Abstract
Stingrays are an important part of the biomass of fishes in shallow, coastal ecosystems, particularly in inter-reefal areas. In these habitats they are thought to be keystone species, responsible for modifying physical and biological habitats through their foraging and predation. Here, we quantify the effects of bioturbation by rays on sand flats of Ningaloo Reef lagoon in Western Australia. At Mangrove Bay we measured 108 pits (length, breadth, depth) on a daily basis for a week. Additionally, an area of 1,000m² of the lagoon at Coral Bay was mapped three times over 18 months, in order to record patterns of ray and pit presence. Over 21 days at Mangrove Bay a total of 1.08m³ of sediment was excavated by rays, equating to a wet weight of 760.8kg, and 2.42% of the total area sampled, or 0.03% of the whole intertidal zone. Based on these calculations we estimate that up to 42% of the soft sediments in our study area would be reworked by stingrays to an average depth of 5.6cm over a year. On average, ray pits persisted in the environment for 4-8 days before being in-filled. An analysis of the change in volume of the pits over time showed high variability in the relationship between pits and years such that only 48% of pits had a negative relationship. Rays play an important ecological role creating sheltered habitats for other taxa in addition to the turnover of sediments and change in the sediment-water interface.