Exyrias belissimus
Small, uncommon goby most reliably logged around Apra Harbor, Guam, in warm 26-29C reef water year-round.
A small goby, topping out around 10 cm, with a mottled brown-grey body that blends into rubble and mud-flecked reef substrate. Look for its habit of propping itself up on its front fins, head raised, scanning the area rather than swimming freely. The body is slender and tapers to a rounded tail, without any bold stripes or bright patches to key on, which is exactly why it is easy to overlook.
Divers most often find it resting on open mud, rubble, or sand patches rather than hovering over coral, propped on its pelvic fins in the classic goby stance. It tends to hold its ground at close range, ducking a short distance rather than bolting, which makes it a workable subject for a patient macro shot. Most sightings come from shallow harbour and reef-flat habitat rather than deep wall dives.
Its perching, sit-and-watch posture on open substrate is a classic goby trait, letting it keep an eye on passing prey and threats without wasting energy swimming in the open.
Observed in water between 26.2 and 29.3C, a narrow, consistently warm range typical of tropical reef flats, and there is no detected seasonal pattern in the sighting data. Treat it as a year-round species and plan your dive around site conditions rather than a particular month.
Most reliably logged in Guam, particularly around Apra Harbor and the Southern Coast, which together account for the bulk of recorded sightings. Smaller but confirmed populations turn up in Singapore's Southern Islands, at Tufi in Papua New Guinea, and in single-site records from Sabah in Malaysia, Koror & Malakal in Palau, and Sulawesi in Indonesia. Guam is by far the strongest bet if you specifically want to find this fish.
Mapping where Mud Reef-goby is logged…
Guam is the standout location, especially around Apra Harbor and the Southern Coast. It has also been recorded in Singapore's Southern Islands, at Tufi in Papua New Guinea, and in single sites in Sabah (Malaysia), Palau, and Sulawesi (Indonesia).
There is no strong seasonal pattern in the sighting data, and it has been observed across a narrow warm range of 26.2 to 29.3C, so it is best treated as present year-round on suitable reef flats.
Look for a small, mottled brown-grey goby around 10 cm long, propped up on its front fins on mud, sand, or rubble rather than swimming in open water. It lacks bright colours or bold stripes, so its perching posture and drab camouflage are the main clues.
They are small fish, typically reaching around 10 cm in length, which along with their camouflaged colouring makes them easy to swim past unnoticed.
No, they pose no danger to divers. They are small, sedentary bottom-dwellers that hold still or retreat a short distance when approached.
Its conservation status has not been formally assessed, so there is currently no listed threat category. There is no specific evidence of population concern from the diving sighting data.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
Slender Goby
803 sites
