Preferential Feeding: An Optimization Strategy in Sea Urchins

Abstract

Strongylocentrotus drobachiensis and S. franciscanus exhibited strong algal preferences in laboratory studies. A generalized ranking of algae from most to least preferred for both sea urchins includes: Nereocystis luetkeana, Costaria costata, Laminaria saccharina, L. groenlandica, Monostroma fuscum, Opuntiella californica, Agarum fimbriatum and A. cribrosum. Similar but weaker preferences were exhibited by S. purpuratus. Preference experiments showed that substance(s) readily sloughed from the fronds of Nereocystis attract urchins whereas substances from A. fimbriatum repel or are not detected by urchins. In the field urchins in contact with or on the fronds of Agarum spp. revealed a low proportion of feeding individuals except during winter. Food preferences were inversely correlated with the caloric content of algae. Although preferences generally were weakly correlated with caloric intake, this relationship is an artifact of the high feeding rates on preferred algae. Absorption efficiencies for the three urchins were strongly correlated with food preference rankings. Mean values for the three species ranged from 84% to 91% on Nereocystis, 77%—83% on Costaria, 64%—78% on L. saccharina, 62%—71% on Callophyllis flabellulata, 28%—56% on J. fuscum, 40%—56% on A. cribrosum and 36%—52% on A. fimbriatum. Growth in diameter and weight of S. drobachiensis fed specific algae for 15 mo correlated positively with algal preference rankings: urchins raised on Nereocystis or L. saccharina grew faster and produced significantly larger tests than animals raised on either species of Agarum. At the end of 15 mo, 50% of the animals were placed on a reciprocal diet from that on which they were originally grown, i.e., from preferred to nonpreferred algae or vice versa. The other half were retained on the original diet as controls. At the end of 5 mo urchins maintained on or transferred to preferred algae produced the largest tests (diameter and weight). Growth rates, however, were highest with urchins transferred from A. cribrosum to Nereocystis. Conversely, poorest growth occurred with the reciprocal transfer. Similarly, reproductive values, gonad indexes, and gonad weights were highest for urchins maintained on a continuous (20—mo) diet of Nereocystis. The second highest reproductivv values occurred with animals transferred from A. cribrosum to Nereocystis. Lowest values occurred with urchins maintained continuously on a diet of Agarum. Field observations on feeding at seven sites showed that S. drobachiensis and S. franciscanus fed primarily on Nereocystis (27%—40%) and ulvoids (Monostroma—Ulva spp.) (24%—28%). Other species accounted for less than 10%; brown and green algae comprised 64% and 26%, respectively, of the diet of these urchins. Qualitative and quantitative (density, frequency percentage, relative biomass) measures of algal availability showed that ulvoids and Alaria dominated shallow (0—4 m) subtidal depths. With few exceptions A. cribrosum dominated deeper depths (4—12 m) at most sites and the areas in which most feeding observations were made. Nereocystis dominated only one site (in terms of relative biomass). Unlike A. cribrosum, ulvoids, Laminaria spp., and other understory species, Nereocystis was not readily available to urchins on the bottom because of its large surface canopy, narrow stipe, and small holdfast. It became more available, however, when dislodged by grazers and storms. Feeding by sea urchins in nature is a compromise between urchin preferences and algal availability. The disproportionately high feeding percentages on Nereocystis relative to its physiognomy and low abundance is due to selective feeding. Similarly, feeding on L. saccharina and possibly filamentous diatoms is interpreted as selective. The high feeding percentage on ulvoids is a reflection of availability. With algae of intermediate preference in laboratory experiments, feeding in nature is interpreted as the result primarily of availability, e.g., Alaria, L. groenlandica and L. complanata. Costaria, although highly preferred in experiments, was taken more in proportion to availability in the field. With the dominant and most available prey (Agarum spp.) and with Opuntiella there was an active avoidance by urchins suggesting that these algae have evolved chemical or less likely physical defensive systems that reduce browsing by sea urchins. Field measurements of sizes (diameter and weight) and reproductive capacities (gonad weights) of S. drobachiensis within food—limited, relatively food—limited, and food—unlimited environments revealed significant differences between populations in all but test diameter. These observations are in good accord with laboratory experiments. Urchins in the food—unlimited and relatively food—limited sites had available ample amounts of preferred and nonpreferred prey, respectively, e.g., Nereocystis, Costaria, and Laminaria spp. vs. Agarum. The growth and reproductive capacities of these populations parallel laboratory experiments. This algal—herbivore interaction has led to important adaptations and strategies by both the sea urchins and algae. Important or dominant subtidal algae in the San Juan Islands have evolved an opportunistic strategy (r—selection), especially those species highly preferred by urchins, or a perennial strategy (K—selection), especially those species not preferred by urchins. Sea urchins, on the other hand, have evolved a keen sense of prey detection, one that increases their intake and absorption of prey, their growth, and their reproductive potential. These measures can be considered 'components of fitness' and the ensuing selective feeding behavior as an optimization strategy. The question of what is being optimized by urchins was examined using Emlen's model. The food value of algae (based on urchin growth and reproduction) was...