Coral reef habitats are changing due to both natural and anthropogenic influences operating at different scales. To better understand and differentiate between these processes, it is important to reconstruct natural variability in reef communities. Here, we examine late Holocene sediments (0–4 kyrs BP) along three vibracores from marginal and central locations, and adjacent to a patch reef, in the lagoon of Rasdhoo Atoll, Maldives. Sediment analysis included quantification of corals, echinoids, and sponges, as well as changes in texture. In addition, growth forms and degree of encrustation of coral fragments were documented. The aim of this study is to identify fluctuations in the proportion of organism remains and sediment composition and possible patterns over time in response to environmental changes. The sediment compositions in the three cores are substantially different. Fragments of reef corals (including Acropora spp., Seriatopora hystrix) were found preferentially in two cores adjacent to reef structures and channels through the atoll rim, suggesting sediment redeposition due to enhanced water circulation compared to the deeper lagoon location. Fragments of echinoids belonging to Echinometra sp., Diadema sp. and Cidaridae, mainly decreased in abundance over time. Different trends in organism proportion in the three cores can be attributed to local changes in ecological interactions among organisms, lagoonal circulation influenced by patch-reef growth and sand-spit development, and regional sedimentological events including tsunamis. Global changes in temperature, El-Niño frequency, and sea level fluctuations did not leave clear imprints in the lagoonal record. This study highlights that organism remains from atoll lagoons have great potential as archives of local to regional ecological and environmental change, as they store information on faunal composition, long-term geomorphological change, and short-term events and should therefore receive more attention in future investigations.

Intertidal burrowing crabs produce distinctive biogenic sedimentary structures on tidal flats and salt marshes of the mesotidal Mira River estuary in southwestern Portugal. This study shows that the morphology and spatial distribution of crab burrows vary depending on the activity of the producer, its age, tidal cycles, tidal range, time of the day, and substrate consistency. The Atlantic shore crab Carcinus maenas was observed during the daylight hours roaming the tidal flats and marshes at neap low tide (1.29 m) as well as occupying burrows and cavities. A variety of traces and tracks are attributed to C. maenas, including cheliped scratch marks, tracks and trackways, burrows/cavities with associated root penetration structures (rhizoliths), and fecal pellets. Scratch marks are understudied in neo- and paleoichnology and are often interpreted as structures resulting from crab grazing and feeding activity (fodichnia/pascichnia), and occasionally from mating behavior. Digit traces and trackways predominantly concentrate around burrows and cavities on the upper tidal flat and resemble Coenobichnus currani, representing crab locomotion (repichnia). With morphology regarded as the most important criteria for ichnotaxonomy, wide burrow apertures and cavities found on the tidal flats are most similar to Psilonichnus upsilon, P. tubiformis, and Spongeliomorpha-type crustacean burrows, representing permanent- to semi-permanent crab domichnia. Finally, cylindrical, semi-consolidated pellets are interpreted as crab fecal pellets (coprolites) similar to Favreina. Carcinus maenas is a prolific bioturbator in intertidal and supratidal habitats; however, the preservation of its traces in the rock record is dependent on the preservation potential of the host sediments.