The Release of Soluble End Products of Metabolism 111
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Copepods Colattukudy), pp. 50-91. Kilvington, C. C. (1977). In. XI. Lipids in Calanus 3 The release of soluble end D Marine Biological Associa- products of metabolism A. E. M., and Blaxter, ROBERT LE BORGNE f marine zooplankton by no gairdneri), Marine Bio- l i , J. R. (1982). Fatty acid i Transactions, 10, 463-4. Introduction i the triacylglycerol, wax The organic matter assimilated by a copepod undergoes a series of caeca of rainbow trout i metabolic reactions that leads to the production of living matter (as s and wax esters. Com- 1 growth, moults, storage and gametes) and energy. The latter is prod- I uced by the oxidation of organic compounds by molecular oxygen ! during the respiration process. End products of these reactions enter the external medium through two physiological mechanisms that are usually studied. separately: (1) respiration, stricto sensu, which is con- cerned with oxygen consumption and the release of carbon dioxide (CO,); (2) excretion, which deals with the liberation of the remaining Fatty acid synthesis and end products. When catabolism is complete, the end products are 37-79. inorganic (ammonium, phosphate) , whereas when it is incomplete or sho, K., and Fujita, S. when the end products are further transformed before excretion (e.g. !&catilis as a living feed urea synthesis), the compounds released are organic. Because both of the Japanese Society for ! excretion and respiration lead to the elimination of catabolic end products, because they have much in common in terms of the methods , and Yone, Y. (1979). used for their measurement and the factors influencing them are j Brachionus plicatilis and identical, it seems logical to consider both releases in the same 'letin of the Japanese Society chapter. le intact lipids of petrel Respiratory exchanges (i.e. oxygen uptake and CO, release) of cope- pods take place on the tegument and at the rectum. Excretion is achieved by the maxillary glands and two to four nephrocytes that are located superficially in the cephalic region. Soluble excretion, secret- ion and moulting losses are distinct processes that are difficult to I differentiate when the products released by a marine animal are analy- sed. Such processes, however, give rise to compounds that differ from those of defaecation, a process concerned with the release of particu- late matter than has not been assimilated (see Chapter 6). During the last 20 years, many studies have been made of the respiration and excretion of planktonic animals, mainly because of the l availability of satisfactory analytical methods and because excretion ! has been recognized as playing an important role in the functioning of ORSTOM Fonds Documentaire I i I 110 The Biological Chemistry of Marine Copepods The relei marine ecosystems. This latter point was stressed by Ketchum (1962) main problem with following the studies of Harris and Riley ( 1956) and Harris ( 1959), so in the amounts of s( giving rise to many studies of the contribution made by zooplankton as small as possible, excretion to the nutritional needs of phytoplankton. In addition to this to be so large that tl kind of study, it has been found useful to combine respiration and the incubation. In c excretion rates in order to characterize the type of substrate being 1 the length of the ex oxidized by zooplankton to produce its energy (Conover and Corner, ~ Thus, for a given tl 1968) and to assess the efficiency of utilization of ingested organic I tion, the longer t matter (Butler, Corner, and Marshall 1969, 1970). influencing the rate Both aspects of the release of soluble end products will therefore be considered. The first aspect is concerned with the proportions of the Short incubations a different kinds of compounds released; the second deals with rate The advantages of measurements and the part played by respiration and excretion in the and short incubatil cycling of organic matter. Most of the studies described here have hours, are that they been concerned with marine copepods, but occasional reference is I closed systems, and made to related work with freshwater species and with other groups in larly as far as diel the marine plankton. ever, to stress effects c introduction into t Methods of measuring copepod respiration and excretion crowding. Moreove animal concentrati Measurements of respiration and excretion are expressed as rates, i.e. errors. Such considc amounts of oxygen or energy consumed, or CO,, nitrogen or phos- rates, and their vari phorus released per unit of time and body weight. Two kinds of tion experiments (: methods have been generally used: the assay of enzymatic activity and I 1974, for Calanus he1 the incubátion technique. Enzyme assays allow respiration rates to be pages ppicus; Ikeda inferred from measurements of the activity of the electron transport Acyocalanus gibber, C system (Packard 1971) and those of ammonium excretion from glut- plumata). amate dehydrogenase activity (Bidigare, King, and Biggs 1982). In the incubation technique, the measurements are those of concentrati- Crowding and confine ons of compounds consumed or released by animals which are left in a of animal concenkr closed, vesse! for a certain period of time. Values from enzymatic that in the natura assays have to be calibrated by incubation experiments to produce the assembling numbe actual rate in the environment (Packard and Williams 1981). Enzy- effect is caused by 1 matic assays are reviewed by Mayzaud in Chapter 5. Accordingly, here, by the ratio betwec detailed attention is given only to the incubation technique, which is flask. According tc still widely used and is the only one providing information about the decrease in rates di types of compounds released by copepods. the animals are sir During an incubation experiment, the respiration and excretion of This may explain tl one or several animals causes variations in the concentrations of oxy- crowding effect. Fi gen or soluble end products, which can be analysed either at regular et al. (1982a) betwc time intervals or at the beginning and the end of the experiment. The A.gibber at concent i .- Copepods The release of soluble end products of metabolism 111 #edby Ketchum (1962) main pr,oblem with this technique is to measure significant differences ) and Harris ( 1959), so in the amounts of soluble compounds so that analytical errors will be made by zooplankton as small as possible, while at the same time not allowing these changes :ton. In addition to this to be so large that they cause drastic alteration to the seawater during mbine respiration and the incubation. In other words, a compromise must be found between ype of substrate being the length of the experiment and the concentration of the copepods. (Conover and Corner. Thus, for a given temperature and species, the lower the concentra- ln of ingested organic tion, the longer the incubation, for both are important factors 370). influencing the rates. )ducts will therefore be the proportions of the Short incubations and high animal concentrations xond deals with rate ln and excretion in the The advantages of experiments that use high animal concentrations and short incubation times, lastirig from several minutes to a few s described here have masional reference is hours, are that they avoid the starvation effect which often happens in Id with other groups in closed systems, and they reflect the actual metabolic activity, particu- larly as far as diel rhythms are concerned. They are sensitive, how- ever, to stress effects resulting from the capture of the animals and their introduction into the vessel, and to the effects of confinement and id excretion crowding. Moreover, physiological state is harder to control at higher animal concentrations and short incubations can lead to analytical expressed as rates, i.e. errors. Such considerations also explain why respiration and excretion O,, nitrogen or phos- rates, and their variability, are generally higher at the start of incuba- veight. Two kinds of tion experiments (see Nival, Malara, Charra, Palazzoli, and Nival mzymatic activity and 1974, for Calanus helgolandicus, Temora sblifera, Acartia clausi and Centro- respiration rates to be pages typicus; Ikeda, Hing Fay, Hutchinson, and Boto 1982a, for the electron transport Auocalanus gibber, Calanopia elliptìca , Eucalanus subcrassus and Pontellìna i excretion from glut- plumata). and Biggs 1982). In :those of concentrati- Crowding and conjnement effects. These are consequences of the increase ials which are left in a of animal concentrations in laboratory experiments compared with thes from enzymatic that in the natural environment. The crowding effect results from iments to produce the assembling numbers of animals far above normal; the confinement Villiams 1981). Enzy- effect is caused by the small size of the vessel and may be represented 7 5. Accordingly, here, by the ratio between the volume of the animal and the volume of the n technique, which is flask. According to Marshall (1973), referring to Zeiss (1963), the nformatioii about the decrease in rates due to crowding would depend upon whether or not the animals are similarly concentrated in their natural environment. ttion and excretion of This may explain the conflicting results’in the literature relating to the oncentrations of oxy- crowding effect. For instance, no difference was observed by Ikeda ysed either at regular et a¿. (1982a) between the ammonium or phosphate excretion rates of -the experiment. The A.gibber at concentrations of one individual in 4 ml and 34 individuals The Biological Chemistry of Marine Copepods The in 100 ml, or for C. ell$tica at similar concentrations. By contrast, , ported that thi Razouls (1972) reported that T. stylifeera displays decreases in respira- ;ADP)/ (ATP tion rate of 22 per cent and 30 per cent when the concentration is and recovers ' increased from 50 to 100 and from 50 to 200/100 ml, respectively.