Focus on - Agarum: The Colander Weed

Agarum: The Colander Weed [Page 3]

We might expect that the benefit of this defense would somehow relate to the intensity of sea urchin grazing. At very low urchin grazing pressure, chemical defense would be of little or no benefit because the chance of being eaten would be very low anyway. At an extremely high grazing pressure, the defense would provide little benefit because it would be likely that most of the plants would be eaten while they were too small to be of any consequence to the herbivore. We would therefore expect maximum benefit from the chemical defense mechanism at some intermediate level of grazing intensity, for under such conditions many plants would escape grazing until they were large enough that they would be left ungrazed due to their chemical content.

This appears to be the case along the Northwest Atlantic coast. In the most heavily grazed areas, only a few short-lived algae survive, although isolated Agarum plants may be found here-and- there. In areas with few urchins, Agarum is confined to the understorey of other large kelps, where its other adaptations are more important. Where Agarum is by far the most abundant, however, is in areas with numerous sea urchins, but where grazing levels are intermediate and/or unpredictable. In Newfoundland, this is below (deeper than) the level of maximum urchin grazing, where Agarum often forms a zone where little else is visible; on boulders, vertical slopes, and the sides of surge channels within the middle depth range (c. 3-10 m). My observations in Labrador, the Bay of Fundy, and the eastern Arctic also support this scenario.

These ideas are, however, largely conjecture, and are unsupported by experimental research. In actual fact we know very little about the ecology of Agarum in the Northwest Atlantic. What factors influence the seasonal timing of reproduction in sporophytes and gametophytes? When do young sporophytes first appear? Are younger, smaller sporophytes more palatable to urchins than the older, larger ones? What roles do other factors such as temperature, light, and nutrients play in the ecology of Agarum? Even, why does Agarum have holes? The lack of answers to these and numerous other questions make Agarum a very good species to study for a masters or doctoral thesis in marine biology.

A dense bed of Agarum at 12 m depth in Conception Bay Newfoundland, Canada. The existence of such dense beds is probably a result of intermediate grazing by urchins combined with Agarum's presumed chemical defense against grazing. This interpretation is, however, open to debate.