Diminutive dwellers of the mangroves, mudskippers and fiddler crabs

IT was an observant proboscis monkey, in a nearby tree, that caught my attention at Bako National Park, as his eyes were focussed, not on me, but on what he was seeing on the low tide mud flats of the mangrove swamp.

I followed his line of vision to see tens of fish skipping, flipping, and crawling around some 5m from my feet.

When I moved towards them, the proboscis monkey took off deeper into the mangrove forest and these mudskippers disappeared down their burrows only to re-emerge when they sensed that the coast was clear.

There are 23 species of this amphibious fish worldwide. Here, I focus on two mangrove swamp mudskippers found in Borneo: the Blue spotted mudskipper (Boleophthalmus boddarti), and the Orange spotted species (Periophtalmus chrysosplios).

Palaeontologists tell us that animal life began in the sea and then migrated to the land’s drier conditions and so these fish are pretty unique, but how do they survive?

Most fish die when out of the sea as their gills shut and become dry, but these mudskippers survive by holding a mixture of water and air in their very prominent gill chambers, enabling them to carry a regular supply of oxygen in a bubble of air to use on land almost like an aqualung.

Blue spotted mudskipper

Its Linnaean name is derived from Dr Pieter Boddaert, an 18th century Dutch physician and natural historian, who collected fish specimens for further study.

The first part of its name comes from the Greek word ‘bole’ meaning ejected, with the suffix meaning eye. In simple terms, it is a fish with bulging eyes mounted like small horns on the top of its head.

These fish very frequently blink as a response to life on land, and keep their eyes moist when out of water. This is achieved by producing a fluid on their corneas by a type of mucus. With lidless eyeballs and a flattened shaped lens, they are almost like land animals enabling them to focus on distant objects.

They can grow up to 20cm in length and are greyish brown to green in colour with distinctive black stripes along their back almost tiger-like and possess iridescent blue spots on their sides.

The fin on their back is normally folded down except for females, which openly display their fin with three or four spines protruding from the top of their fins.

They use their very short pectoral fins for walking. In wet mud, they use these like levers to skip along and use their tails like springs to jerk forward and leap for up to 1m.

These frontal fins are jointed and similar to limbs thus allowing them to crawl and even climb low hanging mangrove branches.

Burrowing and breeding

They dig burrows in the mud at low tide into which they disappear to avoid predators and at high tide, they maintain air pockets inside thus allowing them to breathe.

At breeding time, the males raise their dorsal fins and leap up into the air to attract females and even spar with each other.

Once a female is attracted, the male leads her to his burrow where she lays her eggs to be fertilised by him and she then exits.

The eggs are guarded by the male until they hatch. The young remain submerged until their eyes develop like nodules on the top of their heads and only when their gills fully develop do they emerge onto the mudflats.

Feeding

They feed by scraping the surface of the mud to obtain diatoms and green algae, but also devour small invertebrates, and even snatch insects from overhanging leaves.

While searching for food on a relatively dry mudflat, they cannot exist for much less than a minute without wetting their skin in a nearby pool of water.

The smaller species of mudskipper (Periophthalmus), also found in Borneo, is about 9cm in length and is a carnivore, feeding on very small prey such as small crabs and sea snails, waiting for the snails to emerge from their shells before attacking them.

Detailed accounts of these mudskippers may be found in the book entitled ‘Forest life and Adventures in the Malay Archipelago’ written by Dr Eric Mjoberg. He was a Swedish early 20th century zoologist and former curator of Sarawak State Museum from 1920 to 1922.

This book, first published in English in 1930, is very readable and entertaining.

Fiddler crabs of the mangroves

This crustacean is so named because of its two claws. One set of pincers is very large and the other small.

The small claw swiftly lifts food into its mouth and the frequent movement of this claw resembles the motion of a violinist’s hand when moving a bow up and down!

I have been mesmerised by these creatures at low tides both at Bako, and at Tanjung Aru public beach in Kota Kinabalu where they can reach densities of almost 60 crabs per square metre.

Recent research has estimated that there are 400-plus species of this crab worldwide, but here I shall concentrate only on those found in the mangroves.

Diet

Whilst young, these crabs exist on small worms and organisms living in the mudflats, together with leaves. As they mature, they generally live on the detritus of dead organic material, be it dead leaves or the corpses of other crustaceans, and even eat fresh mangrove leaves, for which they have been seen to climb trees.

When chewing its food, it sifts out edible material before ejecting the indigestible particles onto the sediment in the form of very small mud or silt balls. Trails of these silt balls can be seen around the crabs’ burrows making artistic patterns.

Burrows and mating

Rounded holes are excavated by fiddler crabs on the mudflats at low tide, allowing them to hide at high tide, yet maintaining oxygen levels by plugging the entrance with sediment.

These burrows are also used for mating after a male crab waves his enlarged claw in the air to attract a female.

This large claw is also used to fight off suitors!

The female enters the burrow, remaining there for two weeks before releasing her eggs in the ebb tide. These eggs remain as zooplankton for up to 14 days, before hatching into young crabs.

The lifespan of a fiddler crab is up to two years – unless they are eaten by hawks, macaques, fish, sharks, wading birds, and even humans.

Fiddler crab meat is not advisable for human consumption, for it has been found to contain microplastics from human pollution of the sea and the rivers that flow into the mangrove swamps.

Such mangrove swamps need protection and should be seen as nature reserves to prevent them from being viewed as potential areas for land reclamation for commercial use.

Such timid creatures should be allowed to exist along with a host of wildlife inhabitants.

Insignificant as mudskippers and fiddler crabs may seem to many humans, they are part and parcel of the mangrove system, tolerating the effects of tidal changes and living in symbiosis with their environments.

After all, mangrove trees provide the most natural effective defence systems as natural breakwaters against rising sea levels exacerbated by climate change.