No abstract

Stingrays are an important part of the biomass of the fishes in shallow coastal ecosystems, particularly in inter-reefal areas. In these habitats, they are considered keystone species -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. We measured the daily length, breadth and depth of 108 feeding pits over three 7-day periods, created by stingrays (Pastinachus atrus, Himantura spp. Taeniura lymma and Urogymnus asperrimus) in Mangrove Bay. Additionally, an area of ~1 km 2 of the lagoon at Coral Bay was mapped three times over 18 months, to record patterns of ray and pit presence. Over 21 days at Mangrove Bay, a total of 1.08 m 3 of sediment was excavated by rays, equating to a sediment wet weight of 760.8 kg, and 2.42% of the total area sampled, or 0.03% of the whole intertidal zone. We estimate that up to 42% of the soft sediments in our study area would be reworked by stingrays each year. Based on a model predicting the probability of pit presence over time, there was a 40% probability of ray pits persisting for 4 days before being filled in but only a 15% probability of a pit being present after 7 days. Changes in pit volume over time were static, providing evidence for secondary use. Our results imply that rays play an important ecological role creating sheltered habitats for other taxa in addition to the turnover of sediments.

Although the movements of fishes on coral reefs have been well studied, there are few data on the movement of elasmobranchs on and around cleaning stations. The visitation to cleaning stations by elasmobranchs was documented by direct observation and remote video capture at an oceanic reef in the Coral Sea and the outer Great Barrier Reef at time scales of hours to weeks. Cleaning was only observed at Osprey Reef and duration of occupancy was recorded for all elasmobranch clients. Strong tidal patterns were detected, with 49% of sharks and 59% of mantas engaging in cleaning interactions on ebb tides. Forty-four per cent of non-cleaned sharks were also observed on ebb tides. Some manta rays (n = 19) were individually identified through ventral skin pigmentation to determine site fidelity; three were seen more than once with repeat observations occurring within days. This was consistent among weeks and days within weeks, regardless of time of day. Hypotheses for tidal behaviour are discussed and we argue that these observations are critical in elucidating previously unknown behaviours in elasmobranch ecology. Our study indicates that observations of large elasmobranchs at cleaning stations are another tool to elucidate elasmobranch ecology.

DNA barcoding potentially offers scientists who are not expert taxonomists a powerful tool to support the accuracy of field studies involving taxa that are diverse and difficult to identify. The taxonomy of rays has received reasonable attention in Australia, although the fauna in remote locations such as Ningaloo Reef, Western Australia is poorly studied and the identification of some species in the field is problematic. Here, we report an application of DNA-barcoding to the identification of 16 species (from 10 genera) of tropical rays as part of an ecological study. Analysis of the dataset combined across all samples grouped sequences into clearly defined operational taxonomic units, with two conspicuous exceptions: the Neotrygon kuhlii species complex and the Aetobatus species complex. In the field, the group that presented the most difficulties for identification was the spotted whiptail rays, referred to as the ‘uarnak’ complex. Two sets of problems limited the successful application of DNA barcoding: (1) the presence of cryptic species, species complexes with unresolved taxonomic status and intra-specific geographical variation, and (2) insufficient numbers of entries in online databases that have been verified taxonomically, and the presence of lodged sequences in databases with inconsistent names. Nevertheless, we demonstrate the potential of the DNA barcoding approach to confirm field identifications and to highlight species complexes where taxonomic uncertainty might confound ecological data.