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Chapter 25. Morphological and ethological adaptations of Ostracoda to microhabitats in Zostera beds

Chapter 25. Morphological and ethological adaptations of Ostracoda to microhabitats in Zostera beds

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304 T. KAMIYA



to estimate the functional characters of ostracod morphology, especially that of carapace shape.



THEENVIRONMENT

OF Zosteru BEDS

The eel grass, Zostera marina is one of the most popular and abundant sea grasses throughout

the world. The plant, consisting of several thin leaves, 50-100 cm in length and about 1 cm in width,

is joined to other plants by subsurface stems and grows in stock to form Zostera beds. A leaf has

a short life, falling off within about two months and being swept away from the beds. There is an

obvious seasonal vicissitude in the Zostera beds. The leaves grow thickly from spring to early

summer and decline from late summer to winter.

Samples were collected from the Zostera beds in Aburatsubo Cove which is located near the

southern tip of the Miura peninsula, Pacific coast of central Japan (Text-fig. 1). Aburatsubo Marine Biological Station of the University of Tokyo is located on shore. The water temperature in

the cove ranges from about 27°C (Aug.) to 8°C (Feb.).

Here, Zostera marina grows rankly in shallow water, 30 cm to 2 m deep during spring low tide,

along the shore near the mouth of the cove (Text-fig. 1). The substratum in the Zostera beds is composed of medium- to coarse-grained sand. The surface is covered with a soupy flocculent layer

less than about three millimeters thick.

Three micro-environments were postulated in these Zostera beds where the benthonic Ostracoda

might live. They are the surface of Zostera leaves, the surface of the sand bottom, i.e. in and on the

surface of the flocculent layer, and the subsurface interstices between the sand grains. The Zostera

leaves, which have smooth, flat surfaces, stand upright from the bottom because they hold air



TEXT-FIG.

I-Location of Aburatsubo Cove. Hatched area in the cove shows the distribution of Zosteru beds.

MMBS: Misaki Marine Biological Station of the University of Tokyo.



305



-Formalin



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60% Alcohd



250 mesh



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. . . . . .. . . ..... .



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TEXT-FIG.

24ampling and preparation procedures of Zosteru phytal fauna.



250mesh , Dry



TEXT-nb.34ampling and preparation procedures of sand bottom fauna.



306 T.KAMIYA



bubbles in cavities within the leaves. Because of these features, the habitat formed by the surface of

the Zostera leaves may contrast strikingly with that of the sand bottom surface for Ostracoda.



SAMPLING

AND PREPARATION

Three kinds of samples, Zostera leaves, sand bottom surface and subsurface sand, were first collected separately. However, no Ostracoda were found living in the subsurface samples, and thus

later seasonal and periodic samplings were made from leaves and surface sediments alone.

To collect Ostracoda on Zostera leaves, a Zostera leaf was covered with a plastic bag, and cut

off from the plant with a scissors (Text-fig. 2). The phytal fauna on the leaves was fixed with formalin,

then dipped into 60 % alcohol and the Ostracoda were picked up under the microscope. The Zostera

leaves could be subdivided into old and young leaves or tip and base of the leaves. However, as

there proved to be no difference in ostracod composition among the subdivided samples, they were

then treated collectively.

The surface of the sand bottom was sampled quantitatively by using a small plastic sampler of

box-type (Text-fig. 3). After fixing with formalin, the surface materials were washed through a

250 mesh/inch sieve and dried. They were then separated into light and heavy grains by CCl.,, and

the separations were repeated twice for the sunken residues. Ostracoda were picked up from the

light grains under the microscope. Test samplings were made separately for the bottom samples

according to the thickness of the flocculent layer, but no difference in Ostracoda could be found

in these samples.

Subsurface sand was dug out from a horizon about 1-20 cm below the surface, but no living

Ostracoda were found in it.

Specimens with perfect soft parts were regarded as living individuals and others, such as those

with only the epidermis and some of the appendages remaining, were treated as dead individuals.

Only living individuals were used for later analyses.

Samples were collected once or twice a month from September 1983 to February 1985 around

a fixed point at the centre of the Zostera beds.



OSTRACODA

IN THE Zostera BEDS

As a result of precise samplings, two ostracod faunas were found, one living on the Zostera

leaves and the other on the surface of the sand bottom (Text-fig. 4).

Eight species are found to be living on the leaves of Zostera and constitute the Zostera phytal

fauna. In this fauna, Loxoconchajaponica is predominant from early summer to early autumn, making

up 94-99 % of all individuals. Paradoxostoma coniforme, P. setoense and Cytherois ezoensis are

equally dominant from winter to spring, altogether accounting for 80-95 % of the whole population.

Xestoleberis hanaii, Hemicytherura kajiyamai, Loxocorniculum mutsuense, and Aurila munechikai

are rare throughout the year.

The surface of the sand bottom is inhabited by ten species which are here referred to as the sand

bottom fauna. The dominant species, Loxoconcha uranouchiensis, is particularly abundant from

autumn to winter. Aurila uranouchiensis, Callistocythere rugosa, Cytherois sp. and Loxoconcha sp.

are the main associate species which are commonly found in most seasons. Xestoleberis hanaii is

rare, but consistently found. Several individuals of Hemicytherura tricarinata, Semicytherura

elongata, Keijella bisanensis, and Loxoconcha bispinosa are occasionally found. Xestoleberis hanaii

is the only species common 40 both faunas, although the number of individuals is always small.



Adaptations of Ostracoda to Microhabitats 307



Distribution of these species in other environments was checked in the area surrounding the

Zostera beds. The dominant species of the phytal fauna, Loxoconcha japonica, lives exclusively on

the leaves of Zostera. Paradoxostoma setoense was found to live abundantly not only on the

Zostera leaves, but also on the green alga Ulva in tide pools on the rocky shore. Xestoleberis

hanaii, Hemicytherura kajiyamai and Loxocorniculum mutsuense have their main habitat in the

bushy, calcareous alga Corallina in tide pools. Aurila munechikai lives mainly on the Sargassum

and Zostera which grow along the mouth of the cove. The dominant species of the sand bottom

fauna, Loxoconcha uranouchiensis, lives on the sand bottom in and around the Zostera beds.

Aurila uranounchiensis, Callistocythere rugosa, Cytherois sp. and Loxoconcha sp. are also abundant

on the sand bottom in and around the beds, but the last has a tendency to be more abundant

in the deeper parts. Hemicytherura tricarinata is common on muddy sand bottoms with poor circulation as, for example, in the Zostera beds in the inner part of Koajiro cove (Text-fig. 1). KezJella

bisanensis and Loxoconcha bispinosa live chiefly on the mud bottom at the centre of the cove.

Several pairs of congeneric species were recognised between the phytal and the sand bottom

fauna (Text-fig. 4). Loxoconchajaponica, the dominant species of the Zostera phytal fauna has its

counterpart Loxoconcha uranouchiensis in the sand bottom fauna. Other examples are Hemicytherura kajiyamai in the phytal fauna and H . tricarinata in the. sand bottom fauna. Cytherois ezoensis

and C. sp., and Aurila munechikai and A. uranouchiensis. A parallel relationship in species composition is recognised between the two faunas which live in two adjacent, but distinctly different

microhabitats. This relationship will aid analysis of the functional and adaptive morphology of

the ostracod carapace.



ZOSTERA



PHYTAL FAUNA



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SAND BOTTOM FAUNA

BARREN



TEXT-FIG.

&The



two ostracod faunas in the Zostera beds. Dashed arrows show pairs of congeneric species

betwen the phytal and the sand bottom faunas.



308 T.KAMIYA



CARAPACE SHAPE OF OSTRACODA IN THE ZOSlera BEDS



The species in each fauna have their own common characters of carapace shape (Text-fig. 5).

The carapace shapes of the phytal species are circular or oval in lateral view and rounded, rugbyball-shaped, in posterior view. The carapace shapes of Loxoconcha japonica (Text-figs. 5-1, 5-2),

Loxocorniculum mutsuense (Text-fig. 5-5) and Aurila munechikai (Text-fig. 5-7) are typical

examples. Paradoxostoma spp. (Text-figs. 5-13,5-14) and Cytherois ezoensis (Text-fig. 5-1 1) have

shapes which are somewhat elongated in the anterior-posterior direction, with a ventral contact

margin sharper than is typical.

Conversely, the carapace shapes of the sand bottom species are rectangular in lateral view and

triangular or circular with a flat “ventral plane” in posterior view. The typical representatives with



TJ~XT-FIG.

5-Carapace shapes (lateral and posteriorviews) of Ostracoda in the Zostera beds. 1. Loxoconchajapm-ca

(male). 2. L. japonica (female). 3. L. uranouchiensis (male). 4. L. uranouchiensis (female). 5. Loxocorniculum

mutsuense. 6. Loxoconcha sp. 7 . Aurila munechikai. 8. A. uranouchiensis. 9. Hemicytherura kajiyamai. 10. H.

tricarinnta. 1 1. Cytherois ezoensis. 12. C. sp. 13. Paradoxostoma coniforme. 14. P. setoense. 15. Semicytherura

elongata. 16. CaIli&ocythere rugosa.



Adaptations of Ostracoda to Microhabitats 309



a triangular posterior view are Loxoconchu urunouchiensis(Text-figs. 5-3, 5-4), L . sp. (Text-fig. 5-6),

Aurila uranouchiensis (Text-fig. 5-8) and Semicytherura elongutu (Text-fig. 5-1 5). They have a

distinct edge along the ventral part of the carapace, where the carapace bends inward and makes a

flat ventral plane. Cytherois sp. and Keijella bisunensis are examples of a circular outline in

posterior view. These carapace shapes are shown schematically in Text-fig. 6.

These morphological characteristics common among taxonomically diverse species are thought

to be most probably a consequence of adaptations to life on Zosteru leaves or, for the other fauna,

on the sandy bottom. A pair of Loxoconcha species, Loxoconchu japonicu (Text-figs. 5-1,5-2), the

proper Zosteru species and L. urunouchiensis (Text-figs. 5-3,5-4), the proper sand bottom species,

are ideal for comparison because both species are most abundant and characteristic of their

respective microhabitats. The male of the phytal L. japonicu has an inflated posterior part and a

truncated ventral margin, apparently different from the rectangular lateral view and distinct ventral plane of the sand bottom L . urunouchiensis. This contrast in ventral shape was pursued back to

the early developmental stages (Text-fig. 7). Juveniles of both species grow through seven ecdyses

before the adult stage. At the first stage, they closely resemble each other being distinguishable only

by the shape of their dorsal margins. The differences in the ventral area and the posterior part become clearer as they grow and complete at the final adult stage. The anatomy of the soft parts revealed that the swollen and inflated posterior part of the phytal species provides space to accommodate the enormous penis with its longstem(Text4g. 8). On the contrary, the sand bottom species

with a slender posterior part has a short sexual organ without a long stem (Text-fig. 8).



Shape of carapace

lateral view

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Loxoconcha japonica

Loxocornicuiurn rnutsuense

Paradoxostorna coniforme

P.

setoense

Cytherois ezoensis

Hernicytherura kajiyamai

Auriia rnunechikai

(Xestoieberis hanaii)

Loxoconcha uranouchiensis

L.

bispinosa

L.

SP.

Auriia uranouchiensis

Hernicytherum tricarinuta

Sernicytherura elonga ta

Cytherois sp.

Keyella bisanensis

Caliistocythere rugosa

(Xestoieberis hanaii )



310



L. japonica



L. uranouchiensis



Adult



A-1



A-2



A-3



A-4



A-5



A-6



A-7

TEXT-FIG.

7-Morphological Ehanges during development of the carapaces of the phytal and the sand bottom

species of Loxoconcha. Scale bar 100 pm.



Adaptations of Ostracoda to Microhabitats 31 1

~~



L. japonica



L . uranouchiensis



TWT-FIG. 8-Male sexual organs of the phytal and the sand bottom species of Loxoconchu.



The fact that the differences in carapace shape between the two species become apparent at

the adult stage and are related to the morphology of the sexual organs, suggests a causal relationship between carapace morphologies and reproductive behaviour.



RELATIONSHIP

BETWEEN

MORPHOLOGY

AND ETHOLOGY

Ostracoda generally reproduce through copulation. There are three known couplatory positions-ventral to dorsal, ventral to posterodorsal, and ventral to ventral (Elofson, 1951 ;McGregor

and Kesling, 1969). Little is known of the relationship between the copulatory positions and their

life habits or habitat characteistics.

*

Carapace Morphology and Copulatory Behaviour

According to the writer’s observation in petri-dishes in the laboratory, the sand bottom species,

Loxoconchu urunouchiensis copulates in the ventral to ventral position (Text-fig. 9. P1. 1, figs. 8-1 1).

In the dish, a male actively looks for females and then comes to meet one of them. It climbs on to the

carapace of a favourite female, clings to the lateral side of the female in what is probably courtship behavior and then, if the female opens the carapace, slowly makes the ventral to ventral contact. They copulate lying down or standing on their heads on the substrate, holding on to each

otheqwith their appendages, for more than twenty minutes. In both males and females of this species



312 T. WIYA



the carapace has a flat ventral plane which is suitable for copulation in the ventral-to-ventral position in terms of geometry.

On the other hand, the congeneric phytal species, Loxoconchujuponica copulates in the ventralto-lateral position (Text-fig. 10. P1. 1, figs. 1-7). A male aggressively looks for females and tries to

step on the carapace of a female which is creeping along. If a stepped-on female stops creeping, the

male clings to the lateral side of the female (courtship behaviour). The moment the male quickly

stretches its penis, the female opens the carapace, and firmly clings to the surface of the leaves

to support the male and herself, and the male copulates with the inclining female keeping the same

clinging position. To complete this series of steps usually takes only a few seconds, and copulation

itself lasts less than three seconds. If the phytal species were to copulate in the complete ventral-toventral position holding on to each other by appendages like the sand bottom species, the coupling pair could not support themselves, but would slip down from the Zosteru leaf to the bottom.

This would not be comfortable for them.

The rounded carapace of this species makes such a copulatory position possible. When a female

clings to a Zosteru leaf in an upright position, the rounded ventral margin leaves space between the

leaf for the penis to pass through and be inserted into the opened carapace of the female. A

gentle inflation of the carapace along the ventral margin allows the carapace edges of both sexes to

touch diagonally. Even though the carapace shape is ideal for the ventral-to-lateral position, in



+

-(



0



scores of sec.



several min.

almost 30 min.

TEXT-FIG.9-Copulation of the sand bottom species, Loxocutrchuurunouchiensis. Left: view from above.

Right: side view.



PLATE1-Copulatory behaviour of the phytal and the sand bottom species.

Figs. 1-7. Phytal Loxoconchujuponicu.1-4.5-7. Respective steps in copulation by two pairs of same individuals.

1,2,5. Courtship behaviour. 6. The moment of extension of the hemipenis. 3,4,7. Coupling. Owing to the

strong light and the slippery surface of the petri-dish, they got tired and were steeply inclined or slipped off.

Figs. 8-1 1. Sand bottom Loxoconcha uranouchiensis. 9-10. Successive behaviour of same individuals. 8.

Courtship behaviour. % l l . Ooupling. (m: male; f: female; p: penis).



313



a



TEXT-FTG.

IO-Copulation of the phytal species, Loxoconchujuponicu. Left: view from above. Right: side view.



this position the sexual organs of both sexes are still some distance apart from each other in an

oblique position. Thus it is quite reasonable that the penis of this phytal species is large with a long

flexible stem which can bend easily (Text-fig. 8). The shortness of the mating time is also advantageous in this habitat.



Other Morphological and Ethological Characters

All phytal species generally have rapid and active behaviour. This seems essentialfor life on the

unstable leaves of Zostera. Their morphology is suitable to such a habitat.

Paradoxostoma spp. and Cytherois ezoensis have thin carapaces to lessen body density.

This can be seen when the specimens are dipped in 60% alcohol and sucked up with a syringe.

They float up to the surface together with detritus and fine grains under the influence of a weak

current. They can easily support their body on the inclined surface of the Zostera leaves and walk

fast on the leaves. On the contrary, a swift current is needed to pick up Loxoconcha japonica and

sand grains.

While Paradoxostoma and Cytheroisadapt to the habitat on the Zostera leaves with their low body

density, Loxoconcha japonica, whose carapace is thick and heavy, utilizes unique appendages with

long hooks at their tips (Text-fig. 11). This hook structure is useful for supportingtheir bodies, creeping about and climbing the smoooth and perpendicular surfacesof the leaves. During copulationthe

male hangs on to the female by means of its antennae and first, second and third thoracic legs, all of

which have hooks (Pl. 1, figs. 1-7). The female supports the male and herself with her hooks. Textfig. 12 shows a difference in the behaviour of phytal and bottom species in a plastic bucket just

after sampling. The adults of the phytal species Loxoconcha japonica never stay still, but creep

around quickly and immediately start to climb up the perpendicular side wall. Many of them



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