The mid-reaches of the south seas are months drifting-time away from any shore. The southern hemisphere of this planet is 90% ocean – the seascape there is wider than any desert, barren in every direction, silent all the way around the far horizon. By day the surface is becalmed under relentless sunlight, then by night it chills to mirror the stars of Capricorn and the Southern Cross. Terra firma is not part of the story here. The solid bottom is thousands of feet below, bathed in perpetual darkness.
But this place is not as deserted as it may seem – life abounds, often represented by the hydrozoans. For example – just below the water’s surface a little blue disk may materialize from the depths. It is a single, lone Blue Button Jellyfish, a mid-water, plankton-feeding hydrozoan harbinger of larger such creatures to come.
Long periods of sky-blue monotony here alternate with days of non-stop activity. Sunrise may reveal a new surface – littered as far as the eye can see with small sails. It is a bloom of By-the-Wind Sailors, pushed together overnight by a convergence of air and sea currents. Their sudden appearance is a reflection of the potential of this realm to spring to life from one horizon to the other, when conditions are right.
By-the-Wind-Sailors thrive when the invisible lower level members of the food chain increase in the waters around them. Then the Sailors increase in turn until there are millions of them per acre of surface. Their small rafts, only inches in diameter, appear to be unfolded from cellophane half-circles. Two bottom halves are fused into an oval that sits on the water; short tentacles beneath troll for plankton. The third vane, a top-side sail, stands up high and dry. It is buoyant, so should the creature be up-ended by white-caps, or by a school of leaping dolphins, the heavier tentacles roll back below the lighter sail to keep the ship right.
Like the Blue Button, the By-the-wind-Sailor’s upper sides are deeply colored. This is defensive counter-shading – these animals blend in with the background of the deep dark sea below them when preying eyes pass above. From the other direction they are transparent enough for the daylight to shine through, rendering them invisible against the sky when looking up from underneath.
The Sailors get their nutrients when their micro-venomous tentacles capture their microscopic prey. But they get their energy from the sun. They live bathed in bright light all day long. Thus they are well situated to grow a garden in their transparent tissues. They are part plant – they carry photo-synthetic algae, which share the sugars they produce with their host. Those microscopic symbionts are typical of the free-living plankton in these waters. But as passengers within their transparent home, these single-celled organisms are protected from the sea’s many planktivores.
By-the-wind-Sailors are rudderless, completely passive little vessels. Their tentacles do not reach beyond the edge of their disks, nor down far enough to retard their rotation. So when a puff of west wind raises wakes behind the invisible fingers it draws across the surface, these creatures spin. Under the influence of the breeze, all the short sails rotate around like compass needles to align north-south, perpendicular to the wind direction. In this orientation, the sails are best set to catch the breeze.
Together they all scoot off sideways, heading east to new pastures. The journey will end for many with mass strandings on the continents that lie across the direction of the prevailing west wind. Their bodies on the sand will dry paper-thin, and pile up purple stripes across miles of high tide lines on beaches of Africa and the Americas.
. * * *
At night, with the surface calm and deserted again, a greenish glow materializes in the water. Across the area, luminescent plankton have multiplied in only a matter of hours. They flash when disturbed, and now their glowing bodies outline a rising column of light. Atop the column is a balloon that just inflated below and is accelerating toward the surface. It is a Portuguese Man ‘o War, the largest of the hydrozoans. The creature had deflated itself earlier and settled down into the water. Now its has reinflated its float, which expands and grows tight as it rises and the pressure around it lessens.
The Man ‘o War breaks the surface, and the sail that frills the crest of its float catches the breeze. Its movement across the water is slowed, however, by the sea-anchor of tentacles it extends beneath it. The whole creature is bathed in cold luminescence, which shines up to illuminate its translucent air bladder. Its tentacles below are decorated with tiny, fading sparkles of luminescent plankton.
The illuminated spectacle attracts attention. One of the largest creatures of this realm materializes out of the gloom, and heads for the Man ‘o War. It is an adult female Loggerhead Turtle, seven feet long, black, and a thousand pounds in weight (on land; but in the water, it is weightless). It homes in on the lighted beacon and attacks, devouring the whole Man ‘o War except for a few of the tentacles that diffuse away on the current. As they drift downward, the detached strands will sting and hold whatever creatures they happen to contact, now to no purpose.
At dawn, more of the Men ‘o War pop up all across the surface. The vivid blues of their inflated bodies are embellished further as they catch the color of the sunrise. A few small clownfish appear from nowhere to shelter in the shade of the floats – impervious to the stinging tentacles. The trailing strands become thinner and more transparent when they stretch a hundred feet down to where the light has faded away. At depth, they paralyze, and eventually haul up whatever free-living things have the bad luck to brush against an invisible thread.
A calm-slick appears on the surface along the line where colliding ocean currents meet head-on and plunge straight down side-by-side. The Men ‘o War accumulate along this path. Their flotation keeps them from being sucked downward with the descending flow. They do not pile up together in these calm slicks because they track along their courses at different speeds. Some of them haul up their tentacles, so are pulled farther by the wind; others leave their strands extended, fishing through the passing flow, and the drag of the current becomes the main influence on their course.
Those currents flow in closed loops. Far beyond the places where they dive toward the bottom, they rise back up again. Such up-wellings are the hunting grounds in which the passive hydrozoans multiply. The rising currents bring up nutrients from the bottom. These nutrient-rich columns bloom with the microbes that anchor the planktonic food chain.
The effects of converging and diverging currents, of high vitality in some places and desert-like emptiness in others, provide a geography of different textures, temperatures, and scents. The vast oceanic plains are divided by these differences. Their signs are recognized and followed by all the residents living at the interface of air and water. The sea birds coasting above – shearwaters, petrels, albatross – track along such landmarks. The great Leatherbacks do the same below, as do the even bigger mid-ocean sunfish searching the dappled depths for jellyfish. Only to uninitiated visitors from the land does this surface appear vacant and monotonous.
. * * *
The hydrozoans stand low on the open ocean food chain. Members of the next rung up that chain are less-often encountered. But as the flotilla of Men ‘o War passes, some of the later floats may be seen to be partially collapsed, keeled over in the water. A few are so deflated as to be awash, near sinking. They are under attack from another of their predators – solid-bodied creatures, the open-ocean shellfish.
One of the Men ‘o War appears to be slumped over onto a cushion of bubble-wrap. On the inner edge of that sheet of bubbles hangs an indigo-colored spiral cone. This is the Violet Snail. Its white tip points straight down, toward the rocks and silt far below – where marine shellfish are usually found. But this snail never sinks to the solid substrate – it lives on liquid along with its jellyfish prey. Its stone weight is suspended, buoyed on a raft it has blown of unpopable bubbles that keep it afloat on the open water. When it encounters a Man ‘o War, it docks with it and begins a snail’s-pace attack. It is immune to the stings of the hydrozoan, which it may take days to consume.
The convoy of Men ‘o War is under siege on its far flank as well. There, the predators are Sea Swallows – members of the nudibranch family. They appear like detatched fronds from a purple fern, floating on the surface-tension of the water. They are mollusks, but they shed their shells soon after they hatch from eggs. Several of them will attack a Man ‘o War, and remain until their prey is consumed.
Most nudibranchs live on solid reefs and shoals. They are colorful animals that carry their gills exposed on their backs. Many of them feed on sessile marine flowers, the anemones – which are close relatives of the floating jelly creatures. The nudibranch’s gills are colored like flowers – they may be boldly yellow, or sky-blue grading to orange. But these brilliant petals carry a message of warning. The color advertises the nudibranch’s ability to consume the stinging cells of the anemone, and then pass those cells undamaged, still alive and undischarged, through their bodies. Those stinging cells are finally emplacement among the gills on their backs like the spines of a nettle, facing upward – ready to sting.
A Man ‘o War is an upside-down anemone, anchored to the ceiling of the sea by its float, with its tentacles pointing downward – not up. The Sea Swallow has conformed to this inverted order. It is an upside-down nudibranch, with its foot upper-most – walking on the under-side of the surface, looking for its inverted-anemone prey.
The Sea Swallow has retained the capacity to consume stinging cells without discharging them, passing them through its system, and emplacing them in defensive positions. The body of the Sea Swallow is centered between projecting wings that look and act like the ray-fins of the Lionfish. The animal loads the tips of those projections with the most potent of the stinging cells of its prey – its sting is worse than that of the Man ‘o War.
But the Sea Swallow – unlike the Lionfish or the coral-feeding nudibranchs – does not wear bright warning colors. It wears the open ocean defensive countershading deep blue above, silvery-gray below. It, too, is far from the top of the food chain.
. * * *
The hydrozoan jellyfish are not all members of the surface community. Most of them, including the siphonophores, the longest of the hydrozoans, swim freely below. The thin, transparent siphonophores can be fifty feet in length or more. Their transparency makes it difficult to see all of their length in the same glance – until the night descends and their ribbon-forms are outlined in their own luminescence. Some of their luminescent tips glow red, and jiggle – attracting prey to the adjacent stinging cells.
The siphonophores live below the molluscan surface predators, but are not completely free from them. A mid-water nudibranch that looks like a translucent, free-swimming flatworm hunts through the depths for those hydrozoans.
. * * *
The next morning finds the surface deserted and becalmed once again – even though cirrus clouds are spreading low along the western horizon. The sea lies flat, before the impending storm. The mirror image of the sun’s disk lies just a few feet away, riding as far below the horizon as it’s source – millions of miles away – rides above it.
Off to the northeast, the blue sky and its reflection on the sea grade to shades so similar that the dividing line between them disappears. No birds dip and glide in the middle distance – none of their cries carry across the water. It is so still that the drop of a small jumping fish can be heard. That fish was probably a member of a school. It jumped because something larger was chasing it. Another fish jumps. There is so much life here that it won’t be quiet for long.
Sea Swallow: notes. As we learn more about pelagic ecology, we sort our deeper understandings by coining names. We labeled the passive drifters as the “plankton”, the largest member of which would be the Portuguese Man-‘o-War. Then we devised a new category for the actively swimming open-water community: the “nekton.” We have now further subdivided our categorizations of those water-borne communities defined by their niches, with the designation of the “pleuston” – those organisms that float or live right under the surface. A prominent member of this latter category is the By-the-Wind-Sailor, the only member of the genus Velella. Their sting is proportionate to the size of their prey – it is lethal to microscopic organisms, but it cannot be felt on the hand (though it could sting in your eye if you rub it after handling Vellela). The sails of Vellela orient perpendicular to the wind because a freely rotating airfoil assumes the orientation of greatest resistance to an airflow. The Sailors strand on beaches in Ireland, British Columbia, and other west-facing continental shores. Their larger cousins in the Hydrozoa, the Men ‘o War, strand less frequently. Their courses are more determined by the current than the wind, and near-shore currents tend to be parallel to the beach, as opposed to toward it (like the sea breeze that impels Vellela). Men‘o War can sink into the water, avoiding surface effects (and predators) entirely; then they can inflate their gas bladder, and quickly rise (Whittenberg, 1960). The bladder is filled with carbon monoxide at depth (thus Men ‘o War appear resistant to CO poisoning) but the gasses of the air diffuse in after the creature surfaces. The hydrozoa are a subgroup of the jellyfish, in the phylum Cnidaria. They are preyed upon by members of phylum Mollusca, including the Sea Swallow, which is a free-moving nudibranch (pronounced “nudibrank”) member of the pleuston. The nudibranchs have the capacity to manipulate the living stinging cells (nematocysts) of their prey – moving them intact through their bodies to defensive external placements (Greenwood, 2009). Cephalopyge trematoides is a nudibranch member of the nekton; it is a free swimming predator of sub-surface siphonophore hydrozoans.
Greenwood, P G (2009) Acquisition and use of nematocysts by Cnidarian predators. Taxicon 54, 1065-70
Whittenberg, J. (1960) The source of carbon monoxide in the float of the Portuguese Men-of-War. Journal of Experimental Biology 37, 698-705