Estimation of total gut contents in bivalves from two stage sampling.ABSTRACT Considering that the analysis of total gut contents in large sized bivalve bivalve, aquatic mollusk of the class Pelecypoda ("hatchet-foot") or Bivalvia, with a laterally compressed body and a shell consisting of two valves, or movable pieces, hinged by an elastic ligament. species is extremely time-consuming, the aim of the present study is to give a statistically tested methodology that permits quantitative assessment of gut contents in bivalves from selected subsamples. Thirty Patagonian scallops Zygochlamys patagonica (35-55 mm total height) from Reclutas bed (39[degrees] S to 55[degrees] W) were dissected. The complex "gut + digestive gland digestive gland n. A gland, such as the liver or pancreas, that secretes into the alimentary canal substances necessary for digestion. " was extracted for the analysis of approximately 1/6 of the total content. This procedure was analyzed using the results of the 2-stage sampling theory. The error in the estimation of the mean number of particles per gut was estimated between 15% and 16% using this methodology. KEY WORDS: scallop scallop or pecten, marine bivalve mollusk. Like its close relative the oyster, the scallop has no siphons, the mantle being completely open, but it differs from other mollusks in that both mantle edges have a row of steely blue "eyes" and , gut contents, bivalves, Zygochlamys patagonica, Argentine Sea INTRODUCTION Food quality and quantity in bivalves is related to physiology, growth (Bricelj et al. 1984, Bricelj & Shumway 1991, Davies & Payne 1984, Josefson 1982, MacDonald & Thompson 1988, Hawkins et al. 1990, Bayne & Hawkins 1992, Pilditch & Grant 1999, Lorrain et al. 2000, Chauvaud et al. 2001, etc.) and fecundity fecundity /fe·cun·di·ty/ (fe-kun´dit-e) 1. in demography, the physiological ability to reproduce, as opposed to fertility. 2. ability to produce offspring rapidly and in large numbers. (Bayne & Widdows 1978, Newell & Bayne 1980, Newell et al. 1982, Berg & Newell 1986). Available information that comes from laboratory work with cultured species is related to growth and feeding rates, particle clearance and selection (Bricelj et al. 1984, Jorgensen 1990, Newell et al. 1989, Shumway et al. 1985, Shumway et al. 1997, Hohn et al. 2001, Acosta & Blanco 2003a, Acosta & Blanco 2003b); limited studies about bivalve gut contents in natural conditions have been done. Shumway (1993) and Shumway et al. (1994) studied toxicity in important commercial bivalve species. Some studies (Davies & Marshall 1961, Vernet de Hall 1977, Shumway et al. 1987, Schejter et al. 2002) have shown the importance of diatoms diatoms a series of unicellular algae, microscopic in size, with cell walls containing silica. Members of the family Diatomaceae. Their remains accumulate as geological deposits and are mined. See diatomaceous earth. in the diet of scallops (many Melosira (Paralia), Navicula, Pleurosigma, Synedra, Licmophora and Cocconeis) and other bivalves (Marques Marques may refer to:
MATERIALS AND METHODS Patagonian scallops, Zygochlamys patagonica, were collected from Reclutas bed (39[degrees]S to 55[degrees]W) on June 10th of 1996 using a commercial scallop trawl trawl - To sift through large volumes of data (e.g. Usenet postings, FTP archives, or the Jargon File) looking for something of interest. by the F/V F/V Fishing Vessel F/V Frequency to Voltage Converter "Erin Bruce." A subsample sub·sam·ple n. A sample drawn from a larger sample. tr.v. sub·sam·pled, sub·sam·pling, sub·sam·ples To take a subsample from (a larger sample). from the total catch was preserved in formaldehyde and analyzed in the laboratory; 30 specimens of 35-55 mm total shell height were examined to test the statistical approach. Gut contents were extracted and diluted up to a volume of 2 mL and homogenized ho·mog·e·nize v. ho·mog·e·nized, ho·mog·e·niz·ing, ho·mog·e·niz·es v.tr. 1. To make homogeneous. 2. a. To reduce to particles and disperse throughout a fluid. b. . A subsample of 1 mL of the diluted gut content solution was analyzed using a Sedgwick-Rafter cell (7 rows and 17 columns). Total particles of 6 columns were counted and identified under the microscope (x100-250). This quantity represents approximately 1/6 of total gut content. The number of specimens (30) and the number of columns (6) were considered sufficient to our main objective: to test if the method resulted useful to estimate total gut contents by means of optimizing both sample sizes. A statistical analysis was done to evaluate if the number of sampled scallops and the number of analyzed columns in the sampling cell were enough, looking for precision in the estimation of the mean number of individuals (food items) per gut. In this way, considering the two stage sampling theory, the variance of the mean abundance per gut estimator (number) of a given food item was expressed as a function of the variance between counted columns, the variance between different sampled scallops, the number of columns (m) and the number of sampled scallops (n). Let [d.sub.ij] be the number of individuals for the considered food item, found in the j sampled column (subunit) of the sampling camera, in the i sampled scallop (primary unit). This [d.sub.ij] value is considered to be measured without error. The estimation of the total number of individuals for the food item considered in the gut of the i sampled scallop, is given by: [d.sub.i] = M [[bar.d].sub.i]. (1) where: [[bar.d].sub.i]. = ([m.summation over j=1] [d.sub.ij])/m m: number of sampled columns M: total number of columns (twice the total number of the Sedgwick-Rafter camera). The mean number of particles per gut, of a given food item, considering n sampled scallops and m subsampled columns of the Sedgwick-Rafter camera, is given by: [bar.d](n, m) = [n.summation over i=1] [d.sub.i]/n = M ([n.summation over i=1] [[bar.d].sub.i])/n (2) Considering the two stage sampling context with identical sampling size units ([[summation].sup.n.sub.i=1] [bar.d].sub.i])/n is known as over-all sample mean per subunit (Cochran 1977). Following Cochran (1977) [[bar.d](n, m) is an unbiased estimator of the corresponding population parameter (theorem 10.1), and the variance of [[bar.d](n, m) (10.8 formula) is given by: V([bar.d](n, m))= [M.sup.2][[(1 - n/N)][[S.sup.2.sub.1]/n] + [(1 - m/M)] [[S.sup.2.sub.2/m n]] (3) being: [S.sup.2.sub.1]: variance among primary unit means. [S.sup.2.sub.2]: variance among subunits within primary units. N: total number of individuals of the scallop population under study. Following Cochran (1977), in the demonstration of the theorem 10.2, and considering the estimation sample with [n.sub.0] = 30 individuals and [m.sub.0] = 6 columns, an unbiased estimator of [S.sup.2.sub.2] is given by: [s.sup.2.sub.2] = [n.sub.0].summation over i=1] [s.sup.2.sub.i]]/[n.sub.0] (4) being: [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII ASCII or American Standard Code for Information Interchange, a set of codes used to represent letters, numbers, a few symbols, and control characters. Originally designed for teletype operations, it has found wide application in computers. .] (5) In the corollary of the same theorem, an unbiased estimator of [S.sup.2.sub.1] is given by: [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (6) with: [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (7) In this way, considering (3) and the mentioned results, an unbiased estimator of, V([[bar.d]((n, m)) is given by: [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (8) and considering that n/N is negligible, an estimator of V([[bar.d]((n, m)) could be: [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (9) After the Central Limit Theorem central limit theorem In statistics, any of several fundamental theorems in probability. Originally known as the law of errors, in its classic form it states that the sum of a set of independent random variables will approach a normal distribution regardless of the (Hald 1952), we consider that d (n, m) (average of average values) is normal-asymptotic distributed. This distribution will be approximately normal for moderate values of n (n = 30 in this case). An approximated confidence interval, with a confidence coefficient = 95%, is given by: [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] The semiamplitude of the confidence interval, considered error of the estimation and expressed as percentage of the average estimated value, [[bar.d] (n, m), is given by: Error(%) = 2 cv(n, m) (10) where: [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (11) and cv is the coefficient of variation Coefficient of Variation A measure of investment risk that defines risk as the standard deviation per unit of expected return. expressed in percentage. Finally, the m value was fixed, and the use of the eq. (11) allowed getting curves that represent the cv considering the n sample size. RESULTS AND DISCUSSION These results are representative of scallops between 35 and 55 mm total height. From the analysis of cv it is noted that a small scallop sample (up to n = 10) presents a high cv, that agrees with a big error in the mean number of ingested particles estimation. After n = 30, an increase in the sample size does not generate an important decrease in cv or in the error. In addition, there were not important differences between estimates considering the total counted and identified particles in 3, 6 or 16 columns of the Sedgwick-Rafter cell. For n = 30 and considering 6 columns the error in the estimation of the mean number of particles per gut was between 15% to 16%, depending on the taxonomic group considered (Fig. 1). This statistical analysis was also applied to gut samples collected during August 1997, with similar results, and therefore used to assess yearly variations in Patagonian scallops gut contents (Schejter et al. 2002). In conclusion, the use of this statistical procedure permitted to determine the sufficient number of bivalves (sample number) and proportion of analyzed gut contents (number of columns in the camera) to obtain low error in the estimates; it resulted not only successful but also less time-consuming than analyzing total gut contents. [FIGURE 1 OMITTED] LITERATURE CITED Acosta, C.P., Blanco, J., 2003a. Clearance rate of Pecten pecten: see scallop. maximus. I: effect of flow and food concentration. 14th. International Pectinid Workshop, St. Petersburg, Florida St. Petersburg (often shortened to St. Pete) is a city in Pinellas County, Florida, United States. 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Accumulation of paralytic paralytic /par·a·lyt·ic/ (par?ah-lit´ik) 1. affected with or pertaining to paralysis. 2. a person affected with paralysis. par·a·lyt·ic adj. 1. shellfish toxins by surfclams, Spisula solidissima (Dillwyn, 1897) in the Gulf of Maine The Gulf of Maine is a large gulf of the Atlantic Ocean on the northeastern coast of North America. It is delineated by Cape Cod at the eastern tip of Massachusetts in the southwest and Cape Sable at the southern tip of Nova Scotia in the northeast. : seasonal changes, distribution between tissues, and notes on feeding habits. Nat. Toxins 2:236-251. Shumway, S. E., T. L. Cucci, M. P. Lesser, N. Bourne Bourne, town (1990 pop. 16,064), Barnstable co., SE Mass., crossed by Cape Cod Canal; settled 1627, inc. 1884. Bourne Bridge (1935), across the canal, made the town an entry point to Cape Cod and a resort and commercial center. & B. Bunting. 1997. Particle clearance and selection in three species of juvenile scallops. Aquaculture International 5:89-99. D. HERNANDEZ, (1) * L. SCHEJTER (1,2) AND C. BREMEC (1,2) (1) Instituto Nacional de Investigacion y Desarrollo Pesquero, Paseo Victoria Ocampo 1, (7600) Mar del Plata Mar del Plata (mär thĕl plä`tä), city (1991 pop. 519,707), E central Argentina, on the Atlantic Ocean. It is one of the most popular seaside resorts in South America. Fishing and fish processing are also important industries. , Buenos Aires, Argentina, (2) Consejo Nacional de Investigaciones Cientificas y Tecnicas, Argentina * Corresponding author. E-mail: danielh@inidep.edu.ar |
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