M.R. Reeve

Nutritional Ecology of Ctenophores—A Review of Recent Research

Publisher Summary This chapter summarizes information on nutritional biology (from observations on living animals mostly in the laboratory) of ctenophores and discusses attempts to utilize laboratory data to interpret environmental fluctuations. Fraser, Kremer, Greve have pointed out that ctenophores should not be considered merely wasteful “dead ends” in the food chain. They may act, sometimes, to balance the ecosystem by restraining an overabundance of copepods from virtually eliminating all phytoplankton from the water column at a time when other more useful predators are failing to do this. The phytoplankton would also receive a positive stimulation to renewed growth by virtue of the high percentage of ingested nitrogen being returned in a dissolved form back to the water column. It is possible that the result might otherwise be the production of dead copepod biomass that accumulated in the sediments and was lost to the water column. Growth rate and fecundity of ctenophores are highlighted. The chapter also discusses seasonal variations in ctenophore populations.

Observations on the existence of lower threshold and upper critical food concentrations for the copepod Acartia tons a Dana

Acartia tonsa Dana represents a genus of temperate-tropical inshore copepods which, by virtue of its high biomass and rapid generation times, may be assumed to be an important planktonic primary consumer in terms of total production world-wide; its grazing response to different food concentrations is little known. Using wide ranges of concentrations of phytoplankton cultures, we have found that A. tonsa has a maximum grazing rate of ≈ 10.0 μg chl a1, decreasing to zero below 1.0 μg chl a1. This was confirmed using the more limited range of naturally-occurring particulate material. Although grazing rate became progressively reduced above 10 μg chl a1, ingestion rates continued to increase over the next order of magnitude of food concentration. It appears that A. tonsa is rarely exposed to food concentrations in its natural environment (as measured by conventional techniques) high enough to stimulate the maximum grazing effort. On the other hand, it is suggested that the continued increase of ingestion rates in the laboratory at much higher concentrations may indicate an adaptive mechanism associated with the encountering of ephemeral micro-patches of such concentrations, which would permit rapid filling of an empty gut in an energy-efficient manner. Fecal pellet production was also measured and shown to be a good indicator of ingestion rate.

Conditions of culture, food-size selection, and the effects of temperature and salinity on growth rate and generation time in Sagitta hispida Conant☆

Abstract Methods are described whereby Sagitta hispida Conant was collected, induced to spawn, and the resulting larvae raised to maturity. Selection of food by size increased with age, although the larvae ingested food larger in size relative to their head width than did mature animals. There are wide variations in the size of individuals in growing populations, but mean growth rates are directly proportional, and maximum sizes inversely proportional, to temperatures between 17 and 31 °C. Populations reached maturity in salinites from 25 to 40% and showed less variability with salinity than with temperature. The minimum and maximum time for a population to reach maturity from the egg was 18 and 50 days, respectively.

Digestive efficiency of the chaetognath Sagitta hispida Conant

Abstract A technique is described whereby the digestive efficiency of a plankton carnivore, Sagitta hispida Conant, which tends to feed infrequently on relatively large food organisms, was determined gravimetrically. The mean digestive efficiency in 24 experiments was 80 % (82 % on an ash-free basis) which was not affected by the size of the food organism or pre-starvation period. Since the chaetognath assimilated variable quantities of the inorganic fraction of its food (mean 43 %) calculations of digestive efficiency by Conovers ‘ratio’ method underestimated the true values.

A Large-Scale Experiment on the Growth and Predation Potential of Ctenophore Populations

In reviewing the ecology of planktonic ctenophores, Walter (1976) pointed out how laboratory estimations of such parameters as growth, feeding and fecundity had been utilized to interpret population dynamics of ctenophores in the natural environment. In organisms with short life-cycles, which inhabit the water column, it is usually impossible to follow the growth and mortality characteristics of cohorts by the use of such techniques as size/frequency histograms. The observer can never be sure he is sampling the same population, over a period of time, and even if this were so, continuous breeding and rapid growth tend to quickly obscure the identification of distinct cohorts. It is also not possible to experimentally manipulate the population. This report describes an experiment in which plankton populations were studied in three containers for six weeks following empoundment of a water column and its natural plankton assemblage.

The distribution of organic carbon in a marine planktonic food web following nutrient enrichment

Abstract Low level nutrient enrichment of four enclosed water columns showed an increased production with nutrients but a decrease in transfer efficiency between primary producers and ctenophore production. From an extrapolation of primary productivity levels in the enriched containers to one unenriched container it is found that the nitrogen flux was ~ 1.52 mg-at. N/m 2 /day which allows for an approximate doubling of the nitrogen supply as calculated from winter nitrate levels. A carbon budget for each container was calculated for primary, secondary, and tertiary producers; decrease in transfer efficiencies were accounted for at various points in the food web.

Trophic Interactions and Production Processes in Natural Zooplankton Communities in Enclosed Water Columns

A major advantage of large volume in situ enclosures over small-scale laboratory cultures is that the large enclosures enable study of interactions at higher trophic levels while maintaining environmental concentrations of organisms typical of the natural environment. Small-volume cultures have been used successfully to investigate the dynamics of natural assemblages of phytoplankton species--

The use of large volume, transparent, enclosed sea-surface water columns in the study of stress on plankton ecosystems

A brief account is given of the Controlled Ecosystem Pollution Experiment (CEPEX) program, with results from a recent experiment disigned to ascertain effects of mercury on planktonic ecosystems. The experimental modules are 1300 m3 transparent, flexible, polyethylene cylinders 9.5 m in diameter and 23.5 m deep, floating for periods up to three months at the sea surface of Saanich Inlet, British Columbia. Groups of three cylinders are filled simultaneously by being raised to the surface from 35 m, a procedure shown to produce biological replicability. An experiment is described in which mercury at 1 and 5 μg l−1 was added to two modules, with a third serving as a control. In both polluted modules temporary reductions in heterotrophic activity of micro-organisms and carbon assimilation of phytoplankton were observed, followed by a return to control levels. Only at the higher mercury concentration were any effects observed in the zooplankton, which included reduced feeding inCalanus, mortality and molting failure ofPseudocalanus and decreased growth in young fish.

Introduction and Description of Experimental Ecosystems

The Symposium on Enclosed Marine Experimental Ecosystems took place over 4 days between 13-16 August 1980 at the Institute of Ocean Sciences, which overlooks Saanich Inlet (Sidney, British Columbia, Canada). The location was the site of the CEPEX program (Controlled Ecosystem Pollution, later Populations, Experiment) during its 6 years of operation and had come to be associated with large enclosure work in the minds of many Symposium attendees, either through direct participation, or as visitors over this period. The Symposium was sponsored by the U.S. National Science Foundation as part of its final year of support to the CEPEX program and by the Canadian Research Council. The gathering attracted about 100 participants from 9 countries, and 45 papers were presented.

Visual observations on the process of digestion and the production of faecal pellets in the chaetognath Sagitta hispida Conant

Abstract The passage of food through the gut of Sagitta hispida Conant is described as a batch process and the formation of membranes which surround the faecal material is demonstrated photographically. The utility of such a peritrophic membrane system is considered in the light of the marine ecosystem as a whole.