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Variation of Pistocythereis and Keijella Species in Gamagyang Bay, South Coast of Korea
Variation of Pistocgthereis and Keijella Species in Gamagyang Bay, South Coast of Korea KATSUMI ABEAND KAE-LIM CHOE University of Tokyo, Japan and Korea Institute of Energy and Resources, Korea
ABSTRACT Gamagyang Bay is located on the southern border of the Korean Peninsula. It is a geographically half-closed shallow embayment with a ria-type coastal topography, which provides a variety of environmentsfor ostracod habitation. Among the ostracod species, Pistocythereis bradyi, P. bradyformis and two members of the Keijella bisanensis species group are dominant. Although all these species crawl on the muddy bottom surface in a similar way, their main distribution ranges are limited to either the inner or the outer half of the bay. Each ofthe four species was recognised as having two distinct morphs within the species. Each morph has its own pattern of geographical distribution which is probably controlled by environmental factors or by the origin and history of migration of the species into Gamagyang Bay. Variation in P. bradyi, P. bradyformis and K. bisanensis occurs in the features of the mural elements, the continuity of the ventral ridge and the H/L ratio of the carapace, respectively.
INTRODUCTION Most ostracods are known to have a considerable spectrum of intraspecific variation of carapace morphology. The origin, and thus the biological meaning of such variation, however, may be completely different in each case, and some of them may represent true polymorphism. In Gamagyang Bay, four species of two genera show similar kinds of morphological variation though the variation must have developed independently in each species. This study deals with species of Pistocythereis and Keijella. [For the taxonomy of Keijella, see Abe (1985) in this volume.] Gamagyang Bay is situated at the southern end of the Korean Peninsula (Text-fig. 1). The physiography and other information on bottom sediments, depth, pH, current etc. was given in detail by Chough (1983). The ostracod fauna of the area was studied by Choe (1985).
VARIATION IN SPECIES OF Pistocythereis The genus Pistocythereis from Gamagyang Bay contains two species, P. bradyi and P. bradyformis. The species differ in their distributional range within Gamagyang Bay. P. bradyi occurs over the whole area, but P. bradyformis is limited to the outer half of the bay. The two species differ in the presence of anterior and ventral ridges. Apart from this species diagnosis, two forms are 367
368 K. ABEAND K.-L. CHOE
K O R E A 3 8O
ou L OA
M A OYA N O
-
s
90 KM
0
126' I
3 6'
cHEJD I
I
1280 I
I
1
TEXT-m~.1-Location of Gamagyang Bay.
MORPH
s
c1
MORPH
D
TEXT-ma 2-Features of the mural elements of P. brudyi and P. bra&formis. In both species two morphs are defined by the presence/absence of the additional ornamentation on the top surface of the muri.
Variation of Pistocythereis and Keijella 369
recognized within each species based on surface ornamentation of the carapace. These forms are recognizable in both males and females of the adult population and can be traced back to adult-2 juveniles in P. bradyi, and back to the adult-3 stage in P. bradyformis (juveniles younger than adult-3 were not collected in this area). The difference between the two forms in both species lies in the shape of the top part of the muri. One form has a simple murus, the other a decorative m u m with additional ornamentation. In P. bradyi the additional ornamentation consists of a large number of small cones projecting normally from the summit of the muri. In P. bradyformis, on the contrary, the additional ornamentation consists of wings stretching over the fossae. We call the form with the simple muri “morph S (simple)’’ and the form with additional ornamentation on the muri “morph D (decorated) (Text-fig. 2). Text-figure 3 shows the distribution pattern of these two morphs in Gamagyang Bay. There seems to be no definite difference in the distribution patterns of the two morphs of P . bradyi. There is also no correspondence between their distribution pattern and important environmental factors such as temperature and salinity. As for P. bradyformis all we can do is to present the distributional data at hand because of the small sample size. Moreover, it is possible that the two morphs are in fact two different species.
1
TEXT-FIG. 3-Distribution pattern of the
four morphs of P. bradyi and P. bradyformis in Gamagyang Bay. P. bradyformis is restricted to the outer half of the bay.
Variution of Pistocythereisand Keijella 371
VARIATION IN SPECIES OF Keijella As Abe discusses in this volume, the K. bisanensis species group from the East Asian region consists of four different forms. They can be separated on the basis of size and the H/L ratio of the carapace (Text-fig. 4). Abe concludes that the two groups defined by carapace size should be considered different species. In these two species, two forms can be distinguished by their H/L ratio. Three of these four forms are present in Gamagyang Bay. Text-figures shows their distribution pattern. Distribution patterns of the two forms (form A and form P)belonging to the same species are slightly, but significantly different. I
I
I
I CARAPACE S I Z E
TEXT-FIG. 4-Classification of K. bisanensis species group into four forms.
BIOLOGICAL SIGNIFICANCE AND PERSPECTIVESOF THE VARIATIONS The main habitat of P . bradyformis is restricted to the outer half of the bay. Habitat segregation is stricter between the two species of Keijella. In each species of these two genera, two different forms have been recognized in addition to sexual dimorphism. The speciating process or the origin of two full species can be discussed only in a broader context based on data from a much larger region (e.g. Asia). However, it may be possible to attribute the phenotypic variation within a species either to genetic polymorphism or ecophenotypebecause the two forms in each species are almost sympatric. Morphological variation of P . bradyi and P. bradyformis is similar in that it is based on whether the tops of the muri are simple or decorated. However their biological significance may be quite different. Small sub-conical projections on the muri of P . bradyi are well developed around the pores of sensory hairs which are situated only at the junction of the muri and they are naturally assumed to affect the function of the sensory organ, perhaps by protecting the hair or preventing intrusion by foreign matter from outside. On the other hand, it is difficult to see how the additional ornamentation can affect the sensory hair in the case of P. bradyformis, because the decoration PLATE]-Figs. 1, 3. Morph S of Pistocythereis brudyi. 1. Left valve of adult female, X50; 3. Simple muri, x 540. Figs. 2, 4. Morph D of Pistocythereis bradyi. 2. Right valve of adult male, ~ 5 0 4. ; Decorative muri, x 540. Figs. 5, 7, Morph S of Pistocythereis bradyformis. 5. Left valve of adult female, ~ 5 0 7. ; Simple muri, X 540.
Figs. 6, 8, Morph D of Pistocythereis bradyformis. 6. Right valve of adult male, x 50; 8. Decorative muri, Xi540.
372 K. ABEAM) K.-L. CHOE
TEXT-FIG. 5-Distribution pattern of form A, form P and form G in Gamagyang Bay. Form A is dominant in the north-
western part, form P in the northeastern part and form G is almost restricted to the southern half of the bay.
consists of just a wing stretching out to cover the fossae. Secondary ornamentation of the muri hanging over the fossae is often observed in subtropical and tropical species. Thus, this kind of additional ornament may be temperature-dependent. If the developmental degree of such mural decoration depends only on a physical factor such as temperature, and the decoration itself has no biological meaning and is thus neutral for natural selection, then the m u m might easily change its morphology innately. Variation of Keijella species concerning the H/L ratio of the carapace produces no effect upon the number and the distributional pattern of the reticules and thus the epidermal cells (cf. Abe, 1983; Okada, 1981). In-the group with the smaller value of H/L the elongation is remarkable, particularly in the posterior half of the carapace. Since even in this group the variation is not so striking in juveniles, but is extremely remarkable in adult males, this kind of variation might be related to the reproductive behaviour and sexual selection. What factor controls the distributional range of the two forms as shown in Text-fig. 51 Form P occurs more abundantly in the northeastern part of the bay where the tidal currents flow from the outer sea and form A occurs more abundantly in the very calm environment of the northwestern part. Why then is form P not found in the outer half of the bay? We should investigate the history of migration of these species and forms into Gamagyang Bay.
Variation of Pistocythereisand Keijella 373
In conclusion, several kinds of phenotypic variation have been recognized in the ostracods from Gamagyang Bay. The southern coast of the Korean Peninsula faces the Kuroshio Current and is characterised by numerous post-glacial embayments and nearshore islands forming a typical riatype coast. Thus the sea of this area provides a variety of environments for ostracod habitation. It is likely, therefore, that many ostracod species have developed morphological variations and that such variations have been easily maintained within a species. Study of the morphological variation in this region dealing not only with the carapaces, but also with the inner body will greatly contribute to the understanding of the biological meaning of ostracod variation.
ACKNOWLEDGEMENTS We thank Emeritus Professor Tetsuro Hanai, Professor Itaru Hayami and our other colleagues at the University of Tokyo for their discussion. Professor Sung Kwun Chough of Seoul National University kindly provided the bottom samples for this work. We are indebted to Dr. Paul M. Frydl for reading the manuscript.
REFERENCES CITED ABE,K. 1983. Population structureof KeijeIIa bisanensis (Okubo) (OSTRACODA, CRUSTACEA)-~inquiry into how far the population structure will be preserved in the fossil record. J . Fuc. Sci. Univ. Tokyo. Sec. II 20(5), 443-488. -(inpress). Speciation completed? in KeQeIla bisanensis species group. In HANAI,T., IKEYA, N. and ISMZAKI, K. (eds.). Evolutionary biology of Ostracoda, its fundamentals and applications. Kodansha Scientific, Tokyo. CHOE,K-L. 1984. Recent Marine Ostracoda of Korea (Unpublished doctoral dissertation, University of Tokyo). CHOUGH, S.K. 1983. Marine Geology of Korean Seas. D. Reidel Publishing Company, 157pp. OKADA, Y. 1981. Development of cell arrangement in ostracod carapaces. Paleobiology, 7(2), 276-280.
DISCUSSION Schweitzer: I think that you should show the statistical analyses of shape (H/L) and size (H/L). We need to see the distribution of these values in order to assess the dichotomy that you described. Size ought not to be characterized by one of H or L if shape is characterized by H/L; doing so introduces spurious correlation to your study. Lohmann’s eigenshape analysis of microfossils, a general morphometric procedure for describing changes in form (Mathematical Geology, 1983) might help such an analysis. Reyment: With respect to your observations on the assumedly discrete variational states of the muri (decorated and simple),I should like to mention that a similar condition occurs in intrapopulational samples of Buntonia olokundudui (Recent) from the Niger Delta. With respect to posterior elongation of the carapace, a study by Gilchrist (1960) on Artemia salina may be of comparative significance with respect to the quantitative aspects. I think you might be advised to advance beyond the use of length/height ratios (which obscure the effects of differential growth) and contemplate utilizing some established multivariate statistical technique. In the present examples, the posterior distortion in males, on passing to the adult stage of growth, is doubtless liable to render the L/H ratio unfit as a reference. Moreover, any variational measure based on such a ratio will underestimate the true variance if a covariance term is not included.
ABSTRACT Gamagyang Bay is located on the southern border of the Korean Peninsula. It is a geographically half-closed shallow embayment with a ria-type coastal topography, which provides a variety of environmentsfor ostracod habitation. Among the ostracod species, Pistocythereis bradyi, P. bradyformis and two members of the Keijella bisanensis species group are dominant. Although all these species crawl on the muddy bottom surface in a similar way, their main distribution ranges are limited to either the inner or the outer half of the bay. Each ofthe four species was recognised as having two distinct morphs within the species. Each morph has its own pattern of geographical distribution which is probably controlled by environmental factors or by the origin and history of migration of the species into Gamagyang Bay. Variation in P. bradyi, P. bradyformis and K. bisanensis occurs in the features of the mural elements, the continuity of the ventral ridge and the H/L ratio of the carapace, respectively.
INTRODUCTION Most ostracods are known to have a considerable spectrum of intraspecific variation of carapace morphology. The origin, and thus the biological meaning of such variation, however, may be completely different in each case, and some of them may represent true polymorphism. In Gamagyang Bay, four species of two genera show similar kinds of morphological variation though the variation must have developed independently in each species. This study deals with species of Pistocythereis and Keijella. [For the taxonomy of Keijella, see Abe (1985) in this volume.] Gamagyang Bay is situated at the southern end of the Korean Peninsula (Text-fig. 1). The physiography and other information on bottom sediments, depth, pH, current etc. was given in detail by Chough (1983). The ostracod fauna of the area was studied by Choe (1985).
VARIATION IN SPECIES OF Pistocythereis The genus Pistocythereis from Gamagyang Bay contains two species, P. bradyi and P. bradyformis. The species differ in their distributional range within Gamagyang Bay. P. bradyi occurs over the whole area, but P. bradyformis is limited to the outer half of the bay. The two species differ in the presence of anterior and ventral ridges. Apart from this species diagnosis, two forms are 367
368 K. ABEAND K.-L. CHOE
K O R E A 3 8O
ou L OA
M A OYA N O
-
s
90 KM
0
126' I
3 6'
cHEJD I
I
1280 I
I
1
TEXT-m~.1-Location of Gamagyang Bay.
MORPH
s
c1
MORPH
D
TEXT-ma 2-Features of the mural elements of P. brudyi and P. bra&formis. In both species two morphs are defined by the presence/absence of the additional ornamentation on the top surface of the muri.
Variation of Pistocythereis and Keijella 369
recognized within each species based on surface ornamentation of the carapace. These forms are recognizable in both males and females of the adult population and can be traced back to adult-2 juveniles in P. bradyi, and back to the adult-3 stage in P. bradyformis (juveniles younger than adult-3 were not collected in this area). The difference between the two forms in both species lies in the shape of the top part of the muri. One form has a simple murus, the other a decorative m u m with additional ornamentation. In P. bradyi the additional ornamentation consists of a large number of small cones projecting normally from the summit of the muri. In P. bradyformis, on the contrary, the additional ornamentation consists of wings stretching over the fossae. We call the form with the simple muri “morph S (simple)’’ and the form with additional ornamentation on the muri “morph D (decorated) (Text-fig. 2). Text-figure 3 shows the distribution pattern of these two morphs in Gamagyang Bay. There seems to be no definite difference in the distribution patterns of the two morphs of P . bradyi. There is also no correspondence between their distribution pattern and important environmental factors such as temperature and salinity. As for P. bradyformis all we can do is to present the distributional data at hand because of the small sample size. Moreover, it is possible that the two morphs are in fact two different species.
1
TEXT-FIG. 3-Distribution pattern of the
four morphs of P. bradyi and P. bradyformis in Gamagyang Bay. P. bradyformis is restricted to the outer half of the bay.
Variution of Pistocythereisand Keijella 371
VARIATION IN SPECIES OF Keijella As Abe discusses in this volume, the K. bisanensis species group from the East Asian region consists of four different forms. They can be separated on the basis of size and the H/L ratio of the carapace (Text-fig. 4). Abe concludes that the two groups defined by carapace size should be considered different species. In these two species, two forms can be distinguished by their H/L ratio. Three of these four forms are present in Gamagyang Bay. Text-figures shows their distribution pattern. Distribution patterns of the two forms (form A and form P)belonging to the same species are slightly, but significantly different. I
I
I
I CARAPACE S I Z E
TEXT-FIG. 4-Classification of K. bisanensis species group into four forms.
BIOLOGICAL SIGNIFICANCE AND PERSPECTIVESOF THE VARIATIONS The main habitat of P . bradyformis is restricted to the outer half of the bay. Habitat segregation is stricter between the two species of Keijella. In each species of these two genera, two different forms have been recognized in addition to sexual dimorphism. The speciating process or the origin of two full species can be discussed only in a broader context based on data from a much larger region (e.g. Asia). However, it may be possible to attribute the phenotypic variation within a species either to genetic polymorphism or ecophenotypebecause the two forms in each species are almost sympatric. Morphological variation of P . bradyi and P. bradyformis is similar in that it is based on whether the tops of the muri are simple or decorated. However their biological significance may be quite different. Small sub-conical projections on the muri of P . bradyi are well developed around the pores of sensory hairs which are situated only at the junction of the muri and they are naturally assumed to affect the function of the sensory organ, perhaps by protecting the hair or preventing intrusion by foreign matter from outside. On the other hand, it is difficult to see how the additional ornamentation can affect the sensory hair in the case of P. bradyformis, because the decoration PLATE]-Figs. 1, 3. Morph S of Pistocythereis brudyi. 1. Left valve of adult female, X50; 3. Simple muri, x 540. Figs. 2, 4. Morph D of Pistocythereis bradyi. 2. Right valve of adult male, ~ 5 0 4. ; Decorative muri, x 540. Figs. 5, 7, Morph S of Pistocythereis bradyformis. 5. Left valve of adult female, ~ 5 0 7. ; Simple muri, X 540.
Figs. 6, 8, Morph D of Pistocythereis bradyformis. 6. Right valve of adult male, x 50; 8. Decorative muri, Xi540.
372 K. ABEAM) K.-L. CHOE
TEXT-FIG. 5-Distribution pattern of form A, form P and form G in Gamagyang Bay. Form A is dominant in the north-
western part, form P in the northeastern part and form G is almost restricted to the southern half of the bay.
consists of just a wing stretching out to cover the fossae. Secondary ornamentation of the muri hanging over the fossae is often observed in subtropical and tropical species. Thus, this kind of additional ornament may be temperature-dependent. If the developmental degree of such mural decoration depends only on a physical factor such as temperature, and the decoration itself has no biological meaning and is thus neutral for natural selection, then the m u m might easily change its morphology innately. Variation of Keijella species concerning the H/L ratio of the carapace produces no effect upon the number and the distributional pattern of the reticules and thus the epidermal cells (cf. Abe, 1983; Okada, 1981). In-the group with the smaller value of H/L the elongation is remarkable, particularly in the posterior half of the carapace. Since even in this group the variation is not so striking in juveniles, but is extremely remarkable in adult males, this kind of variation might be related to the reproductive behaviour and sexual selection. What factor controls the distributional range of the two forms as shown in Text-fig. 51 Form P occurs more abundantly in the northeastern part of the bay where the tidal currents flow from the outer sea and form A occurs more abundantly in the very calm environment of the northwestern part. Why then is form P not found in the outer half of the bay? We should investigate the history of migration of these species and forms into Gamagyang Bay.
Variation of Pistocythereisand Keijella 373
In conclusion, several kinds of phenotypic variation have been recognized in the ostracods from Gamagyang Bay. The southern coast of the Korean Peninsula faces the Kuroshio Current and is characterised by numerous post-glacial embayments and nearshore islands forming a typical riatype coast. Thus the sea of this area provides a variety of environments for ostracod habitation. It is likely, therefore, that many ostracod species have developed morphological variations and that such variations have been easily maintained within a species. Study of the morphological variation in this region dealing not only with the carapaces, but also with the inner body will greatly contribute to the understanding of the biological meaning of ostracod variation.
ACKNOWLEDGEMENTS We thank Emeritus Professor Tetsuro Hanai, Professor Itaru Hayami and our other colleagues at the University of Tokyo for their discussion. Professor Sung Kwun Chough of Seoul National University kindly provided the bottom samples for this work. We are indebted to Dr. Paul M. Frydl for reading the manuscript.
REFERENCES CITED ABE,K. 1983. Population structureof KeijeIIa bisanensis (Okubo) (OSTRACODA, CRUSTACEA)-~inquiry into how far the population structure will be preserved in the fossil record. J . Fuc. Sci. Univ. Tokyo. Sec. II 20(5), 443-488. -(inpress). Speciation completed? in KeQeIla bisanensis species group. In HANAI,T., IKEYA, N. and ISMZAKI, K. (eds.). Evolutionary biology of Ostracoda, its fundamentals and applications. Kodansha Scientific, Tokyo. CHOE,K-L. 1984. Recent Marine Ostracoda of Korea (Unpublished doctoral dissertation, University of Tokyo). CHOUGH, S.K. 1983. Marine Geology of Korean Seas. D. Reidel Publishing Company, 157pp. OKADA, Y. 1981. Development of cell arrangement in ostracod carapaces. Paleobiology, 7(2), 276-280.
DISCUSSION Schweitzer: I think that you should show the statistical analyses of shape (H/L) and size (H/L). We need to see the distribution of these values in order to assess the dichotomy that you described. Size ought not to be characterized by one of H or L if shape is characterized by H/L; doing so introduces spurious correlation to your study. Lohmann’s eigenshape analysis of microfossils, a general morphometric procedure for describing changes in form (Mathematical Geology, 1983) might help such an analysis. Reyment: With respect to your observations on the assumedly discrete variational states of the muri (decorated and simple),I should like to mention that a similar condition occurs in intrapopulational samples of Buntonia olokundudui (Recent) from the Niger Delta. With respect to posterior elongation of the carapace, a study by Gilchrist (1960) on Artemia salina may be of comparative significance with respect to the quantitative aspects. I think you might be advised to advance beyond the use of length/height ratios (which obscure the effects of differential growth) and contemplate utilizing some established multivariate statistical technique. In the present examples, the posterior distortion in males, on passing to the adult stage of growth, is doubtless liable to render the L/H ratio unfit as a reference. Moreover, any variational measure based on such a ratio will underestimate the true variance if a covariance term is not included.