Life at the top
The surface of the ocean is a place we associate with waves and weather rather than life, yet those top few metres are the key to the sea’s great food web. A wealth of creatures—some small like this 30 mm leatherjacket, others long salps that resemble 10 metre plastic bags—drift through this vast light-filled domain, seeking to devour smaller organisms and avoid being devoured by larger.
For years, as I snorkelled my way to dive sites my attention was focused only on the sea floor below. Anything near the surface was a blur as inconsequential as roadside scenes on the way to an evening movie. I only awoke to my folly seven or eight years ago while snorkelling near the Poor Knights Islands, off the coast of Northland.
Twenty-four kilometres out from Tutukaka, these islands receive many visitors from the open ocean. On this trip I encountered a shoal of jellyfish and, uncharacteristically, paused for a look. Some members of the shoal were accompanied by tiny jack mackerel. As I approached, these darted behind a jellyfish bell or among the tentacles, seeking refuge in the very jaws of death. They were luckier than me that day, for I received many stings from tentacles that became wrapped around my camera gear. But I came away pumped with more than toxins. Cast up along the high-tide line, jellyfish are little more than amorphous blobs. Out in the ocean they are gossamer kites, billowing and drifting on the currents, their internal architecture on display. I decided to see what else I’d been missing in the top few metres of the ocean.
In many ways the upper ocean is the powerhouse of the planet. Although we know that some organisms get their energy from chemical reactions around deep-sea vents that spew out superheated water and minerals, most life in the sea ultimately depends on photosynthesis. Photosynthesis depends on light, and light diminishes rapidly with increasing depth. Hence, most of the primary production in the sea—light-driven plant growth—takes place near the surface. Where rocks provide a solid substrate in 10 or 20 metres of water, seaweeds capture sunlight and turn it into plant matter, but over most of the ocean, where the water is deep, near-surface phytoplankton perform the vital work of converting sunlight into living tissue.
What phytoplankton lack in size—most range from 1 to 500 microns in diameter—they make up for in abundance. Thousands to millions may be found in every litre of seawater, and there are quite a few litres in the top 10 or 20 metres of the ocean. Murky greenish water indicates rich phytoplankton, but clear blue water—as commonly seen in the tropics—is the maritime equivalent of a desert, despite its inviting appearance and photographic advantages.
Of course phytoplankton are far too small for my lenses to capture, as are the minute protozoa and zooplankton that dine on them. Notable among the protozoa are foraminifera and radiolarians, single-celled organisms that inhabit microscopic calcareous and siliceous casings respectively. A few millimetres in length but still too small to photograph in the open ocean are crustaceans, particularly copepods, which feed on diatoms and other phytoplankton. Copepods in turn are the main course on the menus of a myriad other animals, ranging from shrimps and small fish to sei whales.
Am I ready for a close encounter with a sei whale? Yes, but I am still waiting. Between whales and plankton, however, there is a range of manageably sized and peculiar creatures that are often overlooked.
Small fish can be found sheltering under almost anything that floats. For instance, beneath one piece of drifting weed I investigated recently were tiny sea horses, juvenile leatherjackets and a small pipefish. Each leatherjacket was smaller than a five-cent coin. There was plenty for these fish to eat since the water around them was alive with minute crustaceans.
Many fish begin life as plankton, either as eggs or tiny larvae. A planktonic larval stage ensures that a species becomes widely distributed. Some of our subtropical reef fish may have hatched from eggs laid far “upstream”—around Lord Howe and Norfolk Islands, for instance, back along the East Australian current. Malcolm Francis, a National Institute of Water and Atmospheric Research marine biologist who has studied the distribution of New Zealand fish species, has shown that 10 per cent of the north-eastern North Island’s approximately 170 recorded inshore fish species are tropical strays that make cameo appearances during warm periods. A good example is the lionfish—a gaudy species with large poison-tipped pectoral and dorsal fins—seen in 1990 off Ngaio Rock in the Poor Knights. It had probably arrived on the East Auckland current as a planktonic larva.
Approximately a third of the north-eastern North Island’s fish species are of subtropical origin, their population centres lying astride the tropic of Capricorn. Some breed locally but are boosted by influxes of planktonic larvae from the north. The seven species of Moray eel are examples. They spend two or three weeks as planktonic eggs and then a further few months as larvae a couple of centimetres long—time aplenty to drift to New Zealand waters from off eastern Australia or around Norfolk Island, where they are also native.
Many of our reef fish are planktivores. It is common to see schools of maomao, demoiselles and trevally feasting on plankton. The main treats for these fish are millimetre-scale zooplankton, including copepods, shrimps and larvae (including those of fish). Larger plankton are prey for fish too. I often see leatherjackets chewing their way through large jellyfish and salps.
The best time to visit the sea surface is at night, when it really comes alive. After sunset, copepods and other zooplankton rise to the surface from depths that can exceed 100 metres. It’s like being surrounded by swarms of mosquitoes, except fortunately these little crustaceans don’t bite.
Not only are zooplankton more abundant at night, but their appearance changes too. A delicate luminescence—invisible by day—shines out of many organisms. Turn your lights off and flap your arms and the seawater around you might just light up with bioluminescent plankton. On one night dive I watched underwater sparks shoot through the gloom as if from a grinding wheel, where water was slopping into a sea cave. The brightest displays always seem to be associated with turbulence.
Larger animals patrol the surface zone by night, too. Towards the end of an autumn night dive, one of my colleagues observed a shark, probably a bronze whaler, circling the boat. Later we witnessed something resembling an aquatic electrical storm as luminescent phytoplankton gave off bursts of light, probably the result of collisions with fish. By the next morning the small crustaceans and their followers had returned to the depths.
Why do zooplankton follow this daily rhythm? Several explanations have been advanced. Maybe zooplankton retreat to deep water to avoid predators that hunt by sight. Or, as the metabolic rate of small organisms is reduced in colder subsurface water, nocturnal commuting could be an energy-conservation measure.
Perhaps the largest free-floating consumers of plankton are the fire salps (also called pyrosomes). Last year, while carrying out some close-up photography on a rock wall in the Poor Knights, I had an encounter with one of these giants. I looked up from my camera and noticed my buddy, Tony Kelly, swimming away from me into open water. I left the wall and went after him. A ghostly 7 m-long sock-shaped object came into view. The open end was almost big enough to take my head and shoulders. Examination of the surface revealed a carpet-like sheet of thousands of individual creatures—zooids which are the building blocks of these colonial animals.
With beating cilia, each zooid draws in water through an entrance siphon, then, after removing any food particles, expels it through an exhalant siphon. The zooids are arranged radially so that their exit siphons all point into the internal cavity of the colony. Since one end of the cylindrical colony is closed, exhaust water must exit through the opening at the opposite end. The combined activity of the thousands of zooids produces a gentle pressure that moves the entire colony sedately through the upper levels of the sea. Even so, it is largely at the mercy of currents.
Salps grow as the zooids multiply, and can attain enormous sizes. Back in 1969, fire salp colonies (Pyrostremma spinosum) as long as 20 metres were photographed by divers in the Poor Knights and at Mayor Island. A smaller species (Pyrosoma atlanticum) grows to about 60 cm.
Salps attach to an unexpected branch of the tree of life. Although they are gelatinous and lack bones, they are classified not as invertebrates but as primitive chordates, known as tunicates. Sea squirts, commonly found under intertidal rocks, are also tunicates. Tunicates qualify as chordates—the phylum to which humans belong—because their larvae possess a rigid cord where the backbone lies in the more highly evolved vertebrates.
Despite their taxonomic status, for the most part salps resemble the many primitive invertebrates, such as jellyfish and ctenophores, that account for the majority of pelagic organisms (those that live near the sea surface in open water). Their gelatinous bodies are rich in water and low in solids. A consequence of this watery composition is that they are also semi-transparent. Salps are also among the most brightly bioluminescent of pelagic organisms, producing a blue glow that is visible in the dark for many metres.
Jellyfish belong to the phylum Cnidaria (the C is silent), together with corals and anemones. This is a large and complex group of no fewer than 13 orders with jellyfish-like members. A Portugese-man-of-war (Physalia) with a gas-filled blue float and long tentacles is a quite different sort of beast from the bell-shaped gelatinous objects with a few coloured splotches and skirt-like tentacles that often appear off swimming beaches in summer. And the little by-the-wind sailor (Velella velella), which resembles a miniature wind-surfer when found dried out on the drift line of a west coast surf beach, is different again.
Despite their diversity, however, jellyfish share many features. They are radially arranged (in common with starfish and sea urchins) and carnivorous, relying on stinging cells (nematocysts) to immobilise their prey, which can range from small copepods through shrimps to other jellyfish and small fish. Their gastric and vascular systems are merged to a much greater extent than those of vertebrates, and partially digested food particles are engulfed by cells anywhere in the system of canals that extends from the mouth to the tentacles and round the bell. Strands of muscle around the bell contract rhythmically, imparting jerky propulsion. I have often observed a jellyfish swim vertically upwards, bounce off the surface and then swim down. Perhaps this maximizes the likelihood of encountering food at different levels in the water column. Horizontal movement is pointless anyway, as these creatures cannot outswim an ocean current.
Ctenophores (again, the c is not pronounced), or comb jellies, used to be grouped with jellyfish but are now placed in their own phylum. They tend to be ovoid, cucumber-shaped or spherical rather than bell-shaped. All species have eight bands of cilia that run like lines of longitude from the mouth to the other end and are used for propulsion. The cilia—2 mm long, the largest known in any creature—are arranged in rows across the bands, with the members of each row partially fused to form something resembling a miniature comb, which functions like a blade on a paddle wheel. Like jellyfish, ctenophores are highly translucent but most have only two antenna-like tentacles and few possess nematocysts. Sticky structures (colloblasts) on the tentacles hold prey so it can be transferred to the mouth. All species (100–150 are known) are carnivorous, feeding on small crustaceans, jellyfish and the larvae of fish and invertebrates.
I recently watched a ctenophore I had caught in a plankton net deploying its tentacles. It rocked from side to side as it dropped the two main tentacles, then spread a couple of dozen smaller side branches. Once everything was unfurled, the animal sat motionless, waiting for something to blunder into its sticky net. A tap on the tank caused it to retract its tentacles and move to a different position, where it repeated its unfolding ritual.
Stickiness may not sound like a recipe for success, but don’t be misled. In the 1980s an Atlantic ctenophore (Mnemiopsis leidyi) was introduced into the Black Sea in ballast water. It quickly out-competed indigenous larval fish for zooplankton, and within a decade millions of tonnes of fish had become millions of tonnes of (inedible) ctenophores. Fishermen in six surrounding countries were out of work. Mnemiopsis has since entered the Caspian Sea.
Many of the larger ctenophores—a few species grow to a metre across—are very fragile, and attempts to capture them with towed nets have brought in little more than fragments.
The great phylum Mollusca is also well represented in surface waters. Squid are able hunters amongst the zooplankton and range in length from a few centimetres up to a metre. Fast swimmers with eyes that rival our own for acuity, they devour crustaceans, small fish and other squid. New Zealand is blessed with at least 96 squid species, and I recently encountered one of them Sepioteuthis australis—during a night dive on the western side of the Poor Knights. In the light of my torch it pointed its arms upward like a skydiver and began a slow freefall down through the water. I followed. Occasionally it would pause, open its tentacles and lunge at something in the plankton. The ease with which it descended suggested it was negatively buoyant. Dr Steve O’Shea, a marine biologist at the Auckland University of Technology, has since confirmed that these squid lack air cavities, so that when they stop swimming they sink. They can maintain depth by hydroplaning like a submarine or by directing their siphon downward and hovering.
Less well-known pelagic molluscs include a whole range of gastropods, cousins of the staid limpet and garden snail. After a storm, the thin-shelled violet snail regularly washes ashore, often in the company of the Portuguese-man-of-war, on which it preys. A raft of rubbery bubbles ensures that it lives its life just a centimetre below the surface. Pteropods and heteropods have departed somewhat from the traditional gastropod body plan. The shell has been highly reduced, and the foot, rather than being used for clinging and crawling, has been modified for swimming. Heteropods feed on salps and ctenophores; pteropods eat small plankton, which they catch with mucus nets.
There are clear advantages to being pelagic—it puts you in the midst of the ocean’s fast-food zone—but why are so many pelagic organisms gelatinous? Typically, cnidarians and the like are 96 per cent water, 3 per cent salts and 1 per cent organic matter. Such a body has advantages in the sea. Little effort is needed to stay afloat, in contrast to, say, a squid, which lives life in the fast lane and uses plenty of energy to stay off the bottom. Low levels of organic matter also mean these organisms are not much of a catch from a nutritional point of view—they are only a few calories away from being chewy seawater. Their pace of life is generally sedate, so they can get away without gills—indeed, without most bodily organs. Oxygen diffuses in and waste diffuses out over most of the body surface. Nutritional requirements are low, but if food is plentiful, rapid increases in size or population density are possible.
A watery composition goes hand in hand with transparency—cousin of invisibility and a useful property in a high-visibility environment. Sharp-eyed seabirds patrol the ocean ceaselessly by day, so an organism that is transparent or blue has a better chance of survival. From below, all sorts of nameless fangs rise from the deep, especially at night, and resembling a dinner plate against the light sea surface is imprudent. Transparent is preferable to blue in this context. Whether bioluminescence in surface animals is meant to mimic the glow of the moon or stars is anyone’s guess, but it is thought to be associated with defence. And for me it is one of the bonuses of visiting the peculiar creatures that frequent the surface of the sea once the sun has set.