded in Indonesia only. Among the better known taxa (e.g. molluscs, echinoderms, corals), most of the taxa identified to species level have been recorded to occur throughout north-western Australia, however the diversity recorded in this study is less than other parts of the Kimberley and this is almost certainly a result of the small overall area sampled and the single method of collection utilised. The most important species on soft bottom habitats in terms of biomass was the heart urchin Breynia desorii (up to 326 g.m -2). Sponges were the dominant fauna by biomass (up to 620 g.m -2) on hard bottom habitats and biomass was dominated a by a few large cup and massive sponge species (e.g. Pione velans and two unidentified Spheciospongia). The biomass of other filter feeders, especially ascidians (e.g. Aplidium cf. crateriferum), soft corals (e.g. Chromonephthea spp.), gorgonians (e.g. Junceella fragilis and Dichotella gemmacea) was also high, indicating the importance of these groups in characterising hard bottom habitats. Although low in biomass, crinoids such as Comaster multifidus and Comatula pectinata were abundant in samples that included a high biomass of other filter feeders

Retention and dispersal of shelf waters influenced by interactions of ocean boundary current and coastal geography

Retention and dispersal of shelf waters under the influence of ocean boundary currents is crucial to recruitment processes of many coastal species. In this study, a Lagrangian particle tracking method based on an eddy-resolving, data-assimilating, hydrodynamic model is used to study spatial variations of local retention rates and alongshore dispersal of surface waters on the continental shelf off the west coast of Australia. The circulation on the shelf off the west coast of Australia is dominated by the southward-flowing eastern boundary current, the Leeuwin Current, which is interrupted by episodic wind-driven, northward, inshore surface transport during the austral summer, and by mesoscale eddy formations during the austral winter. Low-retention shelf regions tend to experience high alongshore currents, owing to the near-shore influence of the Leeuwin Current, protruding coastal geography, or formation of mesoscale eddies, whereas high-retention regions are sheltered from the direct influence of the Leeuwin Current by coastal geographic features. Alongshore dispersal also exhibits spatial as well as seasonal heterogeneity, with predominantly southward dispersal during the austral winter, and more symmetrical dispersal during the austral summer. Shelf retention and seasonal dispersal are linked with recruitment processes of invertebrate and fish species off the west coast of Australia

Reproductive biology of the sand dollar,Peronella lesueuri(L. Agassiz, 1841) (Echinoidea: Clypeasteroida: Laganidae), in south-western Australia

The reproductive biology of the sand dollar Peronella lesueuri was studied between 2009 and 2011 in Cockburn Sound, a large coastal embayment in south-western Australia. Individuals of P. lesueuri did not display sexual size-dimorphism, and the population was found to have a sex ratio of 1:1. Maturity occurred over the range of 80–115 mm with all sand dollars larger than 115 mm having distinct gonads. Monthly histological analysis of gonads and changes in oocyte proportions over time indicated that P. lesueuri has an annual reproductive cycle; gametogenesis occurs in spring and spawning in summer. Differences in the rate of gametogenesis between 2009/2010 and 2010/2011 may have been influenced by higher temperatures experienced in 2010/2011. P. lesueuri have large ova (mean = 210 μm), which suggests the species has lecithotrophic larval development

The sea urchin Centrostephanus tenuispinus (Clark, 1914) is an important bio-eroder on a high latitude (32° S) coral reef

Sea urchins are keystone herbivores in many marine benthic habitats and can significantly influence coral-algae phase shifts and reef carbonate budgets. Hall Bank Reef in Western Australia is a unique high latitude reef with high hermatypic coral cover but lacking macroalgae and soft corals. Since the reef status is thought to result from grazing of the urchin Centrostephanus tenuispinus, this study was focused on evaluating bio-erosion by C. tenuispinus with respect to size structure and seasonality. Monthly samples of urchins were collected during 2014–2016 and gut composition was analysed. Gut evacuation rates were calculated using urchins dissected at time intervals up to 96 h. Reworked CaCO3 was calculated using caged urchins in a nearby seagrass bed. Mean percentages of organic, CaCO3, and siliceous components in C. tenuispinus gut contents were 86.3 ± 3.2, 10.3 ± 2.8, and 3.4 ± 1.5%, respectively. Gut evacuation rates for autumn, winter, spring, and summer were 0.70, 0.24, 0.48, and 0.72 day −1. Bio-erosion rates were significantly higher in summer (3.5 g CaCO3 m−2 day−1) than in winter (1.3 g CaCO3 m−2 day−1) and higher rates recorded for large urchins. Urchin bio-erosion was 1 kg CaCO3 m−2 annum−1. Variation in food ingestion rates in response to seawater temperature changes was found to be the main driver for differences in seasonal bio-erosion rates, which likely contribute to the absence of macroalgae and the maintenance of high coral cover on Hall Bank Reef. This study provides baseline data on bio-erosion by a sea urchin at Hall Bank Reef, which will be essential in monitoring and managing reefs in this region, especially under current trends in climate change

Seasonal and diel variation in movement rhythms of sand dollar, Peronella lesueuri (Valenciennes 1841), in Cockburn Sound, Western Australia

Rates and direction of movement in the sand dollar Peronella lesueuri were measured in summer and winter in Cockburn Sound, a large coastal embayment in south-western Australia. P. lesueuri was found to have a diurnal activity pattern throughout the year and had a greater movement rate in the summer (mean of 5.3 cm h -1, day; 3.9 cm h -1, night) than in the winter (mean of 2.7 cm h -1, day; 2.0 cm h -1, night). Seasonal change in temperature and physiological requirements by the sand dollar are the most likely reason for the seasonal differences. Reasons for diurnal variation were not clear. Direction of movement was found to be random at both times of the year. Based on these movement rates, one sand dollar can bioturbate an approximate area of 0.1 m 2 day -1 and 36.4 m 2 year -1. At a conservative density estimate of 0.5 sand dollars per m 2 it takes approximately 20 days for the sand dollars to rework the entire area of the sediments in the habitats they occupy