So, you know, I’m cruising along, trying to determine the diet of the white urchin, Lytechinus anamesus, from the literature. There’s your usual “It eats kelp” papers, a few red algae papers, and nothing else special and then – A PAPER ON LYTICHINUS EATING OTHER SPECIES OF URCHINS.
That’s right, baby, urchin on urchin predation. Could the ocean get much weirder?
Turns out, on urchin barrens (i.e., areas with such ridonculous numbers of urchins – purples, reds, and even whites – that all of the surrounding edible algae has been grazed away) when white urchins get hungry enough, they turn into little pack animals. They mob red and purple urchins (but not each other) and gnaw away at their spines and test until they reach their gooey center.
Or, as gooey as the center of an echinoderm can be.
How intense is this predation? In lab trials, purples were mobbed within 20 minutes. 1/3 were dead by the end of the day. 90% of big fat red urchins tested were severely damaged.
These guys are not kidding around.
Perhaps more strangely, injured whites were left alone. Cannibalism for white urchins, apparently, is not on the menu.
So, dear reader, if you should find yourself hanging out in an urchin barren one day, just remember, don’t turn your back or the urchins will eat you!
(Also of note, the first author of this paper was the man who turned me into a scientific diver. Huh!)
COYER, J., ENGLE, J., AMBROSE, R., & NELSON, B. (1987). Utilization of purple and red sea urchins (Strongylocentrotus purpuratus Stimpson and S. franciscanus Agassiz) as food by the white sea urchin (Lytechinus anamesus Clark) in the field and laboratory Journal of Experimental Marine Biology and Ecology, 105 (1), 21-38 DOI: 10.1016/S0022-0981(87)80027-8