Tip:
Highlight text to annotate it
X
Another visitor to Black Rock and other deep-water sites is the spotted eagle ray.
In some parts of the world eagle rays stay together in schools.
In the Andaman Sea they are normally found alone.
Just behind the short dorsal fin they have up to 6 venomous tail spines
which can inflict serious damage on attackers.
Between its wing-like fins it has a solid, heavy body and a deep head.
The smoothtail mobula is a similar size to the eagle ray
but can only usually be seen by divers in the northern Andaman
at sites like Burma's Tower Rock,
although they are occasionally seen in Thailand
at sites such as Racha Noi.
The mobula is a member of a group termed "devil rays",
so named because of 2 protruding cephalic fins either side of it's mouth.
These fins help to direct plankton and small marine creatures
into the mouth for feeding.
The mobula is a highly social fish
and is often observed in large schools.
The mobula is an impressive sight
but shares our seas with a much larger devil ray:
the king of all rays, the giant manta ray.
Mantas are frequent visitors to sites such as Koh Bon.
Giant mantas can grow to a width of over 6 meters
and a weight of over 2 tonnes.
These pelagic fish are always on the move
and like mobulas, they feed by swimming open-mouthed
and using the 2 cephalic fins to direct water into the mouth.
The gills on its white underside contain rakers
which filter out plankton and small organisms.
The markings on the back and underside of mantas are highly variable
and are useful in distinguishing individuals.
Occasionally the cephalic fins are furled up into cylinders to improve streamlining.
Mantas are one of the most intelligent fish,
with the largest brain-to-body mass ratio of all elasmobranchs.
They often seem to enjoy interaction with humans.
A gentle approach by divers is often permitted,
and mantas will sometimes approach divers,
apparently out of curiosity.
Mantas only give birth to an average of two pups every two years,
and populations have long been in decline.
The gill rakers of mantas and mobulas are used in a Chinese medicine
that is thought to detoxify the blood.
There is no scientific evidence that it works.
Nevertheless the lucrative trade is on the increase.
In November 2011
the International Union for Conservation of Nature
declared giant manta rays as "vulnerable with an elevated risk of extinction".