I love small marine critters. Whenever I go diving, I’ll be like a treasure hunter – looking for the little gems living on rocks, rubbles, seagrasses, corals, giant clams, and yes – the sea urchins.

Sea urchins are close relatives of the sea stars, sea cucumbers, and sand dollars, collectively known as the echinoderms. They may not look very similar, but they are easily recognisable by their penta-radial symmetry (or five-points).

Despite its spiny appearance (and yes, you should not go around handling and touching them!), it may be hard to think of anything wanting to live on the sea urchins. Amazingly, there are numerous and different smaller marine animals living on them, such as shrimps, crabs, snails, and fishes!

Why choose to live on a sea urchin then? Just imagine riding on a mobile food truck – you don’t have to travel around on your own, AND you get free food! In most instances, the host sea urchin may also act as bodyguards of their residents against predation (note that this may not be active protection, since they just stuck themselves to the sea urchins!). It is definitely not worthwhile to get stung by a sea urchin!

Fig. 1. Zebra crab, Zebrida adamsii on the white ball sea urchin. Photo by Neo Mei Lin.

So how about the sea urchins? What do they gain from these hitchhikers? There are at least two different relationship status:

Commensalism – An association between two organisms in which one benefits and the other derives neither benefit nor harm. In such cases, the sea urchin is the latter species that gains no benefits or harm from their hitchhikers.

Parasitism – A non-mutual relationship between two organisms, where one species (parasite) benefits at the expense of the other, the host species. In such cases, the sea urchin is the latter species that becomes detrimentally affected by the parasitic species.

During our recent field surveys on counting sea urchins, we paid closer attention to the possible little denizens on sea urchins. Our observations have paid off and have recently published two short notes describing them! 🙂 But they were both parasitic to the host sea urchins.

The first is a Zebra crab (Fig. 1), which may potentially be parasitic to the sea urchins as we found wounds on the urchin’s body. What’s really super about this observation is that most sightings of a zebra crab is while diving, but here we report spotting this zebra crab while conducting intertidal surveys! No dives required!

The second is a known ectoparasitic snail (Fig. 2), typically found on sea urchins. These tiny snails no more than 1cm long sucks the fluids from the tube feet. When we first saw it, the parasitic snails reminded us of our days picking each one of them from the giant clams! Fortunately for the sea urchins, these snails tend to occur singly.

For more information, please click on the publications in the reference list. 😀

Vitreobalcis sp.
Fig. 2. An ectoparasite snail, Vitreobalcis sp. on the white ball sea urchin. Photo by Lynette Ying.


  • Ying, LSM (2017) Parasitic snails, Vitreobalcis sp., on white sea urchins at Cyrene Reef. Singapore Biodiversity Records 2017: 123-124.
  • Choy, C, LSM Ying, SHR Lee, MS Choo & ML Neo (2017) Zebra crab on a sea-urchin at Changi Beach. Singapore Biodiversity Records 2017: 96.