Cd. Amsler et al., An antarctic feeding triangle: defensive interactions between macroalgae, sea urchins, and sea anemomes, MAR ECOL-PR, 183, 1999, pp. 105-114
A relationship between macroalgae (Phyllophora antarctica and Iridaea corda
ta), the sea urchin Sterechinus neumayeri, and the sea anemone Isotealia an
tarctica in Antarctica is described. Both macroalgal species are chemically
defended against herbivory by S. neumayeri. Where macroalgae and urchins c
o-occur in the field, over 95 % of the urchins use macroalgae as cover and
the vast majority of available drift is held by them. Urchins collected fro
m sites without macroalgae prefer macroalgae over other cover materials in
the laboratory, suggesting that they make an active behavioral choice to co
ver with macroalgae when available. Macroalgal cover acts as a defense agai
nst the major sea urchin predator, I. antarctica. Algal cover significantly
increases the likelihood that urchins will escape from I. antarctica becau
se the anemones' tentacles attach to the algae, which are subsequently rele
ased by the anemone or by both the urchin and the anemone. This defense is
physical as thalli from which defensive chemicals have been extracted are e
qually protective. Macroalgae appear to derive benefit from this relationsh
ip because fertile drift plants are retained in the photic zone where they
can continue to contribute to the gene pool. The urchins also extend the ef
fective horizontal and vertical distributions of the macroalgae, which may
help sustain the range of these algal populations in periods of reduced lig
ht availability. Hence, even though the macroalgae are chemically defended
from urchin herbivory, this relationship is apparently mutualistic. It bene
fits the sea urchins by providing a defense against their primary predator
and benefits the macroalgae by helping to sustain a reproductive population
.