The marine ecosystem is sustained by the roles and niches of different organisms playing key
roles. While the infamous sharks have become rulers of the open ocean,
maintaining balance and adapting well to a pelagic lifestyle,
another group of cartilaginous fish have conquered the seabed in their place. The skates
and rays, making up the superorder Batoidea. A name with origins in Greek, meaning ‘a flat fish’.
With their distinct shape, skates and rays have become highly efficient predators of
life beneath the sediment. Let’s take a closer look at these relatives of the mighty sharks.
The general body plan of rays is very similar to that seen in sharks, with the most obvious
difference being that they are flattened. A shape that results from wide pectoral fins joined to
the head, forming the broad wings they use for swimming. In contrast to sharks, the tail is long
and whip-like. It’s used more like a rudder for balance and steering than for propulsion.
The mouth, nostrils and gill slits are on the underside, while spiracles behind the eyes are
used to draw in water. An adaptation that prevents the gills from getting clogged up with sediment.
The skin of rays is very similar to that of sharks. The sandpaper-like texture is caused
by tiny teeth-like structures called placoid scales or dermal denticles.
These point towards the tail and reduce friction when the rays move,
making them very good swimmers. As a result, rays are capable of hunting fish
in the midwater zone as well as searching the seabed for benthic prey.
Different species make use of different strategies. Bottom-dwelling species such as skates
are often ambush predators, burying themselves in the sediment and lying motionless before
snapping up passing creatures. This behaviour also protects the rays from being spotted by predators.
In contrast, eagle rays and cow nose rays gather in graceful schools. To hunt,
they dig invertebrates from the sediment using their prominent snouts or by beating their wings.
Manta rays are truly pelagic,
spending their lives in the open ocean and feeding in a very different way.
Mantas are the largest species of ray, with wingspans of up to 7 metres (23 feet).
But in a similar fashion to the largest shark, the whale shark, they feed on the smallest of animals.
They are filter feeders, filtering plankton from the water.
Their morphology is remarkably well suited for this way of life, for their mouths are located at
the front of the head rather than underneath. Two long lobes called cephalic horns flank the mouth
on both sides, and funnel planktonic prey into it while the ray moves along slowly.
Often, mantas have been seen swimming in circles together. A technique that concentrates shrimp
and plankton into a small area before the rays lunge through the centre with mouths agape.
Sawfishes are specialised hunters, using their deadly saws to slice through shoals of fish,
and picking them off one by one. They also use it to dig for shellfish in the sea floor mud.
Through their role as large, bottom-dwelling predators, sawfish and other rays play important
roles in the ecology of seabed ecosystems. Most notably, their feeding methods stir
up the sediment and provide access to buried invertebrates for other predators.
This causes bioturbation - restructuring the sediment in a way that allows other
species to flourish. Small fish often trail eagle rays to pick at the unearthed prey.
Rays are also an important source of prey for larger fish,
including hammerheads. A shark species that has adapted an incredible method of detecting
and digging up buried stingrays, using sensitive electrical receptors to pick up signs of movement.
Occasionally, rays gather in mighty swarms, like these devil rays, migrating in their thousands.
Like mantas, they are filter feeders,
trawling for tiny shrimps using their mouths as a net while gliding through the open ocean.
And every now and then, they even take to the skies with acrobatic grace.
It is believed that they do this to corral prey towards the centre of the shoal.
However, they seem to be in direct competition, trying to outdo the jumps of other rays.
It also seems to be mainly males that breach above the waves in this way. Perhaps it’s to attract a
partner, with the loud noises made by re-entering the water sending a signal out to the females.
A sign of their prowess as a mate. This behaviour demonstrates an example of sexual selection,
for those who make the biggest jump have higher chances of getting to reproduce.
Surprisingly, devil rays do not only visit the skies above the waves, but also frequent the
deep sea, being among the deepest-diving animals in the ocean. They have been found to dive as
deep as 2,000 metres (6,560 feet), descending at the incredible speed of 6 metres per second.
The Deepsea skate is found even deeper, at close to 3000 metres down in the pitch black
midnight zone. As denizens of the deep, they make good use of the unique habitats found here.
The abyssal plain is their feeding ground - littered with demersal creatures that act as prey.
At towering hydrothermal vents - fissures in the Earth's surface
that spew forth superheated water – these skates arrive to lay their eggs.
Ochre-coloured sacs that litter the sea floor around the vents in great abundance,
for the skates make use of the structures as incubators to speed up the hatching process.
A method that cuts the usual 4-year development time in half,
and represents the first time this strategy has been documented in any marine animal.
The ghost sharks, or chimaeras, are also residents of the depths,
and are among the closest living relatives of rays.
In fact, they appear as if they are a hybrid of rays and sharks, with an elongated body like
that of a shark, and the broad wing-like fins that are characteristic of rays.
Overall, skates and rays fulfil similar ecological niches to sharks.
Their unique flattened morphology, and adaptations to a life on the bottom
is what allows them to dominate the sea floor as top predators. They help uphold
the abundant biodiversity of sea-floor sediment communities by forming predator-prey interactions,
and causing bioturbation via burrowing in the sediment.
Their importance, much like that of sharks, cannot be understated.
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