The salt marsh fauna includes a broad taxonomic spectrum of invertebrates, fishes, birds, and mammals, but few amphibians and reptiles. Resident fauna are adapted to the land-sea interface, while transient users benefit from the foraging, nursery, and reproductive support functions.
Salt marsh animals cope with inundation regimes that differ seasonally, monthly, daily, and hourly. Vertebrates accomplish this largely through mobility. For example, fishes exploit marsh surface foraging opportunities during high tides and then retreat to subtidal waters. Birds time their use to take advantage of either low or high tide. Residents, such as the light-footed clapper rail (Rallus longirostris levipes), nest during the minimum tidal amplitude. Migrants, such as curlews, move upslope at a high tide and feed during low tide during their seasonal visits. Many invertebrates move away from adverse conditions. Some beetles climb tall plants to escape rising tides. A springtail, Anurida maritime, has a circatidal rhythm of 12.4 h that enables it to emerge for feeding shortly after tides ebb and retreat underground prior to the next inundation. For less-mobile fauna, physiological adaptations are essential. Gastropods avoid desiccation during low tides by sealing their shells. Some arthropods avert drowning by trapping air bubbles in their epidermal hairs during high tides.
Another challenge is fluctuating salinities, which salt marsh residents handle with exceptional osmoregulatory ability. The southern California intertidal crab species Hemigrapsus oregonensis and Pachygrapsus crassipes are able to hypo- and hyperosmoregulate when exposed to salt concentrations ranging from 50% to 150% of seawater (brackish to hypersaline). Tidal marsh fishes also have wide salinity tolerances. Cyprinodontiform tidal marsh fishes can tolerate salinities as high as 80-90 ppt. One species, Fundulus majalis, hatched at salinities up to 72-73 ppt. Lower salinity limits for mussels can be as low as 3gl- and they can tolerate high salinities as well, with mussels able to tolerate losing up to 38% of their water content. Even birds have adaptations for dealing with salt water and saline foods; for example, the Savannah sparrow (Passerculus sandwichensis beldingi) has specialized glands that excrete salt through the nares.
Because salt marshes have continuously changing hydrology, small differences in elevation and topography (e.g., shallow, low order tidal creeks) influence foraging activities of fishes and birds by regulating inundation and exposure times, enhancing marsh access for fishes, and increasing edge habitat. Ephemeral pools of just centimeters in depth provide valuable bird habitat, enhance macroinvertebrate abundance and diversity, and support reproductive, nursery, and feeding support functions for fishes.
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