Bioerosion along a bathymetric gradient in a cold-temperate setting (Kosterfjord, SW Sweden): an experimental study

In the cold-temperate setting of the Swedish Kosterfjord, a 2-year experiment was launched in order to assess bioerosion rates and to investigate the endolithic borer communities in relation to light availability (relative bathymetry), hydrography and exposure time. The inventory of microendolithic traces, studied by SEM analysis of epoxy resin casts of planted bivalve shells, yields diverse ichnocoenoses comprising a total of 21 traces produced by boring cyanobacteria (7), chlorophytes (4), fungi (6) and traces of uncertain affinity (4). The link between the endoliths (biotaxa) and the traces they leave (ichnotaxa) is evaluated by the study of the boring organisms in situ by transmission light microscopy of planted Iceland spar and bivalve shells. Additionally, the activity of various macroborers (foraminiferans, polychaetes, echinoids, gastropods and sponges) is documented, adding to a distinct diversity maximum at 7 m water depth. A highly condensed photic zonation, due to the high latitude (59°) and eutrophic conditions, is recorded by the measurement of the Photosynthetically Active Radiation (PAR) and is confirmed by the bathymetric range of the photic related ichnocoenoses. At 1 m water depth, a mature shallow euphotic ichnocoenosis dominated by cyanobacteria and at 7 m, a deep euphotic ichnocoenosis dominated by chlorophytes, respectively, is developed after as little as 12 months exposure. With the vanishing light availability from 15 m downwards, the ichnocoenoses development is significantly slowed and only immature dysphotic and aphotic borer communities (dominated by fungi) are encountered. Strong fluctuations of salinity (down to 8%) and temperature (0–20°C) in the euphotic zone indicate most phototrophs present to be considerably euryhaline and eurytherm, while most endolithic fungi appear preferentially in the deeper, more stable marine waters.     

The Complex Fungal Microboring Trace Saccomorpha stereodiktyon isp. nov. Reveals Growth Strategy of its Maker

A complex microboring trace of fungal affinity is described in shells as a new ichnotaxon Saccomorpha stereodiktyon isp. nov. and compared with the earlier established ichnotaxon Saccomorpha terminalis Radtke, 1991. The new trace is characterized by a three-dimensional network of tunnels composed of a bifurcate horizontal (parallel to substrate surface) network with an upright (perpendicular to surface) system of tunnels and by the formation of cylindrical to multilobate terminal sporangial swellings. The new trace shares with Saccomorpha terminalis Radtke, 1991 the terminal position of sporangial swellings but differs from this ichnotaxon by its complexity in spatial arrangement, segmented construction, and ramification of tunnels. The horizontal parts of the network in the new taxon adhere to the substrate surface and regularly produce thinner tunnels that explore the interior of the substrate, allowing the producer to participate in digestion of organic lamellae incorporated in the shell. Microborings similar to the new trace fossil have been observed in modern bivalve shells of the Atlantic Ocean, North Sea, Adriatic Sea and Red Sea at depths ranging from the intertidal down to 1,550 m. The fossil record of the trace reaches back to the Jurassic and the type material stems from a Lower Oligocene oyster shell. The study shows that complex microboring traces reflect both behaviour and developmental strategy of their makers.     

Bathymetric zonation of modern microborers in dead coral substrates from New Caledonia—Implications for paleodepth reconstructions in Holocene corals

Microboring endoliths were studied in dead modern corals collected at various depths and in growth position on La Surprise atoll in New Caledonia. Eight taxa of microborers were regularly observed: three cyanobacteria (Hyella caespitosa, Mastigocoleus testarum, and Plectonema terebrans), two chlorophytes (Ostreobium quekettii and Phaeophila dendroides) and three fungal forms (Dodgella priscus, and two unidentified fungi). The relative abundance of each taxon was determined in the modern corals throughout the studied sampling interval (from 0 down to 40 m water depth). This allowed us to propose for the first time a bathymetric model based on microendolith assemblages colonizing in situ hard-substrates of corals. In this model, the occurrence of the cyanobacteria Hyella and Mastigocoleus indicates a bathymetric range from 0 down to a water depth of 3 and 6 m respectively. The chlorophyta O. quekettii, associated with the cyanobacterium P. terebrans and fungi, is dominant from 10 to 40 m depth, which is the lower limit of our sampling. The distribution ranges of individual taxa observed in the dead coral substrates from New Caledonia are much narrower than those reported in the literature for other substrates.The bathymetric model based on distribution of microborers in modern corals was then applied to interpret the trace assemblages of microborers observed in Holocene corals collected by drilling on two reef sites of New Caledonia : La Surprise islet, in the vicinity of the sampling sites of the modern corals, and the barrier reef of Bayes, on the NE coast of Grande Terre. The paleodepth interpretations performed at various levels along the cores allow identification of several shallowing-upward sequences during the last 7000 years of reef growth in New Caledonia. Differences in the thickness of these sequences between La Surprise and Bayes sites were interpreted as differences in the space available for accommodation between the two sites, in turn related to the differential subsidence that affected New Caledonia during Holocene times. The use of the microendolith trace assemblages in paleodepth reconstructions of Holocene reefs from New Caledonia allowed detection of relative sea level variations finer than those deduced from changes in coralgal assemblages.     

The distribution of microborings in molluscan shells from recent reef environments at Lee Stocking Island,Bahamas

Summary The distribution of microbial borings in bivalve shells was assessed between five research sites in windward and leeward environments at the Lee Stocking Island, Bahamas. The research sites are on windward coral reefs (sites B at 2 m, Fat 12 m, and C at 30 m), a tidal channel stromatolite reef (site A at 5 m), and a leeward hard ground reef (site D at 3 m). A total of 22 ichnotaxa have been recognized within 100 samples. Each site contained between 14 and 17 ichnotaxa. Although the diversity of microborings, expressed both as number of taxa and number of individuals per taxon, is similar between sites, the following ranking (in descending order) was found: B-F-A-C-D.Rhopalia catenata was found to dominate at site D, whileReticulina elegans dominated at site C, andFasciculus dactylus dominated at site F. The results are in accordance with the known bathymetric distribution of the boring microorganisms.