 |
Mudskippers are amphibious air-breathing gobies found on intertidal mudflats throughout the Indo-west Pacific. They are the only fishes to carry out many of their major activitiesincluding feeding, courting, and defending territorieson land. In order to manage these feats, they rely on a number of physiological and morphological specializations for life on land, including eyes modified for clear vision in air, leglike fins for walking, climbing, and leaping, and structural modifications to the skin and gill chambers that allow these fishes to breathe in both water and air.
Although the aboveground activities of mudskippers have been studied for over 300 years, little is known about their existence belowground. The ability to leave the water has allowed these fishes to exploit their mudflat habitat, but for many mudskipper species, the watery burrow is a home base from which to launch terrestrial forays and to which it can retreat when threatened by predators. Besides serving as a refuge, the burrow may also be a nursery for developing eggsmembers of two of the four mudskipper genera are known to lay their eggs in the burrow, and quite possibly the other two genera do as well. While the burrow seems essential to the safety of both adults and young, it nevertheless comes with a danger of its ownthe water inside the burrow is perilously low in oxygen. Mudskippers must somehow tolerate or overcome this oxygen shortage, and this video reveals how.
The mudskipper Scartelaos histophorus, also known as the bearded goby because of the short barbels on its lower jaw, builds an air pocket in its mudflat burrow during low tide, when the burrow entrance is exposed to air. When the tide comes in and covers the opening, the bearded gobies can take refuge in their burrows and rely on the air they've stored there for oxygen.
The first segment of the video shows the S. histophorus aboveground air-deposition behavior on a mudflat near the small town of Cardwell, on the northeast coast of Australia (Figure 1). Like a farmer lugging bucket after bucket of water to fill a trough, this bearded goby repeatedly gulps air and transports it into the burrow to create an air pocket for respiration. One sign that the fish has taken a gulp of air is that its buccal and opercular chambers are inflated (Figure 2) and this occurs immediately before the fish enters the burrow. The chambers are deflated when the fish reappears at the surface, indicating that it has released its air breath while in the burrow.
While we were able to observe S. histophorus gulping air at the entrance to its burrow on the surface of the Cardwell mudflats, we still didn't know exactly what was happening inside the burrow. To find out, we designed and built an artificial mudskipper burrow system (Figure 3), the first of its kind, so that we could observe the fish as it created an air pocket in the burrow.
The last segment of the video shows the S. histophorus subterranean air-deposition behavior in our laboratory burrow. A bearded goby with a mouthful of air must swim vigorously to overcome buoyancy; once inside the burrow, we observed, it floats against the ceiling until the mouthful of air is released and it loses buoyancy. The fish may then either settle to the muddy floor of the burrow, make immediate use of the reserve by taking a breath of air, or return to the surface to collect another gulp. This air pocket is probably essential to Scartelaos when confined in its burrow at high tide and would thus be doubly important for mating pairs sharing a burrow. We further suspect that pairs may lay their eggs in or near the air pocket. Work on this project is ongoing and we hope that our experimental burrow system will help to unravel even more details of the belowground activities of these fish.
Figures 1 and 2. (1) The mudflat at Cardwell, Australia. (2) Scartelaos histophorus pair at their burrow entrance. Note the inflated buccal chambers indicating that the fish are holding air breaths.
Figure 3. Artificial laboratory burrow system. The fish can access the mudflat and burrow through the connecting tube.
|
|
|
