Ohba Shigeru

紀要類（bulletin）departmental bulletin pape

Ohba, Shigeru

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

SPATIAL DISTRIBUTION IN THE NATURAL POPULATIONOF A CLAM, TAPES JAPONICA著者 Ohba Shigerujournal orpublication titleThe bulletin of the Marine Biological Stationof Asamushi, Tohoku Universityvolume 9number 4page range 153-156year 1960-03-10URL http://hdl.handle.net/10097/00130998BULI.lHIN OF THE 1\IARINE BIOLOGICAL STATION OF ASAMUSHI, VOL. IX, NO. 4. 1959 SPATIAL DISTRIBUTION IN THE NATURAL POPULATION OF A CLAM, TAPES ]APONICA* By SHIGERU 0HBA Tamano 111arine Laboratory, Okayama University, Tamano, japan (With one figure) Several observations have been reported on the spatial distribution in natural populations of bivalve molluscs inhabiting sandy intertidal zones (Holme, 1950; Furukawa and Suzuki, 1953; Ikematsu, 1957). Nevertheless, our knowledge on this problem is still quite insufficient. In this study an attempt was made to examine the mode of micro-distribution of individual clams in the natural population of Tapes japonica. In summer, Tapes japonica population at the Mukasa-shoal in the Bisan Seta Channel of the Inland Sea was mainly composed of young clams (5-15 mm in shell length) derived from autumnal spawning in the previous year. Besides them, however, some older clams (15-SOmm in length) and other molluscan species, such as Brachidontes sen,~au_sia, were also found. a large number (Ohba, 1959). Field works were performed in August of 1955 and in the same season of 1957. In all, ten samples were taken from different parts distributed irregu-larly in a definite region of the shoal of about 20 X 10m in area. The size of each sample was definitely 100 and it covered the area of 0.1 m2 in all, as shown in Fig. I. On each unit area of 0.001 m2 , the kinds· of molluscs and number of each species were recorded. In some cases, the total weight of them were also measured. The young (0-year old) and adult (1- or 2-year old) Tapes japonica were separately enumerated. The latter may sometimes occur in 1 1 1 1 t r r 1 ·1 •:·~·~···1·1·1·1·1· ---r--T--i---~--+--i---~--f--i--­e:e:•I$I0J9l8]8J$J® ---~--+--~---~--+--~---~--+--~---®l®:®l®l8i&lel&iel® ---~--t--1---~--+--~--~~--t--i--­$:®:•:•:•:•:•:•:•:0 ---,.--T-- ,-- -r-.-,--,- --:r--T- -1---e:ere:®J•r•:• 1•:•1s ---~--+--~---~--+--~--~~--+--~---$]$l$lelelelelel8!8 ---r--~--~---L--~--J---L--~--J---$1$l$:elelelelel®l$ 1 1 1 I 1 1 1 1 I --~r--T--,---r--T--,--~r--r--,--~ ·~·:•:•:•:•:•:•,•:$ ---~--+--~---~--+--~---~--+--~---®1®10lOI$Ielelels:e ---r- -+--1--+--+--~-- -r- -+--1---~~~:®:®:e:e:®:•Je:c ) m Fig. 1. Arrangement of unit divi-sions in each sample. Sand of {I parts (10 cm2 in area, 10 em in depth) was taken for examination. "' Contributions from the Tamano Marine Laboratory, No. 53. !53 154 S. OHBA The total population densities of each species in each sample are shown in Table 1. Table 1 Total number of molluscs obtained from a 100 unit divisions Total area examined is 0.1 m2 (0.001 m2 ·x 100) in each sample. Sample I-III were collected in August of 1955 and IV-X in August of 1957. Number of individuals per 0.1 m2 Sample Tapes japonica B rachidontesl Nassarius B atillaria I Other I Adult I Young senhausia spp.* cummingii molluscs** I 36 82 17 2 4 0 II I 10 26 2 I 0 III 34 105 806 2 I 0 IV 42 2400 45 9 2 I v 4(l 1318 45 4 9 5 VI 30 1260 458 10 4 I VII 31 717 608 13 10 II VIII 11 291 37 5 5 0 IX 36 18 644 29 8 3 X 24 391 I 5 3 2 * Nassanus festwus and Nassanus ktradoens~s. "'"' Sipltonalia jucoides, Umbonium moniliferum, Lunella coronata coreensis, Pyrene varians, etc. Total 141 40 948 2499 1421 1763 1450 349 738 326 Statistical analyses of the mode of distribution in three main groups (young Tapes, adult Tapes and Brachidontes) were attempted on the data o.f a 100 unit records. First, interferences in the distribution of these three groups were examined. The density of the adult Tapes is relatively low, 300-400/m' in usual cases. In comparing the unit records which contains adults with-those without adults, we can never find significanct differences either in mean density or in frequency distribution of young Tapes per unit area. Similar relations were also found between the adult Tapes and Brac!tidontes. This means that the existence of the adult Tapes has no influence on spatial distribution of young Tapes or Brachidontes which set on the same habitat more than a year later.* The densities of young Tapes and Brachidontes fluctuate widely from sample to sample, as seen in Table 1. But no definite relationship in densities can be detected among these two groups. In most cases in which one of the two groups is of low density, the frequency distribution in the other group is not influenced by the coexistence of the former in the same unit area. In samples VI and VII where both groups show relatively high densities, the correlation coefficient between them is -0.408 and -0.132. The latter can not be accepted as to be significantly different from 0 with the significance level of 0.01. That is, the distribution of young Tapes and Bracltidontes may be also independent of each other, except for a case of sample VI in which negative correlation in a slight degree is probable. "' It is evident that all individuals of Bracltidontes are of 0-year old. SPATIAL DISTRIBUTION OF A CLAM 155 From these results it can be concluded that, as rega·rds the distribution of individuals, the above three groups are respectively independent in general, although they exhibit overlapping distributions in the same habitat. Distributions of individual clams were analysed as to the type of distribution function and of distribution structure in space. For these purposes, two kinds of coefficient, "divergence coefficient (V/'ii*)" (="coefficient of dispersion", Holme, 1950) and "sample structm;al correlation coefficient (y)", were calculated according to. the methods reported by Torii (1952). Considering these two coefficients together, exact knowledge of the mode of distribution in natural populations may be obtained (Torii, 1952). The outlines of the results are briefly summarized in Table 2. In the adult Tapes the mode of individual distribution is always of the same type; i.e., random distribution or slight patch-type distribution in which distribution function is of the Poisson type. On the other hand, some different modes of distribution are seen in young groups of the species. In two cases of high densities (samples VI and VII) strong patch-type distribution of concentrating non-Poisson type is observed, while in samples of low densities ('ii < 3) random distribution of Poisson type is usually seen. Furthermore, strong gradient distribution (sample V) and clear uniform distribution (=self-spacing arrangement; sample X) are also found. In Brachidontes strong patch-type distrib~tion is common and r~ndom distribution i" rather rare, without regard to the population density of the samples. Values of coefficient v;•l 7 -1 -0 >I ~o >I >0 <I ~o Table 2 Result~ of mathematical analyses of the type of distribution function and of the type of distribution structure in space Number of samples Distribution Distribution structure Tapes japonica I Brachidontes function in space Adult \ Young senhausia , Poisson type Random or slight patch-type distribution 10 6 3 non-Poisson type Strong patch-type (negative binomial) distribution 0 2 7 " Strong gradient 0 I 0 distribution non-Poisson type I Uniform distribution 0 I 0 (positive binomial) dtfferences are tn stgmficant wrth the level of stgmficance of 0.01 > and < : differences are significant with the level of significance of 0.01 It may be naturally considered that setting of planktonic Tapes larvae on sandy habitats occurs at random, at least in a small area. Ikematsu (1957) re-ported on the random distributions of Poisson type in a dense population of spats * V: unbiased estimate of variance, x : sample mean density 156 S. OHBA of the species just after setting. After three months this type of distribution was maintained in spite of a considerable decrea•e in the population density. The results of the present study indicate that the Poisson distribution is a usual case in both young and adult Tapes when the population density is relatively low. On the other hand, concentrating non-Poisson distribution is observed in cases of high densities in young Tapes. But detailed developmental mechanisms of this type of distribution remain uncertain. Uniform distribution which was found for once in young Tapes may be a special case. Although Holme (1950) reported clear uni-form distribution in a Tellina tenuis population, similar mechanisms can not be c0nsidered in Tapes japonica. In the present case, that special types of distribution re rarely observed (uniform distribution and gradient distribution) may be a result of micro-environmental differences in habitats. Strong concentrating non-Poisson distributions are generally seen in Brachi-dontes senhausia. This is probably due to the mode of sessile life of this species. As it has well-developed byssus, sand granules come to be connected in masses and, as a result of development of the process, several individuals are often found together in a mass. The weight of young Tapes per unit area shows typical normal distributions in samples, IV, V and X. It is impossible, however, to make clear the relation-ships between frequency distribution in number and that of weight in Tapes iapon-ica populations, because the data of weight are scanty. REFERENCE FuRUI<AWA, A. and M. SuzuKI 1953. Bull. Jap. Soc. Sci. Fish., 18: 723-726. {In Japanese) HoLME, N.A. 1950. J. Mar. Bioi. Assoc., 29: 267-280. IKEMATSu, VV. 1957. Rep. Invest. Ariake Sea, No.4: 13-19. (In Japanese) 0HBA, S. 1959. Bioi. J. Okayama Univ., 5: 13-42. ToRII, T. 1952. Seitaigaku-gaisetsu, pp. 202-286. (In J apanesc) 

SPATIAL DISTRIBUTION IN THE NATURAL POPULATION OF A CLAM, TAPES JAPONICA

https://tohoku.repo.nii.ac.jp/?action=repository_action_common_download&item_id=133608&item_no=1&attribute_id=18&file_no=1

SPATIAL DISTRIBUTION IN THE NATURAL POPULATION OF A CLAM, TAPES JAPONICA

Abstract

Similar works

Full text

Available Versions

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ