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Chapter 63. Speciation completed? in Keijella bisanensis species group

Chapter 63. Speciation completed? in Keijella bisanensis species group

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920 K. ABE



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0.8



LENGTH



(mm)



TEXT-FIG.

l-Comparison of the mean values of the carapace size (left valve of adult males) among four forms.

Form A, form G and form P are represented by samples from Gamagyang Bay. The parallelogram is based

on the population of Aburatsubo Cove (cf. Abe, 1983). Specimens of form M are from Misumi-machi, Kyushu.

S u f i number (n = 1, 2, 3,4) indicates adult-n juveniles.



living and fossil specimens has been widely reported from Recent inner bays and post-Miocene shallow marine deposits of East Asia.

Recent specimens of K.bisunensis include four morphs. Text-fig. 1 compares the mean values of

carapace size with the observed range. Each population is represented by the last several moulting

stages. For discussion of sexual dimorphism, see Abe (1983). We now know that populations fall

into two distinct groups based on carapace size. The size difference between the two groups can be

expressed in a generalized fashion such that adult-n (n = 0, 1,2, . . . ) of the smaller group corresponds to adult-(n 1) of the larger group. For instance, adults from the smaller group are

almost equivalent to the adult-1 from the larger group. Here a judgement of maturity is based on

the marginal infold of the carapace and, in living material, on the degree of development of the

copulatory organ.

The second criterion is the ratio of carapace height to length. The smaller value of H/L is due

to the elongation of the posterior half of the carapace. Thus, specimens from various local populations are divided into four forms, using two classifying criteria (see Text-fig. 4 of Abe and Choe's

paper in this volume). All fossil specimens of K. bisunensis in the literature were also found to

correspond to one of the four forms. To simplify the following discussion, the four forms are called

A, P, M and G after the names of the representative places of their occurrence.



+



DISTRIBUTION

OF THE FOURFORMS

Text-fig. 2 illustrates the distribution of the four forms in east Asia during the last several million years. Insufficiency of the fossil record does not allow us to determine when and where each



Speciation in Keijella bisanensis Species Group 921



TEXT-FIG.

2-Classification of the four forms. Forms A, P, G and M are named after Aburatsubo Cove, Great

Peter Bay, Gamagyang Bay and Misumi-machi respectively.



form originated. The distribution patterns of living populations are principally allopatric but

sympatric in part: for instance form A, form G and form P coexist in Gamagyang Bay, Korea

and form A and form M around Misumi in Kyushu, Japan.



PERSPECTIVES

ON SPECIATION

Of the two primary criteria, the carapace size may be more meaningful and significant than the



H/Lratio, because the former divides a population more discretely, while the latter occasionally

yields some intermediates. The difference in carapace size should not be attributed to seasonal

changes. There are two pieces of evidence for this point; first, as the author reported in 1983, reproduction in Aburatsubo Cove takes place within a limited interval and seasonal difference in size,

if any, is negligible. The second line of evidence is the fact that in Misumi, live specimens of both

form M and form A are commonly present in the same sample. Thus it seems natural to think that

the two groups at first differed in terms of carapace size, and that each of these two groups

divided again into two subgroups in terms of H/L ratio. Consequently, a total of four groups have

so far been produced and can be recognized at present.

The fact that the distribution pattern of the four forms is essentially allopatric and sympatric



Speciation in Keijella bisanensis Species Group 923



in a limited area allows us to call them a species group, though it has not yet been decided whether

they are a full species or not.

What is, then, the process by which the species group of the four components has speciated?

Because differentiation in carapace size is independent of the change in H/L ratio, the following

discussion will be focused on the problem of the carapace size only. To generate and fix the larger

and smaller group, two independent and serial steps are required to have occurred in the past.

The first step is the origin of a new group which has a significantly different phenotype from the

existing group. The next step is to subject a variety of new groups to natural selection, so that only

successful groups can find their own habitat. They were distributed at first allopatrically and subsequently sympatrically, in part.

We must explain the origin of two sizes of the ostracod body in the first step of speciation. What

mechanism could produce such a pair? Because the chronological order of appearance of the two

groups is unknown due to an insufficient fossil record, three kinds of answers are possible to the

above question; heterochrony, polyploidy and geographical variation within the normal range.

Those who would like to attribute the process to heterochrony have only to point out that the

size difference between the two groups corresponds just to the difference between the two successive moulting stages in the group. They can explain that the small group has been produced from

the large group by some mechanism like paedomorphosis in which development in two moulting

stages had occurred during only one moulting stage in early ontogenical development. This interpretation, however, has two defects. One is that the ostracods of the small-size group have probably

as many instars as those of the large-size group. The other defect is that the developmental change in

the number and distribution pattern of the reticules and thus the epidermal cells is exactly the

same between the two groups, at least in the last four stages.

The second answer, polyploidy, is not faced with any negative evidence. Polyploidy in animals is

less commonly reported than in plants, yet some cases are known in crustaceans, including ostracods. Karyotype analysis should be applied, but it is questionable whether chromosomes of this

ostracod are amenable to such analysis. Since polyploidy generally produces a larger form than the

original one, if this answer is correct, the larger group will be the descendant of the smaller group.

Geographical variation may be the simplest answer. Here the fact which favoured the first answer turns into a defect. Indeed it is likely that the body size has changed little by little, but isn’t the

actual change beyond the normal range?

Since the change of carapace size and the change of H/L ratio occurred independently, it is most

reasonable to suggest that the origin of the two different sizes of the species group is due to polyploidy, and that the variation of H/L ratio was produced by long-term interaction of ostracods

with a variable environment.

Next, the second step in speciation may be mentioned. The pattern of distribution of natural

populations should reflect the pathway of geographical speciation. The occurrence of two or more

species in an isolated habitat is the result of multiple invasions (Mayr, 1963). As for the history of

the distributional isolation, changes in the coastline and the ocean currents of East Asia will be

important, but at present we have only limited data. When discussing the selection pressure in the

PLATE 1-Figs. 1-6. Form A. 1. Left valve of adult male. 2. Right valve of adult male. 3. Left valve of adult

female. 4. Left valve of adult-1. 5. Left valve of adult-2. 6. Left valve of adult-3. Figs. 7-12. Form P.

7. Left valve of adult male. 8. Right valve of adult male. 9. Left valve of adult female. 10. Left valve of

adult-1. 11. Left valve of adult-2. 12. Left valve of adult-3. Figs. 13-18. Form G. 13. Left valve of adult

male. 14. Right valve of adult male. 15. Left valve of adult female. 16. Left valve of adult-1. 17. Left

valve of adult-2. 18. Left valve of adult-3. Figs. 19-24. Form M. 19. Left valve of adult male. 20. Right

valve of adult male. 21. Left valve of adult female. 22. Left valve of adult-1. 23. Left valve of adult-2. 24.

Left valve of adult-3. (all figures x 50)



924 K . ~ E



local environment, we should not pay much attention to the present-day situation. It should not

be assumed that species have originated where they are now found. It is more reasonable to consider that the same group once existed widely in the East China Sea and that the descendants are

now living in the limited peripheral regions such as the southern coast of Korea and western Kyushu. An embayment or an inward curve of the coastline is an outward protrusion from the viewpoint of marine dwellers. Therefore, each of the local populations can be regarded as a kind of

peripheral isolate.

Although it is not known when and where the diversification of large and small groups occurred,

this may be clarified with sufficient knowledge of the fossil record. However, we can say that at

present, the larger group is living to the north and the smaller group to the south in the East

China Sea, and that here they are in part sympatric. The zonation in distribution pattern of the

three forms in Gamagyang Bay (see Text-fig. 5 of Abe and Choe’s paper in this volume) may

be attributed to two different events. I suppose that a minor change-variation in the H/L ratiooccurred secondarily within a larger group while the two groups of different size coexisted.

Now I conclude that speciation of the K. bisanensis species group is almost completed at least

with regard to the two major groups of different size. And herewith can we not find another attractive theme of study? The two subgroups based on the difference in H/L of the carapace were produced independently in these two full species. Were they produced by chance or of necessity?

Recognising that what evolves is a population and what selection acts on are the organisms or

some unit at the lower level, we should distinguish sharply between the two problems of the population and the individual. Before going on to discuss the evolution of ostracods at the level of populations, we should accumulate more knowledge at the level of individuals. One question must

be considered in evaluating the second step in speciation: how can benthonic ostracods extend

their distributional range, when their whole life is essentially limited to a bottom habitat without any

planktonic stage in early life? The eustatic change of sea level will be the key to this problem. The

other question is whether the speciation is truly completed. Culturing experiments can answer whether fertile offspring can be produced by a pair of different forms. In general, culturing of shallow

marine benthonic ostracods is not easy, but since I have just succeeded in keeping mud dwelling

ostracods alive for more than one year, research in this direction will be possible in the future.



ACKNOWLEDGEMENTS

I thank Professors Tetsuro Hanai, Itaru Hayami and other colleaguesat the University of Tokyo

for their discussion and advice. I am indebted to Dr. Paul M. Frydl for reading the manuscript.



ABE, K. 1983. Population structure of



KeGeZZa bisanensis (Okubo) (Ostracoda, C r u s t a c e a t b inquiry into how far

the population structure will be preserved in the fossil record. J. Fuc. Sci. Univ. Tokyo, Sec. ZZ, 20(5), 443-488.

MAY& E. 1963. Animal species and Evolution. 797 pp. Harvard University Press, Cambridge.



Speciation in Keijella bisanensis Species Group 925



DISCUSSION

Adamczak: Did you follow up the variability of the reticulation in your material? Your material looks to me to be an excellent basis for study of pattern analysis.

Abe: Yes, I have examined the variability of the reticulated pattern before, and pointed out in

some cases when and where a variation pattern had been produced. I have also clarified the microgeographical cline in the frequency of occurrence of variant forms. Please refer to Abe (1983).

Cronin: Could you please comment on the possibility of sympatric speciation with “ecological”

isolation in this group?

Abe: All the components of this species group are considered to crawl in the same manner on

the surface of flocculent mud. Therefore such a possibility is extremely rare.

Reyment: Dr. Abe has provided us with an excellent account of variational patterns in

ornamental features. With respect to the comment on eustatic effects as a migrational driving force,

I can mention that under special, rather rare, circumstances, such as the spread of shallow, narrow

epicontinental seas (the Trans-Saharan seaways of the Late Cenomanian, Early Maastrichtian, and

the Early Paleocene), the relatively rapid advance of the sea in relation to increasing depth of the

encroaching inland sea, can have been a force to be reckoned with, notwithstanding secondary

effects such as algal transport (for phytal species) and episodic spreading by strand-wading birds

in the case of marine ostracods with eggs able to survive for at least some hours in the free air

(i.e. withstand dessication). Nonetheless, even if such transport of eggs, and isolated mud-encapsulated individuals etc. is a possibility, the chances of new demes becoming established by avian

agencies is slight indeed for bisexual organisms. The basic tenets of quantitative genetics (Falconer,

J. (1981) Quantitative Genetics [2]; Oliver and Boyd, Edinburgh) tell us that there is a minimum population size below which new populations cannot have a reasonable likelihood of becoming

established. A single female or male, or just a few individuals, is insufficient for bisexual organisms,

but the chance of success becomes greater for parthenogenetic (hence non-marine) ostracods. Reyment and Brannstrow (1962; Stocht. Dontri. Geol. Vol. 3) found something like a < l o % chance

of success for eggs of a parthenogenetic ostracod species to succeed in establishing a new deme.



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Evolution and Biogeography of Orionina

in the Atlantic, Pacific and Caribbean:

Evolution and Speciation in Ostracoda, 11

THOMAS



M.CRONIN AND



NANCY SCHMIDT



U.S.Georogical Survey, Reston, Virginiaand

University of Arizona, Tucson, Arizona U.S.A.



ABSTRACT

We studied the evolution and paleobiogeography of the marine ostracod genus Orionina Puri,

1953 by examining specimens from 23 Neogene and Quaternary formations and Holocene material

from the Atlantic Coastal Plain, the Caribbean, Central and South America, and the central and

western Pacific. Inter- and intraspecific variation in carapace length, surface ornamentation and

internal features obtained by morphometric analysis and scanning electron microscopy showed

post-middle Miocene Orionina can be grouped into 7 species, including two new species, 0. boldi

and 0. brouwersae. Biostratigraphical and biogeographical data show that geographical isolation

of large populations by the Isthmus of Panama since the Pliocene did not result in morphologic

divergence, but that the isolation of small populations on remote central and western Pacific islands

and atolls resulted in the differentiation of the species 0. flabellacosta Holden, 1976 and 0.

brouwersae n. sp. Inter-and intraspecific variation in Orionina carapace morphology is illustrated

with scanning electron photomicrographs.



INTRODUCTION

Recognizing biological species in marine ostracods requires identification of a consistent range

of morphotypes in time and space, such that intraspecific variation is distinguishable from interspecific variation. In addition, biogeographical patterns should be consistent with the known

ecology and dispersal capabilities of the taxon and the geological and climatic history of the

taxon’s range. This paper is the second in a series of studies designed to investigate speciation and

evolutionary patterns in Ostracoda with emphasis on the geography of speciation and the abiotic

factors influencing the speciation process. In Part I, Cronin (this volume) outlines in detail the

rationale and methodology behind these studies. The present paper focuses on the genus Orionina,

a monophyletic group of species which, with two important exceptions, has been endemic to

tropical, subtropical, and warm temperate regions of the Western Hemisphere from the Oligocene

to the Holocene. Orioninu was selected for detailed study because populations were subjected

to two kinds of geographical isolation : “dumbbell” isolation of large populations by the

Isthmus of Panama during the Pliocene, and “founder” type isolation of small populations

on remote central and western Pacific islands and atolls thousands of miles from the rangeof

ancestral populations.

927



928 T. M.



CRONIN AND



N. SCHMIDT



After its original description based on species from Florida by Puri (1953), Bold (1963) redescribed Orionina in detail, giving a comprehensive list of occurrences from the Caribbean and

Central America. The type species, 0. vaughani was described in 1904 by Ulrich and Bassler based

on material from the Pliocene of Virginia. Although our results and interpretations of some of

the relationships among species differ slightly from those of van den Bold, his work stands as the

underlying framework for all subsequent study of this genus. Gunther and Swain (1976) provided

important occurrence data from the Gulf of Panama and postulated some evolutionary

relationships in Orionina. Other important occurrences of Orionina from the eastern Pacific are

found in Swain (1967, 1969), Swain and Gilby (1967), and Valentine (1976). Holden (1976)

described Orionina flabellacosta from the Miocene of Midway Island, Hawaii. A new species

closely related to Holden's species is proposed below.



LENGTH vs. HEIGHT in OR/O/U/NA

0

400



380



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38C



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TEXT-FIG.

1-Plot of carapace length versus height for Orioninu from various localities. The vuughuni group,



designated by squares, includes specimens assigned by other authors to 0. pseudovuughani and 0 .

serruluta. Holotype specimens are indicated by open stars (measurements of 0. brudyi, 0. sirnilis, 0.

serrulatu, and 0. vuughuni holotypes are taken from Bold, 1963).



880



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Chapter 63. Speciation completed? in Keijella bisanensis species group

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