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Electroreception in the Blue Spotted Ribbontail Ray

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A Blue Spotted Ribbontail Ray (Taeniura lymma) resting under a reef overhang, waiting for the sun to set before beginning a hunt. During high tide, these rays are often seen to migrate inshore to sand flats where they feed on invertebrates and bottom dwelling fish that inhabit these shallower sandy areas.

Rays including the Blue Spotted Ribbontail are cartilaginous fish.  Similar to their close relatives the sharks and skates, these individuals lack any true bones. Skeletal structures made of cartilage are lighter in weight, flexible and allow for faster speed, acceleration and better turning abilities. Members of the Elasmobranchii sub-class, (sharks, skates and rays) benefit from increased performance in these areas, as the majority of species belonging to this class are higher trophic level predators that hunt fast prey.

Blue Spotted Ribbontail Rays are easily spotted during dives, as unlike many other stingrays, they do not bury themselves in the sand. A common feature that is often observed on resting individuals is the movement of their spiracles (respiratory openings) that are located just behind the eyes. The opening and closing of these spiracles is used for ventilation. Breathing using spiracles is an alternative method to using the mouth for ventilation. Although less effective compared to breathing using the mouth (known as ‘buccal pumping’), this method is adopted by several Elasmobranchs. In particular, predators of this sub-class that feed in sandy areas including the Blue Spotted Ribbontail ray significantly benefit from spiracle breathing, as this allows them to keep the underside of their body extremely close to the sand when searching for food.

Like their shark relatives, Rays are able to tune into the Earth’s electromagnetic fields, and are able to detect electrical impulses that are emitted within a local field. Muscular contractions that occur in nearby marine organisms create small impulses that when detected, provide useful information such as the location and size of an individual. This highly specialised mechanism known as ‘electroreception’ can be used during communication, and is a fundamental component used during predation, especially when vision is compromised (e.g. at night or when prey hide under the sand).

Electrical signals are received using highly specialised sensory organs called Ampullae of Lorenzini. Tiny pores can often be seen on rays and sharks, most commonly found on their underside. Each of these pores connects to a canal that is full of highly conductive gel.  When impulses are emitted by an individual, these electrical stimuli arrive at the pores, and travel down the canal. The electrical charge then reaches the Ampullae - a collection of specialised cells connected to several nerves. Here, the electrical signal is transferred into a nerve impulse that then travels to the brain. Each nerve impulse generated from electro-sensory cues provides the ray with information on the location of the individual emitting the impulses. This significantly helps the ray in locating its prey items. Once found, the ray then uses its pectoral fins to direct the prey towards the mouth. Rays do not have any teeth, but instead use fused plates to crush their prey.

The ray’s prey tend to become more active at night time, as they are less likely to been seen by predators. As a result, rays also typically feed at night. In such a low light environment, the probability of locating prey using vision is significantly reduced, and electroreception is essential for successfully locating their prey items.

Rays are important predators of coral reef ecosystems yet several species face significant threats often associated with strong fishing pressures. The IUCN Red List of Threatened Species have classified the Blue Spotted Ribbontail Ray as a result of population effects from significant fishing of the species. Several are also captured for marine aquaria. Ongoing monitoring is essential to provide more information on population trends and threats to this species and is a fundamental element of the Roctopus marine conservation program.

 

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