Mutsunori Tokeshi

DEVELOPMENT OF A FORAGING MODEL FOR A FIELD POPULATION OF THE SOUTH AMERICAN SUN-STAR HELIASTER HELIANTHUS

SUMMARY (1) The objective of this study is to examine analytically size-related predation in a population of Heliaster helianthus in an intertidal rocky shore habitat in central Peru. Among 11243 prey individuals recovered from a total of 1132 feeding predators, the mussel Semimytilus algosus was predominant with an average proportion of 85-5% and 85-7% by number and biomass, respectively. (2) A graph describing the patterns of energy/biomass distribution among different size-classes of mussel prey in the field was obtained and used for comparison with the patterns of prey-size utilization by the predator population. Larger individuals of H. helianthus fed on clumps of Semimytilus algosus containing larger mussels, but the biomass distribution of all S. algosus consumed closely resembled that of the natural prey population, except in the smallest predator size-group (< 109 cm) where small mussels were more strongly represented in the diet. (3) A model was developed to account for the selection of meal sizes in the H. helianthus population. Frequency distributions of observed meal sizes for four predator size categories were compared and the patterns observed were generally in agreement with the model. (4) The study suggests that energy maximization, the core assumption of classic optimal diet choice models, is not relevant to the choice of prey size and meal size in H. helianthus. Difficulties in applying optimal foraging models based on the energy maximization premise to the analysis of foraging patterns in field animal populations are discussed.

On the feeding habits of Thienemannimyia festiva (Diptera: Chironomidae)

Feeding habits of a little‐known tanypod, Thienemannimyia festiva (Diptera: Chironomidae) were analysed using samples collected from Lough Neagh, Northern Ireland. Amongst a total of 215 individuals examined, a vast majority (90.2%) contained food material in the guts, almost always of animal origin. 91.2% of these feeding predators were observed with chironomid prey, 14,4% with harpapticid copepods and 7.7% with oligochaetes. Numerically, chironomids, harpapticids and oligochaetes accounted for 81.5%, 7.3% and 2.3% of all the prey taken, respectively. The modal number of prey items ingested was two for the fourth instar T. festiva (maximum ten items) and one for the third instars (maximum six items). Among the chironomid prey second and third instars accounted for 43.7% and 40.8% in the fourth instar predators and 78.8% and 6.1% in the third instar predators, respectively. Thus significant differences in diet were observed between the fourth and the third instar predators with respect to the number of pr...

Spatial distribution of two chironomid species in the bottom sediment of Lough Neagh, Northern Ireland

Distribution patterns in the larvae of two chironomid species with contrasting ecological traits, Ckironomus anthracinus Zetterstedt and Procladius signatus (Zetterstedt), were examined by taking replicate samples from Lough Neagh, Northern Ireland. The two species showed clear differences in both vertical and horizontal distributions. In terms of vertical distribution P. signatus larvae were restricted to the top 3 cm layer of sediment, while C. anthracinus larvae were most abundant in the 3–9 cm layer. Larvae rarely occurred below 12 cm of the sediment surface. In terms of horizontal distribution the two species departed significantly from random distribution, though the patterns of dispersion were different between the two. P. signatus demonstrated a more strongly aggregated pattern than C. anthracinus. The latter species showed a tendency towards a regular distribution, which may be related to its sedentary, territorial habit. Observed differences in spatial distribution between these two species may ...