Monterey Bay Aquarium Research Institute
Marine Botany

Egregia menziesii   Grazing and Razing

Being a macroscopic alga of the incredibly diverse intertidal, Egregia has some very interesting ecological relationships, especially with the limpet, Acmaea insessa. The crab, Pugettia producta is also commonly associated with Egregia menzeisii.


According to a study by Leighton in 1971, Egregia is the most highly prefered macroalgal food source of a number of invertebrate species, including Aplysia, Norrisia, Haliotis corrugenta, and Haliotis fulgens. Lytechinus, Stongylocentrotus purpuratus, and Stongylocentrotus franciscanus, however, do not tend to graze on Egregia. (1)

Acmaea insessa
One species, Acmaea insessa, is an obligate grazer of Egregia.   Living on and eating only Egregia, this limpet causes extensive damage to the kelp.  Not only is plant tissue lost directly to the feeding organism, but the holes made by the limpet cause weaknesses which lead to increased chance of breakage at that spot.  However, this can actually be quite advantageous to the plant in that it is limited to a reasonable size, and not torn off the rock as easily as non-grazed plants during rough waves. (2)

A. insessa feeds on the epidermal and cortical cells of the rachis and stipe.  The limpets begin appearing on young plants in the spring and summer months when Egregia is growing rapidly, and remain even when the plant is old, tattered, and worn. As the limpet feeds, it forms deep depressions in the plant, which apparently provide some protection for the animal against desiccation, abrasion, and wave action.  According to Black, small limpets tend to settle in large depressions previously made from older limpets who have moved on to fresher portions of the stipe.   In addition, small limpets are much more likely to settle on already crowded rachis, making the difference in concentration of limpets on grazed and fairly ungrazed plants more exteme. (2)

Damage done by the limpets did not show to affect growth rate or reproduction of the algae, but a greater occurance of frond loss due to limpet grazing did lead to an increase in production of fronds by the plant that remains intact. Plants which are not heavily grazed tend to produce so many fronds, which are not as easily torn off, that the chance of the whole plant, including the holdfast, being ripped off is greatly increased due to the greater drag of the dense plant. (2)

Pugettia producta
This dark, olive-green, spider crab is also quite often found on Egregia.  So often, in fact, that it is commonly called the kelp crab.  It has sharp spines on its legs to allow it to hold on to the algae as it is thrown about in the waves. (3)

References for this page:
(1) Round, F.E. The Ecology of Algae. Cambridge University Press, Cambridge. 1981.

(2) Black, Robert.   "The Effects of Grazing by the Limpet, Acmaea insessa, on the Kelp, Egregia laevigata, in the Intertidal Zone." Ecology 57 (1976): 265-277.

© 1999 Sarah Present. Contact for any non-educational use.
Last updated: Feb. 05, 2009