Generalist Grazers Research Articles

The diets of Littoraria ardouiniana and L. melanostoma in Hong Kong mangroves

Littoraria ardouiniana and Littoraria melanostoma are common snails in Hong Kong, living and feeding on mangrove trees. Gut content and stable isotopic analyses were conducted to investigate the littorinid's diets. Gut content analyses revealed these snails ingested bark, epidermal plant cells, fungi, and microalgae, but that broken plant cells were the most abundant food items in the stomach and faecal contents. The gut contents of the two littorinid species, either from the mangrove trees Kandelia candel or Aegiceras corniculatum, were similar and showed little temporal variation throughout the year. Dual stable isotopic analysis, which investigated the δ13C and δ15N values of the littorinids and their potential food items, indicated that these littorinids might feed on mixed diets composed of parts of the mangrove trees and other items available on the trees such as phylloplane fungi, microalgae and cyanobacteria. These epiphytic mangrove littorinids are generalist grazers which graze on the substratum non-selectively as they are constrained on their host trees, and their diets are, therefore, dependent on food availability on the trees themselves.

PLASTID ENDOSYMBIONTS IN THE FRESHWATER CRUSTACEAN DAPHNIA OBTUSA

Symbioses between cyanobacteria (or eukaryotic algae and their plastids) and protists or invertebrates are well known and are important in evolution and ecology. However, no such symbioses have been previously reported in arthropods. An ultrastructural study of a branchiopod crustacean (Daphnia obtusa) from temporary ponds consistently revealed plastids inside gut endocytes. Plastids were most frequent in animals from shaded woodland ponds. Ultrastructure indicated that plastids in D. obtusa (a) are sequestered cyanobacteria and plastids from eukaryotic algae, and (b) senesce, suggesting that these generalist grazers have not closely coevolved with a plastid source. Daphnia obtusa (and other cladocerans?) in temporary ponds may benefit from plastid presence in several ways (nutrition, oxygen, calcium availability); daphnids inhabiting permanent waters may be less selected for plastid uptake. This paper represents an extension of plastid endosymbiosis into the Arthropoda, and indicates more sophisticated evolutionary and ecological interactions between these important crustacean herbivores and their food than previously recognized.

A FUNDAMENTAL TRADE-OFF IN RESOURCE EXPLOITATION BYDAPHNIAAND CONSEQUENCES TO PLANKTON COMMUNITIES

The genus Daphnia (Crustacea: Branchipoda) includes a diversity of species that are generalist grazers on plankton in freshwater lakes. Despite substantial intraspecific variation in body size, explanations of functional differences among Daphnia species continue to emphasize body size. In this study, we ask whether Daphnia in a relatively narrow range of body sizes differ in ability to exploit the array of resources commonly encountered in lakes. We compared juvenile growth rates of several clones and species of Daphnia fed natural planktonic resources that differed in overall dietary richness. We observed, both among and within species, a trade-off between growth rates in rich and poor resource conditions. We compared the ability of different species and clones to exploit natural resources in a lake mesocosm experiment, testing the prediction that clones displaying the greatest growth rates in rich resource conditions would have the highest minimum resource requirements. Using 8000-L whole water column enclosures, replicate populations of D. rosea and two different morphs of D. pulicaria were established separately in monoculture in a thermally stratified lake. An additional treatment without crustacean zooplankton was also established. We quantified resources in the enclosures using multiple indirect measures (e.g., chlorophyll a) and a growth bioassay. Results supported predictions; rankings of resource levels achieved in the different treatments suggest that Daphnia trade off high maximum growth rates for low minimum resource requirements. This field experiment also documented large differences among the species and clones of Daphnia in their impact on phytoplankton resources, water clarity, and formation of calcite particles (whiting). We suggest that these large ecological effects stem from the trade-off in abilities to use rich vs. poor resources and constitute an important aspect of grazer–resource interactions. We review the literature on growth and reproductive responses of zooplankton species to resource level and find general agreement with the concept of a trade-off between abilities to exploit rich and poor resource conditions. A reanalysis of data from a comprehensive study on Daphniidae growth confirms our results and suggests that the breadth of species in this family conforms to the trade-off. Furthermore, the trade-off is only weakly, and not significantly, associated with body size. Our study presents a new framework for understanding the functional significance of daphniid diversity to plankton food webs and illustrates how an ecological trade-off provides a means of linking species diversity to ecosystem function.

Effect of the bat star Asterina miniata (Brandt) on recruitment of the giant kelp Macrocystis pyrifera C. Agardh

The effect of the common bat star, Asterina miniata (Brandt), on recruitment of the giant kelp, Macrocystis pyrifera C. Agardh, was addressed through laboratory grazing experiments, a field experiment, and modeling of feeding behavior. In the laboratory, Asterina miniata significantly decreased the density of sporophytes that developed from 1-wk-old gametophytes as well as the percent cover of 2-, 6-, and 7-wk-old sporophytes. All grazed blades remaining at the end of these experiments subsequently died. Small scale variability in spore settlement and sporophyte development were also evident in the laboratory. Bat star density significantly affected short-term kelp recruitment during a large-scale field experiment. Small scale variability in kelp recruitment and bat star density was also evident during this experiment. Simple modeling suggested that high densities of Asterina miniata could graze nearly 100% of the bottom over the 90-day experiment. However, visible recruitment was seen in less than 30 days, and over this time, 36% of the substratum was predicted to have remained ungrazed. Macrocystis pyrifera of 1 to 3 cm in length may thus obtain a refuge in size from bat star grazing through rapid growth. These results indicate this generalist grazer can affect giant kelp recruitment but that even under high grazing pressure numerous plants survive. Bat star grazing probably does not contribute to large-scale differences in adult plant density but may contribute to small-scale patterns of dispersion.

Primary production, sinking fluxes and the microbial food web

The size distribution of pelagic producers and the size and trophic position of consumers determine the composition and magnitude of sinking fluxes from the surface communities in a simple model of oceanic food webs. Picoplankton, the dominant producers in the model, contribute little to the sinking material, due primarily to the large number of trophic steps between picoplankton and the consumers that produce the sinking particles. Net phytoplankton are important contributors to the sinking materials, despite accounting for a small fraction of the total primary production. These net phytoplankton, especially those capable of nitrogen fixation, also dominate the fraction of the new production that is exported on its first pass through the food chain. The sinking flux is strongly determined by the community structure of the consumers and varies by an order of magnitude for different food webs. The model indicates that generalist grazers, zooplankton that consume a broad size spectrum of prey (including pico-and nanoplankton), play a critical role in exporting particles. The role of generalists that occasionally form swarms, such as thaliaceans (salps and doliolids), can be particularly difficult to assess. Short-term studies probably miss the relatively infrequent population blooms of these grazers, events that could control the average, long-term exports from surface oceanic communities.

Differential survival of macroalgae to digestion by intertidal herbivore molluscs

This study evaluates the capacity of algal propagules to survive digestion by seven intertidal herbivore molluscs. Thirty individuals of Littorina peruviana, 36 of Siphonaria lessoni, 35 of Collisella ceciliana, and 25 each of C. zebrina, Chiton granosus, Fissurella crassa and F. limbata were collected at different dates between October, 1983 and April, 1984. About half of the individuals of each species were examined for gut contents and the other half was used to provide faecal pellets for culture. Diet of these grazers ranged from 8 macroalgal species in Collisella zebrina to 17 in Fissurella limbata. In total, macroalgal propagules of 56% of the 27 algal species found in these gut contents survive digestion. There was no positive correlation between diet and number of algal taxa growing in the respective faecal cultures. Survival through Littorina peruviana and Chiton granosus was low (20–30% of the algal species consumed) while it was high (75–83%) through the two species of Collisella. No significant correlation was found between frequency of algae in the gut contents and its frequency in the faecal cultures. Algal capacity to survive digestion was almost exclusively restricted to opportunistic species, especially in the Chlorophyta and Phaeophyta. It seems that survival of algal propagules through the digestive tract of generalist grazers is a rather random phenomenon.

The relation of food availability and food preferences to the field diet of an echinoid Evechinus chloroticus (Valenciennes)

Echinoids are important in causing disturbance in many marine communities yet little is known of their foraging behaviour in the field. This work investigates the field diet of an echinoid species, Evechinus chloroticus (Valenciennes), in relation to food availability and food preferences. E. chloroticus is a generalist grazer occurring abundantly in a variety of algal and encrusting communities dominated by sponges. In the field, urchins grazed the sponge species according to abundance. In the laboratory, food preferences were demonstrated for two populations of E. chloroticus when presented with 24 species of sponge. One population of urchins came from an intertidal reef where sponges were rarely encountered and the other from a sublittoral habitat where the encrusting community was dominated by sponges. The sponges used as food in the preference experiments came from two different areas: one where urchins were almost absent, the other (from which the second group of urchins were taken) where urchins were abundant and were observed to feed upon sponges. When presented with sponge species unfamiliar to both urchin groups, urchins ranked sponge species similarly. Food preferences did not relate to the diet of the urchins in the field. Reasons for the difference between experimental results and field observations are suggested.