Trench warfare on the shore: interclonal aggression in sea anemones.

mpirical research in behavioral ecolE ogy is dominated by studies of verte brates and insects. The journal Animal Behavior, for example, during the last half of 1995 (Vol. SO), had a total of 48 papers onmammals, 43 on birds, eight on amphib ians and reptiles, 17 on fishes, 33 on terrestrial invertebrates (primarily insects), and five on aquatic invertebrates (only two of which were marine). Thus, it is something of an event when a paper on social organization appears whose subject is a marine invertebrate, a brainless one to boot. Two recent papers by Ayre and Grosbergl.2 r e mind us of how much behavioral biology we can learn from animals that consist of little more than a sack surrounded by tentacles. The organism in question is the sea anemone Anthopleura elegantissima, a conspicuous member of the mid-intertidal rocky shore community along the west coast of the USA. Sexually produced larvae recruit onto empty space on the shore,, but subsequent clonal proliferation potentially allows a single genetic individual to control substantial amounts of space. Clone-mates live cheek by jowl, but genetically distinct groups of anemones are typically separated by an anemone-free zone that calls to mind the no-man’s land between the trenches of opposing forces in World War 1. Like the Western Front, boundaries between clones can remain stable for yearss. The weapons of sea anemones are specialized attack tentacles (called acrorhagi) that are filled with nematocysts tiny spring-loaded and poison-filled harpoons. When two genetically distinct anemones are brought into contact, a characteristic aggressive response generally follows3~4. First, feeding tentacles are withdrawn and attack tentacles swell. Then, one or both individuals apply their attack tentacles to their opponents. The tips of the attack tentacles stick and leave behind a patch of epithelium loaded with nematocysts that cause localized tissue damage. In the case of repeated attacks without the possibility of retreat, the loser may die. Trench warfare is a socially organized behavior, and the biology of these clonal anemones suggests a number of further analogies. First, the anemones at the ‘front h e ’ are specialized for combat, with significantly larger attack tentacles and greater numbers of attack tentacles both absolutely and especially per gram wet weight13zt5, as compared to more centrally