Test Design Lighthouse

In the waves

Swell, rough sea, wind sea, ground swell or cross sea – are indicative of the varying topography of the ocean surface and its wind-driven dynamics. Only rarely is the sea completely calm. Yet even its most obvious concomitant, wind strength, has but a tenuous link with water flow. An object floating in water does not advance along a linear course in the same direction as the waves. Instead, its course takes it up and down, and forwards and backwards at the same time. The shape of the ocean surface makes dynamic progress, but the water does not.


Water particles travel in a circular motion as a wave passes through. Going deeper into the water, this motion decreases drastically.
If the constant movement of air over the surface of the water is interrupted, small differences in pressure produce swirls.

Neuston and pleuston


The surface of the ocean itself is the habitat for a particular community of organisms known as neuston. Independent of the seabed and coast, they live on what collects on the underside of the thin film at the interface between the seawater and the air. Here bacteria and colourless flagellates make use of organic compounds, and are eaten themselves by ciliates and small crustaceans. In the shelf seas in particular, the surface layer is enriched with particles blown from on shore.


Species whose bodies project above the water surface are classified as pleuston. The most astonishing must surely be the Portuguese Man-o’-War (Physalia , siphonophores). These are free-floating colonies of cnidarians, in which the individual members have specialised and reorganised to become organs of a higher-order creature. Using its polyps, which can be a metre long, the Portuguese Man-o'-War can even capture larger prey, such as fish, from the top layer of water. The related species Vellela and Porpita, on the other hand, use the abundance of organisms in the neuston.


The floating bodies of Physalia and Velella are mounted with an asymmetrical sail which enables them to drift with the wind. The fact that there are two forms of each species, a left-sailing and a right-sailing form, gives rise to typical distribution patterns in the different wind zones. Nevertheless, what advantage these species derive from their sailing technique remains unclear. In any given situation, it may possibly prevent one form or the other from becoming beached.



The stinging cells of the Physalia siphonophores are feared.

Equally amazing is the violet snail Janthina, which floats on a foam raft on which it also suspends its eggs, or Glaucus atlanticus, also a snail, which has an air bubble in its stomach to maintain buoyancy, and pushes up to the surface using its foot. Both species feed on by-the-wind sailors.


While insects have not been able to colonise the sea as a habitat on a large scale, nevertheless a handful have found their way onto the ocean surface. The five known types of the water skater genus Halobates are distributed worldwide across the warm oceans. It is possible that they, too, live on by-the-wind sailors, but they primarily consume neuston organisms. They need hard floating objects like sargassum or driftwood on which to lay their eggs.




Back into the sea


Loggerhead sea turtles (Caretta caretta) are at least 25 years old before they lay their first eggs. To do so, they make a long journey back to the exact beach where they themselves were born. (Photo: F.Ritter)


Sea turtles, sea otters, seals, sea cows, whales: the body forms and functions of these animals have undergone numerous special adaptations to life in seawater since their ancestors made their way back into the sea, long ago. Needless to say, they are not just found up among the waves; the majority of these species also exhibit quite extraordinary diving capabilities. But however different their evolutionary trees, their forms and their modes of marine life, breathing air at the ocean surface is one thing they all have in common.


These creatures which have returned to their aquatic origins now vary in their importance for the marine ecosystem. Sea otters and sea cows only nibble at the edges of the marine food web; carnivorous seals seek the open sea, but must return to land to mate, give birth and rear their young. Whales, on the other hand, with their great diversity of species, are completely adapted to the sea and have set themselves at the apex of the food pyramid.



Armoured migrants


There are 7 species of sea turtle to be found in all sub-tropical and tropical waters. During their long dives, they catch squid, crustaceans, sponges or cnidarians. They spend their whole life in water, and only the females venture onto the beach to lay their eggs. Normally this will mean revisiting, for the first time after many years, the place where they hatched from their own egg. Sea turtles cover long distances on their extended migrations, following ocean currents and probably orientating themselves to the earth’s magnetic field.


One of the reasons why sea turtles are endangered continues to be the demand for their meat and eggs, particularly in Asia, as well as for tortoise shell and turtle skin. Furthermore, on some sites, turtles trying to nest are in competition with the interests of the tourism industry, and the destruction of traditional nesting beaches for construction projects. Fishing vessels are also responsible for heavy losses as numerous animals get caught and drown in their nets. General marine pollution also takes its toll.



Seals like these female elephant seals (Mirounga angustirostris) give birth to their young on land. (Photo: L. Fallon)

Flippered predators


The distribution of most of the 35 species of flippered seals is concentrated in the temperate and cold waters of the northern and southern hemispheres. Like sea turtles, all species of seal are amphibious. The aquatic mammals, whose diet consists principally of fish, go onto land to reproduce, to give birth to their young, and to moult.


Even though most of these species are not particularly agile on land, their streamlined, fusiform bodies make them excellent swimmers. The actual duration and depths of their dives depend very much on physical capabilities, and vary from one species to another. It is unlikely that they need to push themselves to the limits of their capabilities very often. Rather, the 'normal' dive depth depends on the particular ocean region, on the food supply and the type of prey. The record-holders for deep diving are elephant seals (Mirounga angustirostris), with average diving depths of 350 - 650 m and a maximum depth of well over 1000 metres.


As air-breathing mammals, both seals and whales have the problem of withstanding water pressure which increases with depth and which powerfully compresses an air-filled cavity like the lungs. To avoid this danger, they exhale before they dive. Sea mammals are also able to store more oxygen in the blood and muscles than land animals, and to reduce their pulse rate during dives.


Video recordings of diving whales and seals have demonstrated that the animals begin their dive very actively, maintaining this for about the first three minutes and then sinking motionlessly into the depths. There is a trick they use to keep their energy consumption as low as possible: as hydrostatic pressure rises with increasing depth, the lungs collapse and the body is likewise compressed. This reduces the animal's volume even though its weight remains the same. The animal’s specific gravity increases and it sinks to the bottom effortlessly.



Whales and dolphins


Adult male humpback whales (Megaptera novaeangliae) are 14 m long on average, and the females, 15 m long. Their weight varies between 30 and 40 tonnes.(Photo: R. Wicklund, NURP)
It is not necessarily pure joie de vivre that makes dolphins leap out of the water whilst swimming fast. Above a speed of around 10 kilometres per hour, the frictional resistance of the water is so high that jumping out of it into the air represents a significant energy saving. (Photo: F.Ritter)


Whales, of which at least 84 species have been recorded to date, are perfectly adapted to life in water. In contrast to seals, their adaptation to the marine environment is so complete that they are unable to survive out of water. Despite this, there is no denying their genetic descent from land-living forms: when in motion, the spine of a sea mammal bends up and down, like that of its forebears. Fish, on the other hand, achieve locomotion by bending the backbone from right to left.


The flipper (or rather, fluke) at the end of a whale's body has a similar cross-section to an aircraft wing. This produces basic thrust and enables the animal to achieve considerable speeds. Blue whales, for instance, which are around 25 metres long and weigh approximately 100 tonnes, are fast swimmers despite their size. If necessary, they can reach a speed of 48 kilometres per hour. It is not necessarily pure joie de vivre that makes dolphins leap out of the water whilst swimming fast. Above a speed of around 10 kilometres per hour, the frictional resistance of the water is so high that jumping out of it into the air represents a significant energy saving.


The shape of their mouthparts reflects the whales' feeding method, and helps to divide them broadly into two groups: the toothed whales, which are generally smaller, and the baleen whales which lost their teeth and have instead developed fringe-like, keratined plates (baleen) on the upper jaw. While toothed whales feed on relatively large prey, like fish and squid, baleen whales live primarily on plankton organisms, in particular krill (Euphausia superba), a species of crustacean approximately 2.5 centimetres in length.


The different feeding methods of whales, which occur in all the oceans between the Arctic and the Antarctic, also lead to differences in behaviour. With the exception of the sperm whale, toothed whales move around relatively little, but baleen whales migrate extensively, shuttling between the feeding grounds in polar waters, and the mating and rearing grounds in warmer latitudes.


Living permanently in the sea is associated with particular difficulties for whales, beyond the adaptations necessary for deep diving. For warm-blooded animals, the regulation of body temperature in what is a predominantly cold medium consumes energy, and a thick insulation layer made of fat (blubber) helps to combat this. And the leap into cold water begins with birth itself, which takes place underwater. Having emerged from the birth canal into the sea, which can be near to freezing, the newborn must swim unaided to the surface to take its first breath. To be nursed by its mother, it must then hold its breath and dive down again.


In water, acoustic orientation is more important than visual, because visibility in the sea is very limited. Sound waves, on the other hand, are diffused better in water than in air. So marine mammals communicate using calls, to keep the members of a school of whales together, for instance, or to signal possible dangers. Humpback whales in particular are known for their complex 'songs', which they break down into recognizable themes and phrases, and regularly repeat. Sound waves also help toothed whales to locate prey; indeed it is possible that the waves emitted are actually powerful enough to stun or even kill the victim.