V. Jormalainen et al., Feeding preferences and performance of a marine isopod on seaweed hosts: cost of habitat specialization, MAR ECOL-PR, 220, 2001, pp. 219-230
The evolutionary hypotheses on plant-herbivore interaction assume that plan
t secondary compounds, such as the phlorotannins of brown algae, function a
s feeding deterrents for herbivores. We studied the effect of seaweed quali
ty on the feeding preferences and performance of the isopod Idotea baltica,
We offered I. baltica 6 species of algae, abundant in the Fucus vesiculosu
s belts where this mesograzer lives, in simultaneous preference tests. The
tests were conducted both with natural algae and with artificial food made
of freeze-dried and powdered algae of the same species, We found clear feed
ing preferences among the natural algae: the order of decreasing preference
was F. vesiculosus > Dictyosiphon foeniculaceus > Elachista fucicola > Pil
ayella littoralis > Enteromorpha intestinalis > Ceramium tenuicorne. The pr
eferences in the test with artificial food, however, did not parallel those
with natural algae, suggesting that the chemical quality of algae is not t
he major determinant of feeding preferences. Furthermore, performance of is
opods when reared on a diet of single algal species did not match the feedi
ng preferences of natural algae: the most preferred brown alga provided poo
r growth rate. Surprisingly, the more phlorotannin a seaweed species contai
ned, the more it was preferred by I. baltica. Moreover, the assimilation ef
ficiency of soluble sugars was generally high when isopods fed on brown alg
ae, and in the 2 species richest in phlorotannins it was not correlated wit
h the phlorotannin concentration of the algal individual. In contrast to th
e conventional assumption of the defensive function of phlorotannins, this
study shows that phlorotannins in seaweeds do not function as feeding deter
rents to I. baltica. Instead, this herbivore readily feeds on phenolic-rich
host plants, which, however, carries a cost in terms of decreased growth r
ate. We suggest that feeding preferences and habitat choice behavior evolve
together; habitat structure, in terms of predation avoidance, and the spat
iotemporal stability of the host algae are more important factors selecting
for feeding preferences in mesoherbivores than the chemical composition of
algae.