Worlds Apart: The Dual Life of Marine Animals

“A person’s a person, no matter how small.” This classic line from the Dr Seuss story Horton Hears a Who! is a lesson for us all. Horton, the elephant, stumbles upon the lives of the Whos in the bustling town of Whoville that exists in all its splendour in an unassuming speck of dust. This marvellous imaginary world created by Dr Seuss inspires us to believe that life exists in the smallest of worlds, even when we cannot see, hear, or feel it.

Similarly, in every drop of seawater, there exists a universe of living beings, hundreds, and thousands (maybe more!) of animals and plants, producing food, releasing oxygen, eating one another, and making babies. Without them, the world as we know it might not exist. Do you believe me?     

As unbelievable as it may seem, every drop of water in the ocean is like Horton’s speck, housing an unseen, vibrant, and dynamic world. Welcome to the weird and wonderful world of plankton. The very creatures that form the basis of our food chains, upon which entire ecosystems are built, and remain unassumingly invisible to the human eye. Plankton communities include unicellular and multicellular, fuzzy, pokey, smooth, phytoplankton (plant-like) and zooplankton (animal-like), bacteria, fungi, viruses, cyanobacteria, to name a few. This seemingly endless list of creatures ranges in size from a few centimetres to near-invisible microns.

It gets stranger still. Some zooplankton species spend their entire lives as drifting miniatures in the blue; we call them holoplankton (examples are diatoms, radiolarians or amphipods and copepods). Zooplankton also comprises animals that only live a part of their lifecycle as tiny larvae before exiting this plankton universe to become adults somewhere else in the ocean. These are called meroplankton.

Introducing the dual or biphasic life of meroplankton! The first phase of their life is spent as larvae, built to survive harsh and unforgiving conditions that come with journeying through the open ocean. Once they find a suitable habitat to settle down, they will undergo a dramatic metamorphosis, “suiting up” for phase two in their new environments.

Where does the story of meroplankton begin? A question I first asked myself a few years ago while swimming near the shore on Havelock Island. I came across an object smaller than a fingernail floating on the surface. I nearly went cross-eyed before I noticed its wing-like fins — the only indication on this bizarre translucent being to suggest it was a larval flying fish and not plastic. The encounter made me realise that most marine animals I had observed before were adults. I had rarely observed juveniles, let alone larvae, eggs, or instances of breeding.

I went down a wormhole to understand marine animal breeding biology, and my mind was blown by the sheer diversity and complexity of the ways they reproduce. Some animals mate in pairs, others in harems. Some species like anemonefish lay their eggs in neat clusters and take care of them well. Others, like jawfish, go to the extent of brooding them inside their mouths or as with seahorses, on their bellies. Then there are spawners (most corals), who release their eggs and sperms into the water in unimaginable quantities and let their gametes take their chances with external fertilisation. The lucky ones that successfully fertilise and escape predators at the site of the spawning aggregation then begin their migration through the open ocean in search of a suitable “forever home” as meroplankton. All this action is from living creatures no bigger than microscopic specks drifting through the vast ocean.

One could argue that instead of propelling their offspring into the vast unknown, it would make more sense for adults to produce fewer offspring and invest in parental care — the way mammals do for the first couple of years (or humans do for the first couple of decades). This does not always make sense for animals in the ocean. Predation and intense competition for space and resources in bustling ecosystems like coral reefs are serious concerns for survival. Some theories also suggest that dispersing away from spawning sites helps with genetic diversity, preventing isolation and extinction.

Considering the risks and benefits of the various environments available for early development, species have evolved different strategies. In direct-developing species, miniature adults hatch out of eggs and settle down close by. Planktotrophic larvae are at the other extreme, drifting and feeding in the water column as they disperse over long distances. Somewhere in the middle lie lecithotrophic larvae that are provided with yolk sacs for nutrition to avoid having to actively feed during their short but substantial dispersal.  

Life in the open ocean is high risk. Beating the odds of survival involves not being eaten (by other plankters, small fish, big fish, whale sharks, whales), finding suitable habitats, and not being swept away. Several species appear unrecognisable as larvae due to the temporary appendages, bizarre shapes, and colours they take on to adapt to the safety, movement, and food-finding challenges of open water.

As if the life of plankton is not tricky enough, 21st-century plankters must also deal with the impacts of climate change on seawater chemistry, acidification, and plastic fragments so small that microscopic creatures have ingested them. Given how prevalent two-part life cycles are in marine animals, larvae, and the habitats they use as plankton are just as important for research and conservation as the ecosystems they live in as adults. For you and me, the next time we find ourselves by the ocean, scoop up some seawater and say hello to the universe thriving in the palms of your hands.