The decline of the European oyster Ostrea edulis across its biogeographic range has been driven largely by over-fishing and anthropogenic habitat destruction, often to the point of functional extinction. However, other negatively interacting factors attributing to this catastrophic decline include disease, invasive species and pollution. In addition, a relatively complex life history characterized by sporadic spawning renders O. edulis biologically vulnerable to overexploitation. As a viviparous species, successful reproduction in O. edulis populations is density dependent to a greater degree than broadcast spawning oviparous species such as the Pacific oyster Crassostrea (Magallana) gigas. Here, we report on the benthic assemblage of O. edulis and the invasive gastropod Crepidula fornicata across three actively managed South coast harbors in one of the few remaining O. edulis fisheries in the UK. Long-term data reveals that numbers of O. edulis sampled within Chichester Harbour have decreased by 96%, in contrast numbers of C. fornicata sampled have increased by 441% over a 19-year period. The recent survey data also recorded extremely low densities of O. edulis, and extremely high densities of C. fornicata, within Portsmouth and Langstone Harbours. The native oyster’s failure to recover, despite fishery closures, suggests competitive exclusion by C. fornicata is preventing recovery of O. edulis, which is thought to be due to a lack of habitat heterogeneity or suitable settlement substrate. Large scale population data reveals that mean O. edulis shell length and width has decreased significantly across all years and site groups from 2015 to 2017, with a narrowing demographic structure. An absence of juveniles and lack of multiple cohorts in the remaining population suggests that the limited fishing effort exceeds biological output and recruitment is poor. In the Langstone & Chichester 2017 sample 98% of the population is assigned to a single cohort (modal mean 71.20 ± 8.78 mm, maximum length). There is evidence of small scale (<5 km) geographic population structure between connected harbors; the 2015 Portsmouth and Chichester fishery populations exhibited disparity in the most frequent size class with 36% within 81–90 mm and 33.86% within 61–70 mm, respectively, the data also indicates a narrowing demographic over a short period of time. The prevalence of the disease Bonamiosis was monitored and supports this microgeographic population structure. Infection rates of O. edulis by Bonamia ostreae was 0% in Portsmouth Harbor (n = 48), 4.1% in Langstone (n = 145) and 21.3% in Chichester (n = 48) populations. These data collectively indicate that O. edulis is on the brink of an ecological collapse within the Solent harbors. Without effective intervention to mitigate the benthic dominance by C. fornicata in the form of biologically relevant fishery policy and the management of suitable recruitment substrate these native oyster populations could be lost.

The considerable quantities of dead wood in the intertidal zone of mature mangrove forests are tunnelled by bivalves of the family Teredinidae. When the surface of heavily tunnelled wood is broken open, cryptofauna are able to use tunnels as refuges. In this study, the exploitation of this niche during low tide by the dartfish Parioglossus interruptus was investigated. The majority of tunnels offer a close fit falling within the range of typical dartfish diameters. The fish found within wood tended to be smaller than fish found swimming between mangrove roots at high tide. Dartfish were found in tunnelled wood even where it was emersed for over 11 h d −1 , but favoured wood in the lower intertidal. Within the wood, daytime thermal maxima were reduced by 6.5°C compared with adjacent tidepools. Wind-tunnel observations indicated that this lowering could be due to evaporative cooling. However, dartfish were found to be notably tolerant of high temperatures, with a critical thermal maximum that exceeded temperatures reached in tunnelled wood and pools. Nonetheless, such tolerance may impose a metabolic cost that would be reduced by occupying tunnels. Teredinid tunnels are also likely to give dartfish protection from desiccation and predation. During high-tide, free-swimming dartfish were observed to favour areas of Rhizophora roots over open creeks. In aquaria, fish swam actively during the day, but took refuge in teredinid tunnels at night. Sampling of wood at low tide and direct observations at high tide indicate that a substantial proportion of the dartfish population takes refuge in wood during low tide. Thus, teredinid-tunnelled wood is a key low-tide refuge especially for younger fish, which would otherwise be exposed to predators. This study provides an example of a mechanism whereby mangrove forests support intertidal biodiversity.

Previous work has shown that the intertidal seagrass macrobenthos at three geographically and ecologically disparate localities (in the north-east Atlantic, south-west Indian and south-west Pacific Oceans) possess similar relative species occurrence distributions and uniform species densities. These common features are here demonstrated to be related to the presence in those assemblages of: (1) similar functional diversities and evennesses, (2) the same set of dominant component functional groups, and (3) similar ranked relative occurrence distributions both of those groups and of the component genera within each of the larger groups. The two lower-latitude systems were particularly similar in all these respects. Although sharing the same subset of individual functional groups, however, the relative importance of members of that subset varied from locality to locality and even within a single locality, whilst still maintaining the same ranked relative functional-group occurrence distribution. Therefore the broad structure of available macrobenthic functional roles and the relative occurrences of the component taxa in intertidal seagrass beds (and hence, granted stochastic assembly, the total numbers of taxa supported by unit area) are likely to be linked causally, although the form of the relationship is unclear.

Substantial amounts of dead wood in the intertidal zone of mature mangrove forests are tunnelled by teredinid bivalves. When the tunnels are exposed, animals are able to use tunnels as refuges. In this study, the effect of teredinid tunnelling upon mangrove forest faunal diversity was investigated. Mangrove forests exposed to long emersion times had fewer teredinid tunnels in wood and wood not containing teredinid tunnels had very few species and abundance of animals. However, with a greater cross-sectional percentage surface area of teredinid tunnels, the numbers of species and abundance of animals was significantly higher. Temperatures within teredinid-attacked wood were significantly cooler compared with air temperatures, and animal abundance was greater in wood with cooler temperatures. Animals inside the tunnels within the wood may avoid desiccation by escaping the higher temperatures. Animals co-existing in teredinid tunnelled wood ranged from animals found in terrestrial ecosystems including centipedes, crickets and spiders, and animals found in subtidal marine ecosystems such as fish, octopods and polychaetes. There was also evidence of breeding within teredinid-attacked wood, as many juvenile individuals were found, and they may also benefit from the cooler wood temperatures. Teredinid tunnelled wood is a key low-tide refuge for cryptic animals, which would otherwise be exposed to fishes and birds, and higher external temperatures. This study provides evidence that teredinids are ecosystem engineers and also provides an example of a mechanism whereby mangrove forests support intertidal biodiversity and nurseries through the wood-boring activity of teredinids.