There are three species of zebra crab, and their specific care may vary a little bit from one to the other, but they should be fairly similar.
Despite the claims that they can live without an urchin, I would not get one without an urchin for them to live on - these are obligate symbionts (they are actually thought to be parasitic to the urchins, feeding at least somewhat on their tubefeet) with urchins in the wild, so even if they can survive without one, they would likely be incredibly stressed and susceptible to health issues as a result.
Males are more likely to leave the urchin and wander around while females are thought to pretty much just stay put on their urchin.
Some species of urchin that are known hosts for at least one of the zebra crab species:
"Diadema setosum, Tripneustes
gratilla (cf. Yamamoto, 1973; Suzuki and Takeda,1974), elegans, and Asthenosolma ijimai (cf. Doki, 1972), Salmacis virgulata (cf. Daniel and Krishnan,1979), Toxopneustes pileolus (cf. Saravananet al., 2015),
T. pileolus and Salmacis bicolor (cf. Rathbun, 1910), Anthocidaris crassipina (cf. Urita, 1926), Acanthocidaris sp. (cf. Balss, 1956) and Pseudocentrotus depressus (cf. Rathbun, 1910; Urita,
1926a; Balss, 1956; Doki, 1972; Yamamoto, 1973; Suzuki and Takeda, 1974)."
Source:
https://www.researchgate.net/public..._LINNAEUS_1758_REPORT_FROM_KERALA_COAST_INDIA
