Sargassum mats are a safety stop treat
Poppy Julianna Nowicka discovered many delights as a dive guide on the Caribbean island of Grand Cayman - none more surprising than the wonders of a seaweed mat
There is a special treat waiting for you on safety stops in Grand Cayman. Look up for a mat of tangled sargassum seaweed floating on the surface, because under, inside it and clambering all over it is a host of marine life.
Depending on the wind and wave conditions, you can often find large mats of sargassum seaweed, which originates in the Sargasso Sea in the middle of the Atlantic, sitting on top of the water. The leaves of the sargassum act like Velcro sticking it together in great mats, which then float on the surface due to the gas filled bladders in the plant, forming an ecosystem for marine life.
An array of species such as sea turtles, crabs, eels, shrimp and fish can be found using it as a base to feed, hide and spawn in. And you never know what larger predators may be basking in the shade underneath!
For some, it is their only home. For others, it is a great place to hang when they are growing up. And some, just regularly visit to try their luck.
One specialist resident is the sargassum frogfish (Histrio histrio). Usually, pale yellow to brown in colour, it grows from the size of a fingernail to the size of your hand. But it is tricky to spot as this master of camouflage relies on being able to mimic its host while unsuspecting prey venture too near. The sneak predator can also be a ruthless cannibal. One individual was dissected and found to have 16 juveniles in its stomach.
Another habitat specific species often seen in Grand Cayman is the sargassum crab (Portunus sayi). It has swimmerets on its back legs, allowing it to propel itself through the water and survive in the sargassum on the surface. It is similar in appearance to other swimming crabs, but is green and brown in colour to blend in with the sargassum.
The sargassum triggerfish, bright blue in colour, is commonly seen on the reef during dives in Grand Cayman. However, it is almost unrecognisable from its juvenile state. Before this fish matures and moves to the reef it spends the majority of its life in sargassum where its brown and green colours allow it to be perfectly camouflaged. Spotting this triggerfish on dives makes you realise just how well adapted this species is to its surroundings in both its juvenile and adult states.
Filefish in various stages of their lives head for the sargassum. Particularly common is the planehead filefish (Stephanolepis hispidus), but lots of other relatives hide in the weeds.
The slender filefish (Monacanthus tuckeri) is one of the most commonly seen juvenile fish living in the sargassum until it gets bigger and moves to the reef. It is slimmer and darker in colour than the planehead and divers will often see it on the reef in the later stages of its life. Once it moves to the reef, it protects itself from predators by lurking in he shadow of fire coral and gorgonians. They have even been found hiding in between the dorsal fins of oblivious lionfish!
The mats of weed tend to be infested with amphipods (or skeleton shrimp) providing a food source for all the other organisms nearby.
It isn’t just macro life that you will be spoilt with - the mats provide cover and shade for larger and predatory fish which can usually be seen lurking underneath. Species include pompano, pilot fish, juvenile permits, goggle eye, horse eye jacks, crevalle jacks, almaco jacks, amber jacks, lionfish, barracuda and occasionally even mahi mahi a little further offshore. These fish use the sargassum floating on the surface as a source of shade and shelter, and also as a source of food, preying on the juvenile fish living there. You can occasionally see remoras waiting underneath the sargassum for something bigger to come along that they can hitch a ride on, such as turtles or sharks.
Green sea turtles and hawksbill turtles, which can be seen on most dives in Grand Cayman, will often spend time underneath the sargassum, grazing on it and using the shelter it affords and juvenile turtles use it as a refuge. Jellyfish get tangled in the sargassum, providing an ideal snack for loggerheads turtles.
All this marine life attracts its fair share of predatory birds. Gulls and petrels land on the floating sargassum mats and pick off any crabs and fish that they can reach. Pelicans can sometimes be observed diving down to get the fish hiding under the seaweed.
And the perfect partner for your safety stop would be one of the large barracuda that like to lurk in the shade ready to ambush the unwary. Beats staring into the blue any day - so search out a sargassum tangle and end your dive in style.