The devil is coming: Feeding behavior of juvenile Munk’s devil rays (Mobula munkiana) in very shallow waters of Punta Descartes, Costa Rica

Introduction: Identifying critical habitats for vulnerable elasmobranch species is crucial for effective conservation measures. The Munk’s devil ray (Mobula munkiana) is endemic to the Eastern Pacific, but yet little is known about its biology, ecology, and habitat use. As filter feeders, it is assumed that this species concentrates at high-productive upwelling regions, such as the Costa Rican Dome. Like many elasmobranchs, its populations are highly depleted and require urgent information to inform better conservation measures. Objective: The study was conducted to gain information on a unique behavior observed in juvenile M. munkiana, so further information can be provided on early life stages of this vulnerable species. Methods: From June to September 2017 and in August 2018, the feeding behavior of juvenile Mobula munkiana was observed in two shallow bays located at Punta Descartes, North Pacific Costa Rica. Individuals were captured using a non-lethal method to obtain data on size, weight, and sex distribution. Plankton samples (n = 100) were taken at both bays throughout the months to infer diet composition. Results: Munk’s devil rays showed a repetitive swimming movement parallel to the beach, feeding exclusively in the shallow breaking zone of the low tide waves at depth <50cm. A total of 12 M. munkiana (11 live and one found dead) indicated a juvenile feeding aggregation ranging from 490 – 610mm in disk width and 1400 – 2300gr in weight. The sex ratio (males to females) was 3:1. Zooplankton of the order Mysidacae was found in the highest abundance in the breaking zone. Conclusions: The specific behavior and seasonal occurrence of juvenile Munk’s devil rays in this area seem to be driven by prey abundance. More research is needed to conclude the presence of reproductive adults at deeper depths and the year-round habitat use of Punta Descartes. The area is threatened by unsustainable development and requires realistic management strategies to guarantee the survival of vulnerable species and their critical habitats.

A feeding aggregation of Omura‘s whale Balaenoptera omurai off Nosy Be, Mozambique Channel

A feeding aggregation of Omura’s whales Balaenoptera omurai was documented off Nosy Be Island at the northeastern entrance of Mozambique Channel in November 1994. Underwater photographs of live individuals illustrated sub-surface skimming as main feeding behaviour, with small crustaceans, small jellyfish, and other gelatinous micronecton identified as prey. A precise description of the whales’ pigmentation patterns completes previous descriptions from the recent literature.

The movement of fly (Diptera) larvae within a feeding aggregation

Dipteran larvae from a number of families feed in aggregations. Rotation of blow fly (Diptera: Calliphoridae) larvae within an aggregation has been reported anecdotally many times. However, there is a lack of quantitative data on such larval movement, which is necessary to better understand the advantage of this gregarious behaviour. A recent development in tagging methods provided an opportunity to address this gap in knowledge. In 15 aggregations of 500 Lucilia sericata (Meigen) (Diptera: Calliphoridae) larvae, the location of four-tagged individuals was recorded at 10-minute intervals. All larvae were seen to rotate, alternating between the periphery and within. There was much variation in the relative proportions that larvae were seen in these two locations among aggregations (χ2=78.4, df=58, P=0.038), perhaps as a result of differences in mass shape and, therefore, surface area: volume ratio. There were also differences between larvae within aggregations (χ2=25.6, df=14, P=0.029), which may give rise to differences in development rate, perhaps as a result of intraspecific competition. Further work would be required to verify this competition, and to establish whether the limited resource is temperature, food, oxygen, or some other requirement.