About the coconut crab and rare gobies on the island of Iriomote
In small and crystal-clear streams we observed many fish species while snorkeling at water temperatures of 22-24 °C. We observed longarm shrimps and carried out water analyses. With only about 2000 inhabitants, the island has large, untouched rainforest areas and a correspondingly diverse fauna.
We investigated three different river systems and their inhabitants. Most of the rivers come from the mountains and therefore contain quite cold but very clear water.
Small rapids make it repeatedly difficult for the fish to get upstream. In the last pond before a waterfall there are only shrimps and no fish. As soon as the predatory fish have gone, the shrimps come out of their hiding places and swim freely. In the ponds further downstream containing the predatory fish not a single shrimp was seen during the day. However, some were hidden under the stones.
The kingdom of colourful gobies
As regards colouring it would be hard to beat some of the Stiphodon goby species we encountered. In contrast the neon tetra looks like a black and white drawing! Those of us who managed to lay motionless in the water (at 22 °C, without a diving suit, only for the toughest) saw mating and fighting males in their full, multicolour splendour. But the really magnificent colouring only came from three species.
Stiphodon atropurpureus, male
The other species were more on the sandy side. Their adaptation to their habitat was noteworthy. Not even strong currents stopped the slender fish from grazing diatoms from the stones. The silicic acid content (silicates, SiO2) was correspondingly high at over 6 mg/l.
Other gobies (Rhinogobius) are more predatory and even eat snails!
The gobies’ social behaviour was also very interesting. In the Stiphodon species the females lived in groups of up to 8 animals. In all other species males or females were always found individually. As soon as two animals met, there was troubles.
Stiphodon semoni, 2 ♂
1 ♂, 5 ♀
Interesting water values of the Iriomote rivers
Measuring the water in your aquarium at home can be tedious, but it’s really exciting in the wild! You see fish and invertebrates and you really want to know what kind of water they live in. After all, our goal is still to learn more about our aquarium fish so that we can breed them and thus reduce the amount of fish taken from the wild.
Water temperature (surface)
1.5 ° dKH
> 6,0 mg/l
2.0 ° dGH
2.0 ° dKH
The low general and carbonate hardness values show that hardly any calcium and magnesium is present. The conductivity value of around 100 µS/cm, on the other hand, suggests that, in addition to calcium and magnesium, another, non-measurable metal is responsible for much of the conductivity. It can’t be potassium, because the K value showed 0. It could be sodium, but this is not measurable! The nature of the stones leads to the high silicate value and the high flow velocity with rippling "rapids" leads to the high oxygen content.
The fish in the rivers love JBL food!
Before departure each participant received some granulated food for freshwater fish (JBL GranoMix). In the various biotopes we tested how the fish (and what size of fish) reacted to this food, previously unknown to them. The acceptance of the JBL food surprised the participants again and again. The fish pounced on it as if they had never received anything else!
The king of all fresh waters: the marbled eel
With a length of a good two meters the giant mottled eel (Anguilla marmorata) was by far the largest freshwater fish in Iriomote. The huge ones had to be females, as the males only grow to 70 cm in length. The eels were also day-active and we were able to observe them swimming. However, their chances of success in hunting are much better at night and spotted flag tails (Kuhlia marginata) resting on the ground are good prey.
Fish from brackish water in freshwater
It is always refreshing on expeditions when we come across fish in biotopes where, according to research, they are not supposed to be. But fish can’t read, so we found spotted scats (Scatophagus argus), which are described as brackish water inhabitants, in pure fresh water with hardness 1-2 °dGH and a conductivity of 100 µS/cm! There they were, grazing the diatom coating off stones, without a care in the world.
They were eating the same as the small Stiphodon gobies, but in a slightly different habitat. The scats always sought out the quieter parts of the river. There their size made them superior to the gobies, and they could not eat in the strong current because of their body shape. The gobies were better adapted to that. This is a nice example of the biotope mountain river with its different habitats (strong current and low flow areas). Snappers (Lutjanus goldiei) can also penetrate much deeper into freshwater zones than is commonly believed.
We also observed the snappers in pure and soft fresh water. Moonyfish (Monodactylus argenteus), on the other hand, shied away from the soft water area and really seemed to prefer brackish water.
Neither freshwater nor marine water – the fascination of brackish water
Assuming a river doesn’t plunge through a waterfall straight into the sea, there is always an area where the freshwater and the marine water mingle. This brackish water area is the habitat of those fish species whose physiology enables them to tolerate changes in salinity.
One of the cutest inhabitants of this transitional region is the mudskipper, and it is one of the very few fish species to live on land too. These little guys can be found jumping around everywhere.
Besides the mudskippers, crabs are the most noticeable mangrove inhabitants. If you keep mangrove crabs, you may now want to convert your terrarium to adjust to our new findings: Some crab species climb up to FOUR meters high into trees!
At up to half the marine salinity, you’ll even find completely marine fish. Pomacentridae (damselfish and clownfish), snappers and even some surgeonfish swam relatively far up the river. Only when the salt content sinks further are you left with the completely brackish water inhabitants, such as moonyfish, puffer fish and grunters. The grunter Mesopristes ivani wasn’t noted on the island of Iriomote until 2002! The Japanese seem to know more about sushi than they do about their own fish species…
With such soft water it was hardly surprising that the snail species we saw displayed a lot of shell defects. If there is very little or no calcium (general hardness), they can’t build their shells properly.
We saw snail spawn, which we often find as white dots in our aquariums, everywhere. The snail larvae are washed into the sea and find their way back into the freshwater after the larval phase.
Water temperature (surface)
1.5 ° dGH
1.0 ° dKH
Out in the jungle at night
After dinner starts the most exciting time for terrarium friends. Armed with lamps, cameras and flashlights, you have half the team running around in the rainforest. Normally it’s good not to have too many people are at the same place at once, but when searching for animals our motto is, the more eyes to search, the more animals we’ll find!
Green frogs are hard to find in green leaves. Brown frogs on the other hand were jumping around practically everywhere.
We found three snake species, one of which was poisonous. Our snake specialist Maik Figura caught the snakes with a skilled hand, so we were able to have a closer look at these shy species. One snake (Dinodon rufozonatus walli) was trying unsuccessfully to eat a baby turtle (Cuora flavomarginata evelynae). Of course, you should not intervene in nature, but we saved this little turtle's life, because the snake wouldn’t have managed to eat it anyway!
The insect and spider fauna of Iriomotes is really exciting. As soon as the sun goes down it creeps and crawls everywhere. We photographed this tiny and highly enjoyable creepy crawly (except in bed) using macro lenses!
After several night walks we still hadn’t seen the biggest terrestrial hermit crab in the world. The coconut crab can live up to 60 years and weigh 4 kg! So we elicited help from some local guides and found a beautiful, blue coconut crab! These crabs can actually open coconuts with their enormous claws! You have to watch your fingers - cutting off fingers is easier than opening a nut. The southern Ryukyu Islands form the northern border of coconut crab distribution. Unfortunately they are considered a delicacy, even though this can have deadly consequences if eaten, probably because of the plant toxins they contain.
Biofluorescence of land animals
Fluorescent photography is really exciting. A special lamp (blue light) is used to render any animals capable of biofluorescence luminous. A yellow filter in front of the camera lens then makes the effect visible.
Animal discoveries during the day
During the day we also discovered some lizards (7 species), snakes (9 species), butterflies and other insects. This was not particularly species-rich, but still very interesting!
As on every expedition, the biotope analyses of the terrestrial habitats were also on the agenda. With the help of a UV-measuring device we determined the UV-A and UV-B values separately. A luxmeter indicated the amount of light and a data logger, attached to a tree in the shade, provided a record of air humidity and air temperature.
The green curve shows the relative humidity (62-92.2 %) during the 3 days of measurement, the blue line shows the air temperature (23-30.1 °C), and the purple line shows the dew point.
GPS data: longitude
GPS data: latitude
Sand: 31.5 °C
Water temperature (surface):
The most beautiful skeletons on planet earth
This may sound strange, but it's really true! On some beaches in Iriomote there is a unique sand that contains beautiful skeletons of foraminifera. These are unicellular organisms with shells, of which there are still 10,000 known species and about 40,000 fossil species! The beauty only really became visible under the Bresser microscope:
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