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Crystal-clear rivers in the jungle on Iriomote

In contrast to Ishigaki, the neighbouring island Iriomote is hardly developed and only sparsely populated. The island is impassable in the interior and is covered by subtropical jungle and crossed by rivers. On the coast there are mangrove swamps. Iriomote is home to numerous endemic animal species, the Iriomote cat is the best known, but also many reptiles and insects belong to it. On three days we investigated different rivers on Iriomote. There, besides water tests, light measurements were made and algae samples were examined under the microscope. In addition to green algae, abundant diatoms growing on the rocks are the main food source for various Aufwuchs eating fish and invertebrates. In the clear, sometimes fast-flowing water, fresh water pipefish, Macrobrachium shrimp and snails live alongside various colorful neon gobies. There are also seawater fish that migrate with the tides into the rivers. Large mullets, for example, have also been found in areas with pure fresh water! The further you get up the river, the more waterfalls and whirlpools are there. Here, in small pools, the large shrimp are well protected from predatory fish. The algae are grazed upon by snails. The further one gets up the river, the more colourful the neon gobies become. Butterflies visit  the rocks on the riverbank to absorb minerals; large colorful dragonflies are hunting above the river.

We used the evening on the island for a night excursion. Directly next to the canoe rental we find the first crabs and snakes. Pointing our flashlights into the mangroves, we find crabs sitting high up in the trees, in the water we see numerous fish, eel and brackish water shrimps. The whole evening we search for the largest land crab in the world, the coconut crab. But at first we witness another drama - a snake tries to swallow a young tortoise. Since the prey is much too big and both animals may die, we intervene and set the tortoise free. We also find spiders and insects in the shrubs. The crickets and locusts are well suited to document the autofluorescence of insects. Illuminated with a UV flashlight and photographed with our FluoClear camera filter, the crickets are glowing in a bright green. Also a land hermit shows this phenomenon clearly. But the coconut crab is the most impressive. It is a particularly beautiful, light blue colored specimen. The whole carapace is fluorescing clearly under UV light!