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Chapter 26. Morphological variations of Cytheromorpha acupunctata (Brady) in continuous populations at Hamana-ko bay, Japan
320 N. IKEYA
made such statements as “Surface of the shell thickly covered with small, impressed, circular
puncta”, as well as drawing fine punctation all over the surface of the carapace in his figures.
Hanai (1961) noticed that the hinge morphology of this species is gongylodont, and transferred it to the genus Cyfheromorpha. Later, Ishizaki (1968) reported C. acupunctata in his
study of the Recent ostracod associations in Uranouchi Bay, Shikoku, Japan, and also proposed
C.japonica as a new taxon. His description and figures suggest that the name C. acupuncfafais applied to individuals with developed reticulation, and C. japonica to those that have only punctation.
C. japonica, as referred to by Ishizaki, is very similar in morphology to C. acupunctata as described
by Brady. Hanai et af. (1977) therefore concluded that C. japonica was a synonym of C. acupuncfafa,and C. acupunctata as described by Ishizaki (1968) is the same species as C. acupuncfafaas described by Brady (1 880), although there is a considerable difference between these two forms. They
also pointed out that “two forms differing in the coarseness of surface ornamentation” are found
in the same sample.
Okubo (1978) dissected specimens of this species from the Inland Sea of Japan, and described
their soft parts. He also noticed that the outline of the carapace differs widely between males and
females, and that there are two distinct morphological types in the males which differ in surface
ornamentation. Occurrences of this species were also reported by several authors, from a number
of other localities in Recent inner bays, and from inner bay sediments formed in the Pleistocene and
later, in various parts of Japan. Ikeya and Hanai (1982) concluded, from their research on the
ostracod assemblages in Hamana-ko Bay on the Pacific coast of central Japan, that this species is
among those that are characteristic of silty bottom sediments of the inner bay biofacies.
Regarding intraspecific morphological variations in the genus Cyfheromorpha, some differences
have been observed in surface ornamentation between specimens of C. paracasteana (Swain) from
different localities. In this species Sandberg (1964) reported that “from low salinities-[it] is thin
shelled and weakly ornamented with faint to moderate reticulations. The same species from nearly
marine salinities has a distinctly thicker shell and strong, high-walled reticulations.” In C. acupunctafa, however, both these morphological variants are often found together in the same sample, and thus these variants are not considered allopatric ones. Nor is this diversity an example of
sexual dimorphism, generally conspicuous in the carapace of the Cytheromorpha species, as is
shown by the fact that the different morphological variations are found among the males for this
In order to investigate the morphological variations and their possible causes in C. acupuncfafa,
a fixed station in Hamana-ko Bay was chosen as a sampling locality. As this is not a very large bay
and its mouth is rather narrow, and because there is also no other bay in the neighbourhood, it is
unlikely that there will be any drastic change in the gene frequencies of the population at the sampling station caused by immigration from other populations. This makes it particularly suitable
for examining the influence of changes in the environmental factors.
The senior author has previously investigated the sediments and sedimentary environments of
Hamana-ko Bay (Ikeya and Handa, 1972), as well as the ecology of the ostracod faunas (Ikeya and
Hanai, 1982). The latter work has revealed that both the population density of Cyfheromorpha
acupuncfata and the percentage of this species in the living ostracod assemblage is higher at ‘Station 48’ than at any other station. For this study, we selected a sampling station about 200 m northwest of this ‘Station 48’ (137”36’42’’E, 34”42’40’”) (Text-fig. 1). Here the average water depth
is around 2.5 m and the bbttom sediment consists of moderately well-sorted sandy silt.
Mophological Variations of Cytheromorpha acupunctata 321
Between March 1977 and February 1978, twelve samples were collected at this station at
intervals of about a month. Below, sampling methods and preparation are only briefly described.
Details are given in Ikeya, Ohishi and Ueda (1986) which also records the seasonal changes in the
ostracod ‘faunas. Each monthly sample consisted of a quantitative sample collected by means of a
Phleger-type core sample (3.6 cm#), and a non-quantitative sample by a modified Ockelmanntype bottom net sampler. The core sampler was dropped three times for each monthly sampling, and
the top 1 cm layer of each core sample was taken for examination. In this research the samples
taken from the three cores were combined and treated as a single sample with a surface area of 30.5
cm2and a volume of 30 cm3.All the samples were fixed with 10 % neutralized formalin, and washed
with water through a 200-mesh sieve (opening 74pm). A part of each net sample was then preserved in 70% alcohol and the rest of the sample was dyed with Rose Bengal and then dried.
Ostracods were picked from all the Phleger core samples, but not enough individuals of C. acupunctutu were obtained for statistical analysis. Additional ostracods were therefore picked from the
dried net sample to bring the total number of specimens of this species up to at least 100 each
month. The dates of collection of the samples, the numbers of C. acupunctata individuals, and other
relevapt information are listed in Table 1.
322 N. IKEYA
AND H. UEDA
OF Cytheromorpha acupunctata SPECIMENS
TABLELIST OF SAMPLING
OF TWO DIFFERENT
&RE SAMPLER AND MODIFIED
Number of specimens
Study of the seasonal changes in the ostracod fauna using these Phleger core samples (Ikeya,
Ohishi and Ueda, 1986) has revealed that the samples collected over the year contained 12
species with living specimens and 31 species in total. Among the living species, the four species of
Spinileberis quadriaculeata (Brady, 1880), Cytheromorpha acupunctata (Brady, 1880), Hemicytherura tricarinata (Hanai, 1957), and Semicytherura sp. were dominant in this order. In any month
the sum of these species made up more than 95 % of the total living population. Except in May and
June, the composition of the living population was relatively stable through the year. The June sample was markedly different from those of other months. This may in some way be related to the
fact that the June sample was composed of silty sand, whilst in all other months the samples
consisted of sandy silt. C. acupunctata, which was the second most common species in all other
months, was the dominant species in June. These facts suggest that the June sample was collected
from a subtly different environment, which is turn implies that either the sampling station of that
month was slightly off the fixed station or a large amount of sandy sediment was brought in from
In order to understand the degree of intraspecific morphological variation within an ostracod
population, it is essential to have some preliminary knowledge of the life cycle of that species, i.e. :
1) Clarification of the mode of growth is required because the morphology changes with moulting,
and differs between the tw'o sexes.
2) Knowledge of population dynamics is needed because the seasons of spawning and growth,
providing different environmental elements, result in different morphologies.
Study methods : All the specimens found in the Phleger core and bottom net samples collected
every month at the fixed station were measured. A preliminary investigation revealed that the right
valve is slightly larger than the left, although the difference is almost negligible. It was decided to
measure only the right valves. The length and height of each specimen was measured three
times using a digital micrometer, and the mean values were calculated (Text-fig. 2). On the basis of
these values, the moult stage of each specimen was determined and the relative growth formula was
Mophological Variations of Cytheromorpha acupunctata 323
calculated. All complete carapaces were examined, and those specimens found to contain nearly
complete appendages in their carapaces when observed in transmitted light were considered to
have been alive when they were collected.
Mode of growth: The 1254 complete carapaces and 316 separated right valves measured were
plotted as scatter diagrams (Text-fig. 2a, b). In the diagram for complete carapaces, the plotted
points form ten separate groups (a-j) as shown in the figure, whereas in the diagram for separated
right valves, the separation of groups b and c, and that of d and e, as seen in the diagram for complete carapaces, was not clear. This implies that some of the separated right valve specimens
may have been distorted. In order to secure precision, therefore, we dealt only with specimens
with complete carapaces.
Each group (a-j) of points represents a particular moult stage and a particular sex. Groups a and
b all correspond to adult stages, whereas groups c to j represent juvenile forms. This was con6rmed
by observation of the duplicature, which is fully developed in groups a and b. Dissection of alcoholpreserved specimens of groups a, b, c, and d revealed that groups a and b are adult males and
of complete carapace
2--Length/Height diagrams for right valves of Cytheromrpha acupunctata. 2a: Measurements based on
complete carapaces. Each moult stage and sex is shown as one of ten separate groups (a-j). 2b: Measurements
based on separated valves. Distinction between two consecutivemoulting stages is unclear at later moult stages.
324 N.IKEYAAND H. UEDA
MOULTSTAGSAND/OR SEXES AND THEIR
females, respectively, since the male copulatory organ and eggs were found in the corresponding
specimens. Immature male copulatory organs were found in the specimens of group c, which is
thus considered to represent males of the A-1 stage (Pl. 1, fig. 1; P1. 2, figs. 1, m). Group d
is considered to represent females of the A-1 stage (Pl. 1, fig. m; P1. 2, figs. n, o), taking into
account the carapace size of this group and the differences between adult males and females.
Group e represents the A-2 stage, but spreads rather widely, suggesting that it may reflect the
inclusion of both sexes at a time when sexual dimorphism is just beginning. In C. acupuncrara, the
eight moult stages from the A-7 to the adult stage were recognised relatively clearly by length/
height measurements. Although the earliest moult stage recognised by observation of existing
specimens was the A-7, the size of the probably mature eggs (approximately 75 pm, confirmed by
dissection) indicates the possible existence of the A-8 stage.
In order to formulate the average mode of growth of C. acupunctata throughout the year, an
allometry formula was applied on the length (L) and height (H) of the carapace.
H = bLo; where a = relative growth coefficient, and b = relative growth constant.
The values of a and b were calculated by regression using the reduced major axis method, because L and H are independent of each other (Imbrie, 1956). Since the mode of growth differs between males and females in the advanced moult stages, growth formulas were determined separately
for 1) males of A-1 and adult stages, 2) females of A-1 and adult stages, (in both of which obvious
sexual dimorphism could be observed) and 3) individuals belonging to the A-2 stage or younger
(Text-fig. 3). The mode of growth up to the A-2 stage is represented by groups of points corresponding to each moult stage aligned at regular intervals on the graph, suggesting that the growth rates
of L and H are almost constant at every moulting (L: approximately x 1.22, H: approximately
x 1.17). This regularity agrees with Dyar’s law, which is often applicable to the mode of growth
Population dynamics: Of the 1254 complete carapaces, 728 were judged to have been alive when
they were collected. Among these, the living specimens in the Phleger quantitative samples totalled
182 individuals. Seasonal changes in the number of individuals of each moult stage in the quantitative samples were studied (Text-fig. 4a). Since living juvenile individuals were not abundant in
the Phleger samples, juveniles of all stages were treated collectively. Whereas the numbers of
adult individuals, both male and female, are relatively stable throughout the year, the number of
juveniles shows marked changes with the season, increasing in April and decreasing in summer.
Mophologcal Variations of Cytheromorpha acupunctata 325
3-Length/Height diagram on logarithmic coordinates and allometry formulas for males, females
and sexually undifferentiated juveniles.
Adult males were not observed alive in December and January. The relative stability of the adult
population over the seasons in comparison with that of the juveniles probably indicates that the
life span of the adult is considerably longer than the juvenile period. In the samples collected in
June, both adults and juveniles were abundant, but, as stated in ‘Materials’, these samples may
have been collected slightly off the fixed station, and the population is likely to have been smaller
at the right station.
The percentage of individuals of each juvenile stage was examined every month (Text-fig. 4b).
Since the juveniles in the Phleger core samples were not abundant, those from the bottom net
samples were added. Individuals of the A-5 stage and older occur throughout the year, and no
significant tendency was observed such as a particular moult stage being concentrated in any one
month. Living specimens of the A-6 stage were not observed from May through July, and those of
the A-7 stage were not found from February to March, or from May through September. However,
a female adult was found carrying probably mature eggs, which, together with the numbers of individuals of the A-5 stage and older confirmed every month, implies that individuals of the A-7 stage
also exist throughout the year, as well as those of the A-6 stage. It is inferred, therefore, that
C. acupunctata spawns all year round, although the number of eggs produced may vary with
the season and is less in summer. The length of time needed to grow from hatching to maturity and
the life span after maturity are not yet exactly known. However, we estimate that the growth
period, although we think it fluctuates with the seasons, takes around three months on the basis
of the time lag between juvenile and adult peaks in Text-fig. 4a.
326 N. IKEYA
AND H. UEDA
4-Seasonal distribution of living Cytherornorphu ucupunctutu individuals. 4a: Change in number of
individuals in the Phleger quantitativesamples. 4b: Change in the percentage of each juvenile stage in the total
samples including net samples.
The carapace of C. acupunctata varies widely from one individual to another in size and surface
ornamentation. Particularly, it has already been pointed out that there are two morphotype patterns in their carapace sculpture.
Variation in size: Variation in carapace size was analysed using the measurements of all the
samples from all months mentioned in the preceding chapter. The distributions of the two parameters L and H are nearly normal in nine of the ten groups of points in the graph described above
(Text-fig. 2a) that were formed on the basis of the moult stage and the sex, the only exception
being the group representing the A-2 stage. The rejection ellipse method was applied to each of
these groups (Text-fig. 5). It will be seen that only the ellipse corresponding to the A-2 stage is
rather stretched out and does not represent well the actual distribution of the points. This suggests
that both males and females coexist in the group representing the A-2 stage, and thus the morphological characters of both sexes are reflected here.
Variation in surface ornamentation : Individuals with less developed reticulation have punc-
Mophological Variations of Cytheromorpha acupunctata 327
aoo - -
eio f r m )
5-Lengthweight diagram with series of rejection ellipses of ten groups computed from five values
(means and standard deviations of both Length and Height, and orientation of plotted distribution) with 99 %
tation all over their carapaces. About 500 puncta occur on each valve, and each punctum is roughly
the same size (about 70 pm in diameter). Reticulation is formed by the development of muri
surrounding several (two to five) puncta, and somewhat obscure puncta are often seen inside
each fossa. The reticulation in this species, therefore, is an example of second-order reticulation
(Sylvester-Bradley and Benson, 1971). The location, shape and size of each fossa is consistent
among individuals of this species, probably because the number of muri included in each fossa
is roughly constant (macroreticulation in Liebau, 1977). However, the degree of development of
the muri of the corresponding fossa differs widely from one individual to another; a continuous
variation was observed between weak and low muri that are not much different from puncta, and
those that form high and steep projections surrounding faint puncta. It was also observed that the
degree of development of the muri is not consistent all over the carapace; a mixture of puncta and
fossae was found in a number of individuals.The area in which reticulation develops also varies from
one individual to another. Generally, however, reticulation develops most often in the posterior third
of the shell. These observations revealed that the surface Ornamentation of the carapace in this
species is not a character with two distinct forms but rather has a wide and continuous variation.
For convenience, we recognised three morphotypes based on the relative development of reticulation seen under the microscope. Those with reticulation covering almost the entire carapace
(80 % or more) were classified as Coarse Type, those with only punctation as Fine Type, and those
between these two extremes as Middle Type. All 1570 specimens were placed in one of these three
categories. It is to be remembered that these are only for convenience and do not represent distinct
polymorphic features, because the development of reticulation is in fact a continuous character.
Observation of broken sections of adult male carapaces by SEM revealed a clear positive relationship between the degree of development of reticulation and the thickness of the carapace. In the
posteromedian area, the carapace thickness of Fine Type specimens was about 2 pm on puncta and
4 to 5 pm around them, whereas that of Coarse Type specimens was 4 to 5 pm on puncta and
up to 15pm in muri parts. Moreover, the carapace thickness of each specimen is almost constant all
over, except in the marginal zone. Even in Middle Type specimens, where reticulation and puncta-
328 N. IKEYA
AND H. UEDA
tion exist together, there was no fluctuation in thickness associated with the degree of external
surface ornamentation in any part of the carapace. In summary, the more reticulate the carapace,
the thicker it is; a highly reticulated carapace becomes almost twice as thick as a carapace with
Development of the surface reticulation: In order to know how the degree of carapace surface
ornamentation changes with growth and how it is related to carapace size, rejection ellipses for all
individuals of each ornamentation type, Coarse, Middle, and Fine, and the measurements of specimens of the corresponding type were drawn on the same graph (Text-figs. 6a, b, c). Fine Type specimens occur in all moult stages, whereas more coarsely reticulated specimens of the Middle and
Coarse Types are found only in the advanced moult stages. Fine Type specimens, on the contrary,
decrease significantly in the adult stage. This indicates that no individual of C. ucupunctata is reticulated on hatching, and that individuals with developed reticulation increase with growth or
moulting. The degree of surface ornamentation, therefore, may be considered to be a morphological variation associated with growth. Regarding sexual difference, a wide range of variation is
observed in adult males from individuals with no reticulation at all to those that have reticulation
all over the carapace. In females, on the other hand, individuals with only punctation are rarely
seen, (i.e. ornamentation is generally coarser than in the male, as stated in Sandberg, 1964), and the
range of variation is not so wide.
Some difference in surface ornamentation is seen to be associated with adult carapace size. The
coarser the surface ornamentation, the smaller both L and H; the weaker the reticulation, the larger the size. This tendency is observed in both sexes in the adults, but is not very clear in juveniles.
The measurements L and H in adults of both sexes were analysed statistically with respect to each
type of surface Ornamentation (Table 3).
Seasonal change in morphological variations : The preceding sections show that there are two
kinds of morphological variations in the carapace of C. ucupunctutu, namely carapace size, and
degree of development of surface reticulation. It was also found that these two are likely to be related to each other. On the basis of these results, we then examined how these variations change
with the season, using living specimens from the samples collected every month. The ratio of Coarse,
Middle and Fine Types was calculated with respect to the adult individuals found in all the samTABLE
AND STANDARD DEVIATIONS)
OF ADULT Cytheromorpha
51 1 .O
I-Cytheromorpha acupunctata (Brady, 1880) (all figures x 100).
Figs. a, b. External lateral view of F-type male, right and left valve, IGSU-0-458.
Figs. c, d. External lateral view of M-type male, right and left valve, c: IGSU-0-460, d: IGSU-0-461.
Figs. e, f. External lateral view of Ctype male, right and left valve, e: IGSU-0-463, f: IGSU-O-464.
Figs. g, h. Interior lateral view of M-type male, left and right valve, IGSU-0-466.
Fig. i. Dorsal view of F-type male carapace, IGSU-0-459.
Fig. j. Dorsal view of M-type male carapace, IGSU-0-462.
Fig. k. Dorsal view of C-type male carapace, IGSU-0-465.
Figs. 1, m. Dorsal view of A-1 carapaces, 1: male, IGSU-0-478, m: female IGSU-0-481.
bLength/Height diagrams with the same series of rejection ellipses as in Text-fig. 5.6a: Measurements
based on F-type specimens (figured specimen: right valve male, IGSU-0-482). 6b: M-type specimens (figured
specimen: right valve male, IGSU-0483). 6c: Gtype specimens(figured specimen: right valvemale, IGSU-O484).