Feeding preferences and performance of a marine isopod on seaweed hosts: cost of habitat specialization

Abstract

The evolutionary hypotheses on plant-herbivore interaction assume that plant secondary compounds, such as the phlorotannins of brown algae, function as feeding deterrents for herbivores. We studied the effect of seaweed quality on the feeding preferences and performance of the isopod Idotea baltica. We offered I. baltica 6 species of algae, abundant in the Fucus vesiculosus 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 feeding preferences among the natural algae: the order of decreasing preference was F. vesiculosus > Dictyosiphon foeniculaceus > Elachista fucicola > Pilayella littoralis > Enteromorpha intestinalis > Ceramium tenuicorne. The preferences in the test with artificial food, however, did not parallel those with natural algae, suggesting that the chemical quality of algae is not the major determinant of feeding preferences. Furthermore, performance of isopods when reared on a diet of single algal species did not match the feeding preferences of natural algae: the most preferred brown alga provided poor growth rate. Surprisingly, the more phlorotannin a seaweed species contained, the more it was preferred by I. baltica. Moreover, the assimilation efficiency of soluble sugars was generally high when isopods fed on brown algae, and in the 2 species richest in phlorotannins it was not correlated with the phlorotannin concentration of the algal individual. In contrast to the conventional assumption of the defensive function of phlorotannins, this study shows that phlorotannins in seaweeds do not function as feeding deterrents to I. baltica. Instead, this herbivore readily feeds on phenolic-rich host plants, which, however, carries a cost in terms of decreased growth rate. We suggest that feeding preferences and habitat choice behavior evolve together; habitat structure, in terms of predation avoidance, and the spatiotemporal stability of the host algae are more important factors selecting for feeding preferences in mesoherbivores than the chemical composition of algae.

Trophic diversity of idoteids (Crustacea, Isopoda) inhabiting the Posidonia oceanica litter

“…From this, we could expect that these species search for specific food items such as microbial colonization of the food. However, Jormalainen et al (2001) suggested that in I. balthica, habitat structure, in terms of predation avoidance, and the spatiotemporal stability of the habitat are more important factors selecting for feeding preferences than the quality of the food.…”

“…Hence, the coexistence of these species in the same biotope raised the question about trophic diversity in this particular environment. To our knowledge, no data are available about the feeding ecology of I. hectica and C. prismatica, while I. balthica food includes both vegetable (plant and algae) and animal materials (Naylor 1955;Nicotri 1980;Jormalainen et al 2001). Laboratory observations suggested that I. balthica actively consume living and decaying P. oceanica material (Lorenti and Fresi 1983b).…”

Inducible resistance to herbivory in Fucus vesiculosus—duration, spreading and variation with nutrient availability

“…After the juvenile stage its diet consists mainly of Fucus vesiculosus (Salemaa 1987, Schaffelke et al 1995, Jormalainen et al 2001a) and, to a lesser extent, of filamentous epiphytic algae. The presence of I. baltica in F. vesiculosus habitats is highly variable over the course of the year.…”

Seasonal variability in the grazing potential of the invasive amphipod Gammarus tigrinus and the native amphipod Gammarus salinus (Amphipoda: Crustacea) in the northern Baltic Sea

“…Large and efficient herbivore species such as sea urchins, periwinkles or limpets are lacking and the low diverse communities are often dominated by resource generalists such as gammarid amphipods and idoteids (Salemaa 1979;Jormalainen et al 2001;Orav-Kotta and Kotta 2003). The native Gammarus salinus Spooner and the invasive Gammarus tigrinus Sexton are the prevailing gammarid amphipods in the coastal range of the northern Baltic Sea (Hällfors et al 1981;Herkül and Kotta 2007).…”