![]() ![]() Here, unlike the tropics, the seas change with the seasons. In spring thousands of mobula rays gather in Mexico's Sea of Cortez. At night, in a previously unseen event, tiny organisms that light up when disturbed react to their wingbeats, creating an enchanting bioluminescent firework display. Phytoplankton produce as much oxygen as all the plants on land and lie at the base of marine food chains everywhere. Where the plankton thrive, fish thrive too, and ocean travellers will migrate thousands of miles to take advantage of these productive seas. Predatory false killer whales off the coast of New Zealand are in search of dolphins. But when they find them, the whales team up with the dolphins to form super-pods - a formidable army to take advantage of the bounty of these seasonal seas. In temperate seas around the globe, spring brings greening oceans. In Japan, a kelp-covered shipwreck is home to the Asian sheepshead wrasse, or Kobudai. At the start of summer a male mates with the females. But when a female reaches both a critical body size and age, it can undergo an extraordinary metamorphosis. Females change gender, and a new male challenges an older male to a face-off. Toward our planet's poles, the ocean's surface is locked in ice. But in the Arctic, a warm current from the south keeps some Norwegian fjords ice-free all year round. Here, in winter, pods of orcas use dramatic tail slaps to stun herring, and humpback whales follow the noises to find the feast. Ocean currents move heat around our planet and maintain a climate favourable for life. But our ocean system, in relative equilibrium for millennia, is changing at a worrying rate. Deep in the polar north, we meet walrus mothers and their newborn calves, searching for an ice floe to rest on. But with rising temperatures, summer sea ice is retreating - their battles to survive are becoming ever harder. As we begin to understand the true complexity of the lives of our ocean creatures, so do we recognise the fragility of their home.Even in the dark zone of the ocean there is some light. If you turn off the submersible headlights, you can see a pyrotechnic display in the deep sea. These flashing lights are generated by animals - this is bioluminescence. A deep sea angler fish flashes in the darkness. The light is generated by bacteria that live permanently inside the fish's lures, and attracts prey to sets of murderous teeth. There are all sorts of lures out in the darkness.
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