Easy Riders: Cruising with the Currents

“Is there a current?” a diver asked me as our boat moored above a dive site. “Hold on, let me check,” I said. I leaned over the side of the boat and spat into the water. After staring at the spitball for a few seconds, watching it disintegrate in the water but not travel much, I said, “Nope! No current”.

If a wad of saliva, or any floating object for that matter, drifts away the moment it touches the water, it’s a good indication of the presence of a current. The speed at which you see your saliva leave your line of sight can tell you just how strong a current is. There may be times when you need to hold on to a rope line to stay in the same place, and other times when you feel like a piece of taught fabric on a flagpole holding on for dear life! Currents are a phenomenal reminder of the might with which the ocean is constantly moving, mixing ingredients, and providing passage to living beings between sites, regions, and across the world.

Being a drifter in the ocean, going where the water takes you, is not necessarily a bad thing. “Not all who wander are lost,” as JRR Tolkien powerfully stated. For a very large group of organisms in the ocean, wandering is a way of life. Nearly three-fourths of planet Earth is covered by the ocean. More than half of this is a vast, deep, open ocean with no sea floor, structures, or shelter in sight. A seemingly endless azure hue gradually transitions into a deeper blue until it slowly turns into darkness at the bottom of the ocean. In such an environment, resources are sparse and scattered. A lot of time is spent searching, and there is often a lot of distance to be covered. Tracking the tides and moving with currents is often energy efficient and might take an animal to patches with essential resources faster.

Here are a few ocean inhabitants who spend their lives traversing the open ocean and are equipped with strategies to make the most of the opportunities that come their way.

Jellyfish

Jellyfish or jellies (they are unrelated to fish) are expert ocean wanderers. These gelatinous animals have umbrella-shaped bells lined with tentacles protecting their mouth and essential organs. Oral arms extend from the mouth and trail behind as they move. Jellyfish (Class- Scyphozoa) move by rhythmically contracting the muscles in their bells; this expels water and thrusts them forward. Their efficient locomotion allows them to cover great distances using minimal energy. Some species can control the direction of their movements better than others. A jellyfish pulsing through the ocean is on a constant quest for food. Jellyfish tentacles are packed with touch-sensitive, venom-laced stinging cells called cnidocytes that help them capture anything from zooplankton and invertebrates to small fish. 

Comb jellies

Comb jellies or “ctenophores” are another group of fascinating ocean inhabitants that spend their lives in the blue. Besides their jellylike texture, comb jellies have very little in common with true jellyfish. Comb jellies are named for the rows of comb-like cilia (ctenes) that line the outer structure of their bodies. Rhythmically beating these cilia propels them through the water. Comb jellies are carnivores but do not use venom to immobilise and capture their prey. Instead, their tentacles contain “colloblasts”, which produce a sticky secretion that captures zooplankton. 

Salps

Superficially resembling jellies and comb jellies is another set of beautifully bizarre animals known as salps. Salps are gelatinous, barrel-shaped tunicates (sea squirts). Tunicates are a very interesting group of animals that are more closely related to vertebrates than invertebrates. Unlike other tunicates which attach to substrates, salps spend their whole lives propelling themselves through the open ocean. They are filter-feeding creatures that feed on phytoplankton. When salps come upon a large bloom of phytoplankton, their populations become aggregations spanning from tens to hundreds of kilometres. Individual salps clone themselves, adding more individuals to an ever-growing chain. As the chains grow larger, salps within and across chains begin to reproduce sexually, each being a sequential hermaphrodite (starting as females and transforming into males).

Blue button

Some animals traverse the oceans only on the water’s surface. The blue button (Porpita porpita), which looks like a delicate floating flower, is an exquisite example. So far, we have talked about creatures that wander solo (jellies and comb jellies) and those that come together in congregations or blooms (salps). Blue buttons in contrast, are entire colonies of small animals (Hydrozoans) living together. Many individuals called “polyps” are attached to each other to form branches and branchlets connected to a circular float in the centre that bobs the colony along the ocean’s surface. Blue buttons are carnivores seeking zooplankton as they drift with the currents. Being related to jellyfish, anemones, and corals (Phylum: Cnidaria), blue buttons also possess venomous cnidocytes (stinging cells), strategically placed at the end of each branch and triggered on touching potential prey. 

The community of ocean drifters is a fascinating and eclectic assemblage of form and function. They offer us a stunning showcase of survival strategies with lethal stingers, food filters, swimming cilia, and surface floats, to illustrate a few. Although most of these species have been around for hundreds of millions of years, they continue to be sensitive to environmental changes, with potentially drastic consequences to the functioning of entire ecosystems. Changes in water chemistry through eutrophication (increase in nutrients that support some forms of life) and rising water temperatures combined with overfishing are resulting in the increase in frequency and size of jellyfish blooms across the world. Warmer waters and an increase in food availability encourage jellyfish to reproduce in large numbers, with fewer and fewer predators to keep their populations in check.

On the other hand, salp aggregations we now know might play a significant role in combatting the ongoing climate crises. When salps feed on carbon-rich phytoplankton close to the ocean’s surface en masse and descend to the ocean’s depths to escape predators, they end up depositing this carbon in the form of heavy faecal pellets. A study conducted in the Pacific Ocean modelled what the contributions of carbon export by global salp aggregations might be. Their estimate suggested that salps and other tunicates export 700 million metric tonnes of carbon to the deep sea annually, which is equal to the emissions from over 150 million cars.

There are other kinds of ocean drifters in the ocean. Translucent bell-shaped plastic bags, barrel-like plastic bottles, and floating flip-flops, to name a few. Peculiar things to find when you are out at sea, but they are becoming far too ubiquitous to ignore. In typical wanderer fashion, they go where the currents take them but end up poisoning, clogging and choking the waters they inadvertently inhabit.

As we try to fathom the magnitude of this phenomenon, imagine a blue sea dragon (Glaucus sp.) swimming on the ocean’s surface. This nudibranch loves feeding on blue buttons and charts the seas in pursuit of these hydrozoan colonies. Depending on where in the ocean it is and the currents its prey latches onto, this blue sea dragon could travel far out into the high seas or find itself nearing a coastline. There is a good chance it might end up in a current system that lures it into one of the many Great Garbage Patches (vast collections of debris floating in the ocean) bobbing in the sea, where many other drifters end up — animal and plastic.