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Biotic stress may operate in concert with physical environmental conditions to limit or facilitate invasion processes while altering competitive interactions between invaders and native species. Here, we examine how endolithic parasitism of an invasive and an indigenous mussel species acts in synergy with abiotic conditions of the habitat. Our results show that the invasive Mytilus galloprovincialis is more infested than the native Perna perna and this difference is probably due to the greater thickness of the protective outer-layer of the shell of the indigenous species. Higher abrasion due to waves on the open coast could account for dissimilarities in degree of infestation between bays and the more wave-exposed open coast. Also micro-scale variations of light affected the level of endolithic parasitism, which was more intense at non-shaded sites. The higher levels of endolithic parasitism in Mytilus mirrored greater mortality rates attributed to parasitism in this species. Condition index, attachment strength and shell strength of both species were negatively affected by the parasites suggesting an energy trade-off between the need to repair the damaged shell and the other physiological parameters. We suggest that, because it has a lower attachment strength and a thinner shell, the invasiveness of M. galloprovincialis will be limited at sun and wave exposed locations where endolithic activity, shell scouring and risk of dislodgement are high. These results underline the crucial role of physical environment in regulating biotic stress, and how these physical-biological interactions may explain site-to-site variability of competitive balances between invasive and indigenous species.

Different kinds of experimental calcareous substrates were exposed at Lee Stocking Island (Bahamas) and One Tree Island (Great Barrier Reef, Australia) to study which endolithic bacteria, algae and fungi contribute to bioerosion and what their bioerosion rates are. The sites at Lee Stocking Island were several leeward shallow water and several windward shallow and deep‐water positions (from the Acropora palmata reef at 2 m down to 275 m depth). At One Tree Island, the experiments were conducted in patch reefs treated with P and N to study the influence of mineral nutrients on bioerosion. The exposure periods ranged from 1 week to 2 years. The micritic carbonate substrates exposed on Lee Stocking Island contained 6 genera with 15 species of cyanobacteria, green and red algae, and different kinds of microendolithic heterotrophs. The mean values of bioerosion rates measured between 1 to 2 g/m 2 /y at 275 m and 520 g/m 2 /y at one of the leeward sites. The composition of the endolithic community and the bioerosion rates changed over time. At One Tree Island, shell pieces of Tridacna were used as substrate exposed for 5 months to endolith activity. Five genera and 6 species of cyanobacteria, green and red algae and different kinds of heterotrophic microendoliths were found with bioerosion rates of 20–30 g/m 2 /y. There are differences in abundance of taxa between Lee Stocking Island and One Tree Island. The introduction of nutrients had no apparent impact on the microborer community. Controlling factors for the distribution and abundance of microborers are mainly light, but also the kind of substrate and, possibly, the biogeographic position. The results support the paleoecological importance of microendoliths.

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This study examines microendolithic community patterns in experimental carbonate blocks in shallow waters between 0 m and 30 m adjacent to Eilat, Israel. We set up two different habitats per depth: one in full light and one shaded. After 6 months of exposure we observed 23 species of which five are unknown to science and herein described as forms. Differences in community patterns between open and shaded habitats were clearly visible at 0 m, indistinct between 6 m and 15 m and indiscernible at 30 m. Three modern producers of key ichnotaxa were confirmed in our experiments within their paleobathymetrical range: Hyella balani (Fascichnus acinosus), Conchocelis (Palaeoconchocelis starmachii), and Ostreobium quekettii (Ichnoreticulina elegans). For Fascichnus dactylus we found six possible producers. We dismiss Scolecia filosa, Eurygonum nodosum and Rhopalia catenata as potential key ichnotaxa because of the broad bathymetrical range of their producers.

Measurements on modern coral reefs at Lee Stocking Island(Bahamas) illustrate that boring cyanobacteria species make a major contribution to microboring bioerosion rates. Borings attributed to cyanobacteria also occur in fossil environments. Bioerosional studies on Permian and Triassic reefs show similar intensities to those observed on modern equivalents. The importance of borings assigned to cyanobacterial activity is even more apparent in paleobathymetry. Comparison of the bathymétrie ranges known from modern and fossil microborings demonstrates a preference of boring cyanobacteria for shallow marine environments. Furthermore, some traces are linked to distinct portions of the shallow euphotic zone. They significantly contribute to characterize typical microboring assemblages, which are used for paleodepth reconstructions. In contrast to these stenobathic species, one cyanobacterial species turned out to be eurybathic. It has been recorded as deep as the dysphotic zone but may even extend to the aphotic zone.