Disturbance of primary producer communities disrupts the thermal limits of the associated aquatic fauna.

Environmental fluctuation forms a framework of variability within which species have evolved. Environmental fluctuation includes predictability, such as diel cycles of aquatic oxygen fluctuation driven by primary producers. Oxygen availability and fluctuation shape the physiological responses of aquatic animals to warming, so that, in theory, oxygen fluctuation could influence their thermal ecology. We describe annual oxygen variability in agricultural drainage channels and show that disruption of oxygen fluctuation through dredging of plants reduces the thermal tolerance of freshwater animals. We compared the temperature responses of snails, amphipods, leeches and mussels exposed to either natural oxygen fluctuation or constant oxygen in situ under different acclimation periods. Oxygen saturation in channel water ranged from c. 0 % saturation at night to >300 % during the day. Temperature showed normal seasonal variation and was almost synchronous with daily oxygen fluctuation. A dredging event in 2020 dramatically reduced dissolved oxygen variability and the correlation between oxygen and temperature was lost. The tolerance of invertebrates to thermal stress was significantly lower when natural fluctuation in oxygen availability was reduced and decoupled from temperature. This highlights the importance of natural cycles of variability and the need to include finer scale effects, including indirect biological effects, in modelling the ecosystem-level consequences of climate change. Furthermore, restoration and management of primary producers in aquatic habitats could be important to improve the thermal protection of aquatic invertebrates and their resistance to environmental variation imposed by climate change.

Diel oxygen fluctuation drives the thermal response and metabolic performance of coastal marine ectotherms.

Coastal marine systems are characterized by high levels of primary production that result in diel oxygen fluctuations from undersaturation to supersaturation. Constant normoxia, or 100% oxygen saturation, is therefore rare. Since the thermal sensitivity of invertebrates is directly linked to oxygen availability, we hypothesized that (i) the metabolic response of coastal marine invertebrates would be more sensitive to thermal stress when exposed to oxygen supersaturation rather than 100% oxygen saturation and (ii) natural diel fluctuation in oxygen availability rather than constant 100% oxygen saturation is a main driver of the thermal response. We tested the effects of oxygen regime on the metabolic rate, and haemocyanin and lactate levels, of velvet crabs (Necora puber) and blue mussels (Mytilus edulis), under rising temperatures (up to 24°C) in the laboratory. Oxygen supersaturation and photosynthetically induced diel oxygen fluctuation amplified animal metabolic thermal response significantly in both species, demonstrating that the natural variability of oxygen in coastal environments can provide considerable physiological benefits under ocean warming. Our study highlights the significance of integrating ecologically relevant oxygen variability into experimental assessments of animal physiology and thermal response, and predictions of metabolic performance under climate warming. Given the escalating intensity and frequency of climate anomalies, oxygen variation caused by coastal vegetation will likely become increasingly important in mitigating the effects of higher temperatures on coastal fauna.

Extreme philopatry and genetic diversification at unprecedented scales in a seabird.

Effective conservation requires maintenance of the processes underlying species divergence, as well as understanding species' responses to episodic disturbances and long-term change. We explored genetic population structure at a previously unrecognized spatial scale in seabirds, focusing on fine-scale isolation between colonies, and identified two distinct genetic clusters of Barau's Petrels (Pterodroma baraui) on Réunion Island (Indian Ocean) corresponding to the sampled breeding colonies separated by 5 km. This unexpected result was supported by long-term banding and was clearly linked to the species' extreme philopatric tendencies, emphasizing the importance of philopatry as an intrinsic barrier to gene flow. This implies that loss of a single colony could result in the loss of genetic variation, impairing the species' ability to adapt to threats in the long term. We anticipate that these findings will have a pivotal influence on seabird research and population management, focusing attention below the species level of taxonomic organization.

Enemies with benefits: parasitic endoliths protect mussels against heat stress.

Positive and negative aspects of species interactions can be context dependant and strongly affected by environmental conditions. We tested the hypothesis that, during periods of intense heat stress, parasitic phototrophic endoliths that fatally degrade mollusc shells can benefit their mussel hosts. Endolithic infestation significantly reduced body temperatures of sun-exposed mussels and, during unusually extreme heat stress, parasitised individuals suffered lower mortality rates than non-parasitised hosts. This beneficial effect was related to the white discolouration caused by the excavation activity of endoliths. Under climate warming, species relationships may be drastically realigned and conditional benefits of phototrophic endolithic parasites may become more important than the costs of infestation.

The combination of selection and dispersal helps explain genetic structure in intertidal mussels.

Understanding patterns of gene flow, selection and genetic diversity within and among populations is a critical element of predicting how long-term changes in environmental conditions are likely to affect species distribution. The intertidal mussel Perna perna consists of two distinct genetic lineages in South Africa, but the mechanisms maintaining these lineages remains obscure. We used regional oceanography and lineage-specific responses to environmental conditions as proxies for gene flow and local selection, respectively, to test how these mechanisms could shape population genetic structure. Laboratory experiments supported the field findings that mussels on the east coast (eastern lineage) are physiologically more tolerant of sand inundation and high temperatures than those on the south coast (western lineage). Temperature loggers mimicking mussel body temperatures revealed that mussels experience higher body temperatures during aerial exposure on the subtropical east coast than on the temperate south coast. Translocations showed that, on the east coast, the western lineage suffered higher mortality rates than local individuals, while on the south coast, mortality rates did not differ significantly between the lineages. Nearshore drogues showed remarkably little overlap between the trajectories of drifters released off the south coast and those released off the east coast. Physiological tolerances can thus explain the exclusion of western individuals from the east coast, but they cannot explain the exclusion of the eastern lineage from the south coast. In contrast, however, ocean dynamics may limit larval dispersal between the two lineages, helping to explain the absence of eastern individuals from the south coast. We emphasise the importance of a multidisciplinary approach in a macro-ecological context to understand fully the mechanisms promoting evolutionary divergence between genetic entities. Our results suggest that phylogeographic patterns of Perna perna may be maintained by a combination of local conditions and the isolating effect of the Agulhas Current that reduces gene exchange.

Relevance of mytilid shell microtopographies for fouling defence--a global comparison.

Prevention of epibiosis is of vital importance for most aquatic organisms, which can have consequences for their ability to invade new areas. Surface microtopography of the shell periostracum has been shown to have antifouling properties for mytilid mussels, and the topography shows regional differences. This article examines whether an optimal shell design exists and evaluates the degree to which shell microstructure is matched with the properties of the local fouling community. Biomimics of four mytilid species from different regional provenances were exposed at eight different sites in both northern and southern hemispheres. Tendencies of the microtopography to both inhibit and facilitate fouling were detected after 3 and 6 weeks of immersion. However, on a global scale, all microtopographies failed to prevent fouling in a consistent manner when exposed to various fouling communities and when decoupled from other shell properties. It is therefore suggested that the recently discovered chemical anti-microfouling properties of the periostracum complement the anti-macrofouling defence offered by shell microtopography.

Scales of mussel bed complexity: structure, associated biota and recruitment.

Hierarchically scaled surveys were carried out on beds of the brown mussel Perna perna (Linnaeus) on the South coast of South Africa. The object was to assess spatial and temporal variations in the complexity of mussel beds and to investigate relationships between mussel bed complexity and mussel recruitment. Complexity was divided into three components: physical complexity; demographic complexity; associated biota. A series of variables within each component were recorded at two different scales (10 and 50 cm) within nested quadrats on three separate occasions. The nested ANOVA design explicitly incorporated spatial scale as levels of the ANOVA. These scales were: shores (areas 1 km in length separated by 25 km); transects (areas 20 m in length separated by 100s of meters); 50x50-cm quadrats separated by meters and 10x10-cm quadrats separated by cm) This approach was intended to generate hypotheses concerning direct associations between recruitment and complexity versus co-variation due external processes. Three main questions were addressed: (1) At what scale does each variable of complexity exhibit greatest significant variation? (2) At these scales is there similar ranking of variables of complexity and recruitment? (3) Within this/these scales, is there any significant relationship between the variables measured and mussel recruitment? On two occasions (Nov. 97 and Mar. 98) the majority of variables showed greatest significant variation at the transect-scale. On a third occasion (Oct. 97) most variables showed greatest significant variation at the quadrat-scale and the site-scale. On all occasions a markedly high percentage of the variation encountered also occurred at the smallest scale of the study, i.e., the residual scale of the ANOVA analyses. Some similarity in the ranking of variables occurred at the transect scale. Within the transect-scale, there was little indication of any relationship between variables of complexity and recruitment. Relationships were inconsistent either among transects or among sampling occasions. Overall, the results suggest that a high degree of variation in mussel bed complexity consistently occurs at very small scales. High components of variance generally also occur at one or more larger scales; however, these scales vary with season. Mussel recruitment does not appear to be directly affected by complexity of mussel beds. Instead it appears external factors may influence both complexity and recruitment independently. In addition recruitment may influence complexity rather than vice versa.

The influence of desiccation and predation on vertical size gradients in populations of the gastropod Oxystele variegata (Anton) on an exposed rocky shore.

Oxystele variegata (Anton.) exhibits a vertical size gradient contrary to the model proposed by Vermeij (1972) for low/mid intertidal species, as shell size increases in an upshore direction. Settlement occurs in the lowest zones and juveniles are restricted to the lower shore by conditions of desiccation higher up the beach. Juveniles suffer rapid water loss due to a relatively large opercular surface area and circumference and have a much lower resistance to water loss than adults. This leads to high mortality under conditions of low humidities, and juveniles caged at the top of the balanoid zone, where adults normally occur, die within a few days. As animals increase in size their resistance to desiccation rises allowing them to migrate upshore. This is a response to high rates of predation by the whelk Burnupena delalandii in the lower balanoid zone. Predation is so intense as to override the advantages of higher food availability which lead to a greater body weight for adults protected by cages on the lower shore.