Lebetus guilleti
A 1cm goby seen mostly in Brittany, France, in water as cool as 9.8C. Here's how divers spot this uncommon micro-fish.
At barely 1 cm long, this is a fish you find by scanning slowly, not by expecting it to swim into view. Look for a slender, near-translucent body resting on rock or coarse sand rather than hovering mid-water, with a rounded head and no bold colour blocks to catch the eye. There is no reliable colour pattern documented for divers to key on, so size and habit (still, low, easily overlooked) are the main clues.
Divers log this goby resting on the bottom rather than swimming in open water, consistent with its tiny size and low-profile lifestyle among rock and coarse substrate. It shows no particular reaction to divers, most likely because at 1 cm it is simply hard to notice unless you are looking closely and slowly. Expect it low on the reef structure rather than anywhere in the water column.
At around 1 cm, this is one of the smallest fish species divers in Brittany are likely to log at all, and its uncommon-to-rare sighting frequency at nearly every known site suggests it is hard to find rather than simply under-recorded.
There is not enough seasonal data to point to a particular month, so treat this as a year-round possibility rather than a seasonal target. Recorded water temperatures span roughly 9.8 to 17.7C, which fits Brittany's cool Atlantic conditions across most of the diving season rather than a narrow warm-water window.
This one is very much a Brittany fish: almost all logged sightings come from sites along the French Atlantic coast there, making it the clear stronghold for divers hoping to find it. A single record from Lanzarote in the Canary Islands shows it can turn up further south, but France, and Brittany specifically, is where you actually have a realistic shot.
Mapping where Guillet's Goby is logged…
Brittany, France, is by far the best-documented place to find it, with a single additional record from Lanzarote in the Canary Islands. If you're specifically hunting for it, Brittany is the region to focus on.
There isn't a clear seasonal pattern in the sighting data, so it's treated as a year-round possibility. Recorded water temperatures range from 9.8 to 17.7C, matching Brittany's cool coastal conditions across much of the year.
This species tops out at around 1 cm, making it one of the smallest fish a diver could realistically log on a dive.
Look for a tiny, slender, near-translucent fish resting still on rock or coarse sand rather than swimming freely. Its small size means it is usually spotted by careful, slow scanning rather than at a glance.
No. It poses no risk to divers and is far too small to be anything other than a curiosity to spot.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
Slender Goby
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