Plankton & Beasts from the Deep
Extracted from The Sea that Never Sleeps by Douglas David Seifert, read the full feature in the DIVE night diving special
You cast the beam of your dive torch over the feeding corals and in the light you can discern the tiny blizzard of plankton carried on the gentle currents. As you focus on the tiniest of visible animals, you think you may be going cross-eyed trying to concentrate upon tiny copepods smaller in size than a terrestrial flea. On close inspection, you can discern other body shapes that are worms, larval fish, larval invertebrates, strange, self-propelled gelatinous creatures called comb jellies with long filaments and inexplicable lighting emanations.
Some of the plankton are attracted to your torch, especially the worms. They can congregate in a dense swarm if your light stays in one place for too long. Yet you are seeing but a tiny fraction of the plankton universe as you meander across the reef; the real action is taking place a few kilometres away, in the offshore, where the bottom is hundreds to thousands of metres below, but by night the action shifts to just below the surface.
During World War II, US Navy ships’ sonar detected a mysterious, shifting, phantom sea bottom and after much study determined it to be the sonar signature of a dense, expansive migration of plankton, deepwater fish and deep pelagic organisms that embark upon a migration towards the surface at night and a descent back into the darkness by day. This was named ‘the deep scattering layer’ and over time has become known as the diel vertical migration of plankton (diel meaning ‘daily’), which is now understood to be the largest migration of animals on the planet, with the biomass of animals estimated at more than one billion metric tonnes, with some animals travelling as many as 800m in each direction.
The reasons for the migration are believed to be the enticing abundance of food in surface waters, primarily phytoplankton – the algae, dinoflagellates, diatoms, coccolithophorids and cyanobacteria.
These self-nourishing organisms create organic compounds, such as carbohydrates and sugars, from carbon dioxide dissolved in water using photosynthesis (and, incidentally, producing oxygen as a by-product) that form the base of the trophic food web from which all life depends. And the zooplankton feed upon the phytoplankton.
The consumption of phytoplankton by zooplankton is so intense that the entire population of phytoplankton is consumed every two to six days, thus necessitating a continuous population replenishment of phytoplankton on a scale unimaginable in terms of sheer numbers of individuals and the infinite generations over time.
Zooplankton are comprised of diverse and diminutive invertebrates: crustaceans – copepods, mysids, shrimp and krill, amphipods, isopods, ostracod, larval stomatopods and larval decapods – chaetognaths; pteropods (sea angels and sea butterflies); tunicates; ctenophores – and larval fish.
So much of the upper water column is comprised of larval fish fry and recently fertilized but unhatched embryonic fish eggs. The vast majority of reef fish species and most pelagic species reproduce via pelagic broadcast spawning, a reproductive strategy where a tremendous number of eggs are produced by the female and fertilized externally by the male or multiple males, and the thousands or tens of thousands of fertilized eggs are taken by the current to drift into the vastness of the sea. It’s a numbers game, with the majority of the spawn not surviving to adulthood; but enough do survive to maturity to keep the populations from collapsing.
Just as phytoplankton is relentlessly consumed by zooplankton, zooplankton are, in turn, the prey of just about everything larger than themselves: jellyfish and other gelatinous predators; pelagic fish; megafaunal planktonivores such as manta and mobula rays; megamouth-, whale- and basking sharks; various deepwater fish, especially as the mind-bogglingly prolific lanternfish (Myctophidae) – the biomass of which is estimated to be as much as 65 per cent of the deep water fish stocks and a major component of the diel vertical migration. Lanternfish make the journey of 800m or more every 12 hours. Flying fish and squid also feed upon the nocturnal plankton assembly and in turn are the prey of night-hunting marine mammals and sharks. Thresher sharks rise from the deep and use their elongated tails in the manner of a whip to stun fishy prey. Scalloped hammerhead sharks leave their schools at defined areas off seamounts for solitary cephalopod hunts into the offshore waters. Dolphins and their cousins, the pilot whales and melon-headed whales, hunt fish and squid all night long and move to inshore bays or shallow flats by day to rest.
In the darkness all these animals out in the open water are engaged in the eternal, frenzying struggle to eat prey while trying to avoid being eaten themselves, with the act of sporadic reproduction when the chance occurs. They rely on serendipity and on senses unfathomable to humans, such as chemoreception, electroreception, a hybrid sense of smell and taste, pressure sensitivity through lateral lines, visual information in spectrums beyond the vision of humans.