Variable fang blennies Petroscirtes variabilis are small elongated fishes that can be encountered in coral reefs of Singapore’s Southern Islands. Apart from discarded glass bottles, they have also been found making their homes within the shells of large snails. Watch this video as the blenny leaves its bottle, makes an about-turn, and swims back into its bottle backwards!
Variable fang blennies can grow up to 15cm long but according to Wild Singapore, those seen around Singapore are mostly 5-7cm long. They are identified by their elongated body and blunt squarish snout. It gets its name from the pair of long curved fangs that grow on its lower jaw. It doesn’t have scales and has a continuous dorsal fin along the body length that it can flare impressively when startled. Colours are usually a drab olive green with pale markings, and a yellow underside. It has six large irregular dark blotches along its upper side. The males are orange-brown while females, like the one in the video, are sea-green above and lighter below. 
According to FishBase, the Variable fang blenny eats mainly small crustaceans and occasionally nips off scales from other fishes! Some are even known to scrape algae off broad seagrasses! How opportunistic!