The odd relationship between brittle stars and hairy jellies

I am always excited for our Palawan, Philippine snorkeling trips. One never knows what to expect as the chances of seeing anything from the largest fish to the smallest invertebrate! On this trip, however, we encountered a group of organisms that does not inspire excitement from our snorkelers. Bacuit Bay, El Nido, Palawan faces the South China Sea (West Philippine Sea). Except for a few islands that stand guard along the western perimeter of the bay, it is open to receive anything that is brought by winds and tides. Every few years, the bay experiences an invasion of some of the larger of the tropical jellies, hairy jellies (Lobonema smithii).

Their name derives from the thick tentacles that cover the entire bell. Sometimes referred to as a cannonball jelly, the bell can reach almost a meter in diameter, and with the trailing tentacles included, can exceed over a meter in length. Though a bit intimidating, they generally are not harmful as their sting is almost non-existent.

What’s most interesting about them is not necessarily their size or beauty (they come in a variety of pastel colors) but the hitchhikers that they bring. It is still a mystery as to how the brittle stars (often dozens are on each individual jelly) find their way onto their pelagic host. My guess is that, as larva, seek out the jellies (using some sort of chemical sensory to track them down), and upon settlement grow into the adult. What is known is that brittle stars not only gain protection from the jelly, but also get a free ride from the open ocean to their ultimate habitat: under rocks and boulders on the coral reef. How do they know when to leave their host? It is also my guess that brittle stars abandon the jelly while it is near death and with no longer having the ability to swim, rolls helplessly along the reef.

This fascinating migration compliments the awe at seeing dozens of brittle stars entwined in the tentacles of the jelly or crawling over the massive bell. For our group of snorkelers, fear of these jellies was immediately conquered by the desire to simply swim with these giants and marvel at their beauty, size, and, yes, their hitchhikers.