Feeding ecology

Gut contents analysis (GCA) is a conventional method for examining the diet of a species. Direct examination of the gut contents can provide a “snapshot” of recent food intake for an animal. GCA in manta rays is only applicable to dead animals so has limited use for studying these large vulnerable marine animals.

Indirect methods of diet analysis to inform ecology and biology include biochemical approaches, such as stable isotope (SI) and signature fatty acid (FA) analyses. These methods are non-lethal and require only a small tissue sample to be taken from the animal. SI analysis provides information on the assimilated food intake of a consumer based on nitrogen and carbon values present in its tissues. FA analysis provides information about an animal’s trophic position, owing to the signature profile of the prey, influencing the profile of the consumer.

Satellite oceanography is the use of remote sensing environmental data to provide spatial and temporal representations of variables such as bathymetry, sea surface temperature and chlorophyll-a concentrations (a proxy for productivity). This technique can be combined with animal tracking and modelling to highlight areas of residency that are potentially important for foraging activity.  

Direct observation and sampling of the environment is another approach to analysing species diet. In manta ray species, the relationship between feeding dynamics, prey availability and prey density can be elucidated by measuring and quantifying plankton in the presence of feeding or non-feeding behavior. These can also be correlated with environmental parameters (such as tidal phase, water temperature and salinity) to examine drivers of zooplankton availability. Plankton work on manta rays is carried out in collaboration with the CSIRO Plankton Laboratory. 

Gut content analysis

Analysis of the contents of a preserved stomach of a reef manta ray from the Great Barrier Reef in 1935 revealed the presence of large calanoid copepods (62%), trypanorhynch cestodes (35%) and minor contributions from arrow worms, barnacle larva and a nematode. Manta rays appear to either preferentially ingest large copepods, or the filters used to extract prey from the water are selective for prey items over 0.8 mm in length. This research also revealed that the copepods originate from offshore open ocean areas, suggesting that either animals are feeding offshore of the oceanic copepods periodically move onto the reef near Whitsunday Island where the animal was collected.

Biochemical analyses

Measured SI values have confirmed that the reef manta ray is a secondary consumer – i.e. they eat the herbivorous zooplankton. Results indicated a flagellate-based food source in the diet, which likely reflects near-surface feeding on zooplankton. However, certain FA values in reef manta ray tissue suggest that they do not feed solely on pelagic zooplankton, but rather obtain part of their diet from another origin. The closest match was with demersal zooplankton, suggesting that manta rays primarily feed from the bottom of the ocean rather than the top.

Satellite oceanography and animal tracking

Satellite tagging of reef manta rays at Lady Elliot Island (LINK TO TAGGING PAGE) revealed that manta rays spent significant time in an offshore region characterised by the mesoscale cyclonic Capricorn Eddy. Mesoscale eddies are known hotspots for foraging activity in a variety of marine species, and analysis suggests this is an important foraging ground for reef manta rays off eastern Australia. Examining changes in sea surface temperature and chlorophyll-a concentrations revealed heighten productivity in this area, supporting the findings from the model (Jaine et al. 2014).

Zooplankton sampling, oceanography and direct observation

The foraging behaviour of reef manta rays at Lady Elliot Island was analysed in relation to zooplankton populations and local oceanography, and compared to long-term sighting records of reef manta rays from the dive operator on the island. Reef manta rays fed at Lady Elliot Island when zooplankton biomass and abundance were significantly higher than other times. The study established a critical prey density threshold that triggered reef manta ray feeding, and showed that zooplankton size and community composition had no significant effect on feeding activity. Higher zooplankton biomass and reef manta ray feeding activity were observed prior to low tide and when the water was cooler.

Understanding the energetic requirements of manta ray species is an important goal for improving our understanding of how they meet their energy requirements in tropical oceans. Establishing the metabolic rate of large, highly mobile marine animals remains a challenge, however, future research could examine energetic modelling and employ the use of acceleration data loggers to monitor the difference in energetic output between feeding and non-feeding activity. This method, combined with local sampling, would provide an insight as to whether feeding at that location was energetically profitable and/or sustainable, or if feeding was likely occurring elsewhere.