An antarctic feeding triangle:defensive interactions between macroalgae, sea urchins, and sea anemones

Abstract

A relat~onshlp between macroalgae (Phyllophora antarchca and Irldaea cordata), the sea urchln Sterechnus neumayen and the sea anemone Isotealia antarctica in Antarct~ca is descnbed Both macroalgal specles are chemically defended against herb~vory by S neumayen Where macroalgae and urchins CO-occur in the field, over 95 % of the urchins use macroalgae as cover and the vast malonty of available dnft is held by them Urchins collected from sites without macroalgae prefer macroalgae over other cover materials in the laboratory suggesting that they make a n actlve behavoral c h o ~c e to cover with macroalgae when available Macroalgal cover acts as a defense against the major sea u ~c h i n predator, I antarctica Algal cover s ~g n ~f ~c a n t l y increases the likelihood that urchlns wlll escape from I antarchca because the anemones tentacles attach to the algae which are subsequently released by the anemone or by both the urchin and the anemone This defense IS physical as thalh from which defensive chemicals have been extracted are equally protective Macroalgae appear to derive benefit from this r e l a t ~o n s h ~p because fertile drlft plants are retalned in the p h o t ~c zone where they can continue to contnbute to the gene pool The urchins also extend the effective honzontal and verhcal distributions of the macroalgae, which may help sustaln the range of these algal populat~ons in periods of reduced l ~g h t availabihty Hence even though the macroalgae are chemically defended from urchln herbivory t h s relationsh~p IS apparently mutual~stic It benef~ts the sea urchlns by providing a defense against t h e ~r pnnlary predator and benefits the macroalgae by helplng to sustain a reproductive population