Nutritional ecology of the ascidian Pyura stolonifera: influence of body size, food quantity and quality on filter-feeding, respiration, assimilation efficiency and energy balance

Abstract

Effects of body size, cell density and food type on filtration rate, respiration rate and assimilation efficiency in the ascidian Ppra stolonifera were determined using a continuous-flow circulating seawater system. Filtration and respiration rates were related to body size according to the regressions: FR (m1 min-') = 8.15 and RR (m1 h-') = 0.14 W9? Assimilation efficiency was independent of body size for the consumption of the alga Dunaliella prirnolecta but was correlated (AE = 21.1 + 3.3 W, r = 0.66) with size for natural food. Different types of food, such as phytoplankton species and kelp detritus which occur in the natural environment, were filtered by the ascidian at equal efficiency and rate. Under controlled conditions, the organic fraction of kelp particles is assimilated at 42 % efficiency while D. primolecta cells are assimilated at 75 % efficiency. Food in nature, comprising a high proportion of detritus with varying amounts of silt, was assimilated at 34 % efficiency. Filtration. respiration and assimilation rate were all independent of cell density which varied between 1.0 and 70 X 106 cells 1-I of D. primolecta. At the lower cell densities there was a 'switching-off' of filtering. High silt loads (25 mg 1-l , 3 % OM) or high density of large-sized particles (> 65 ~m) resulted in 'backsquirting' to reject this material through the inhalent siphon. Additions of silt which produce particle concentrations found in the natural environment of P. stolonifera did not affect filtration rate. Net energy balance and the conversion efficiency for artificial and natural diets were estimated. The potential for utilization of natural food resources by P. stolonifera is compared with other members of the filter-feeding community.

Seasonal variation of in situ feeding rates by the temperate ascidian Halocynthia papillosa

“…The experiments covered the natural range of prey concentration (10 to 50 pg llve carbon 1-', 19 to 813 pg C 1-' of detrital origin, Rlbes 1998), and within this range H. papillosa CRAFDw was not significantly affected by food concentration. These results agree with previous studies carried out with graphite particles (Jsrgensen 1955) and with food that resembles the composition of natural seston (Klumpp 1984). However, in laboratory studies.…”

“…These potential food sources are retained by the mucous net secreted by the endostyle in the pharyngeal cavity and afterwards transported as a food cord to the intestine (FialaMedioni 1978b, Flood & Fiala-Medioni 1981. It has been pointed out that large particles (>l00 pm) usually do not reach the brachial cavity because of retention by the oral tentacles (Millar 1971, Klumpp 1984. However, some studies which examined the gut contents of ascidians have shown that some species consume a variety of invertebrate larvae and eggs (Young 1988, Bingham & Walters 1989.…”

“…As mentioned in the 'Introduction', temperature, food concentration, body size and oxygen tension have been suggested as the main factors that affect filtration rate (FialaMedioni 1978a, b, c, Robbins 1983, Klumpp 1984, Petersen & Riisgdrd 1992). We did not examine the effect of oxygen tension on filtration rate of Halocynthia papillosa because a.11 experiments were performed in the field, where oxygen concentration was always fully saturated or slightly supersaturated.…”

“…Halocynthia papillosa specimens were removed from their natural substratum, and attached to PVC posts (2 cm in diameter, 2.5 cm tall) with an inert cement 1 mo before the experiments and kept in their natural environment with conspecifics. All specimens were selected to have a sim.ilar size (0.25 i 0.08 g ash-free dry welght, AFDW; 0.33 i 0.1 l g dry weight, DW; 0.09 + 0.03 g DW without tunic) in order to avoid the effect of size on the feeding behavior (Robbins 1983, Klumpp 1984, Petersen & Riisgdrd 1992. At the beginning of each experiment, an H. papillosa specimen was placed on the base of the experimental chamber and both experimental and control chambers were closed while allowing outside water to circulate throughout the chambers (Fig.…”

“…As a group, they are considered to be non-selective filter feeders able to capture particles from 0.5 to 100 pm (Fiala-Medioni 1987), with high retention efficiency for particles larger than 0.6 pm (Fiala-Medioni 1978a, Randlov & Riisggrd 1979, Stuart & Klumpp 1984. The filtration rates of ascidians have been shown to be dependent on external factors such as temperature, oxygen tension, and food concentration, as well as on morphological factors such as body size (Fiala-Medioni 1978c, 1979a, b, Robbins 1983, Klumpp 1984, Petersen & Riisgard 1992. However, most of these studies have been carried out under laboratory conditions and with simplified diets (FialaMedioni 1978c, Ra.ndl0v & RiisgArd 1979, Robbins 1983, Petersen & RiisgArd 1992, Petersen et al 1995.…”