Metasesarma obesum
Marble Crab turns up uncommon to rare on Taiwan's reefs, especially Xiaoliuqiu, in warm water year-round. Here's where to look.
Look for a small, mottled crab with a rounded, marbled carapace in shades of brown and grey that breaks up its outline against rock and rubble. It moves in short, deliberate bursts rather than swimming, tucking into crevices when approached. Size is on the small side for a sesarmid crab, so divers usually spot it by scanning ledges and gaps closely rather than seeing it out in the open.
Divers tend to find it wedged into rocky crevices or under ledges rather than out foraging in the open, which tracks with its uncommon-to-rare logging frequency at these sites. It doesn't react much to divers beyond retreating deeper into a gap if disturbed, so a slow approach and a torch for peering into dark spots pays off. Most sightings come from close-up reef searching rather than casual mid-water encounters.
Its mottled, marble-like shell pattern works as camouflage against the rocky, encrusted surfaces it hides in, which is likely why it reads as uncommon or rare even where it's present.
Sightings don't show a clear seasonal pattern, and the water it's found in runs from about 24.6 to 30.6 C, consistent with year-round tropical reef conditions in Taiwan. Plan on any month being fair game rather than chasing a specific season.
This one is currently a Taiwan speciality in the logged data, most reliably found around Xiaoliuqiu and also reported from Kenting. If you are diving either of these regions, keep an eye on rocky crevices and reef nooks rather than open sand or coral.
Mapping where Marble Crab is logged…
Logged sightings are currently concentrated in Taiwan, particularly around Xiaoliuqiu and Kenting. Search rocky crevices and reef ledges closely rather than open water in these regions.
There's no detected seasonal pattern, so sightings are treated as year-round. Water temperatures where it's found range from about 24.6 to 30.6 C, consistent with warm tropical reef conditions throughout the year.
Look for a small crab with a rounded, mottled brown and grey carapace tucked into rocky crevices or under ledges. It doesn't swim and tends to retreat rather than move across open substrate when disturbed.
No, this species poses no danger to divers. It's a small, retiring crab that hides in crevices rather than interacting with divers.
Exact size data isn't confirmed, but as a sesarmid crab it stays on the small side, typically recognised by its shell pattern rather than its bulk.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
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