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Chapter 32. Distribution of Recent Ostracoda in Ise and Mikawa bays, Pacific coast of central Japan
414 A.-M. BODERGAT
AND N. IKEYA
The study of Ostracoda has recently advanced from merely reporting sporadic discoveries of
local occurrences to defining the distribution areas of each species. This, in turn, has opened the
way for the discussion of speciation processes. However, clarification of speciation processes calls
for a large amount of data. In areas around the Japanese Archipelago, however, data on the occurrence of ostracod species are too scarce, partly because of the short history of their study. There is
now a definite need to collect more data in order to understand better the Japonic Realm ostracod
faunas as well as to enable us to discuss the specific relationships between this and other areas.
The purpose of this study is to clarify the distribution of Recent ostracod species in Ise and the
adjacent Mikawa Bay on the Pacific coast of central Japan. This kind of study will enable us to
make precise statements about the autoecology of the ostracod species, as well as about the extent
to which the environmental parameters influence the species associations.
According to Yoshimura (1934), the large bays around Japan are classified into two types on the
basis of their depth; namely, bays with depths of more than 1,000 m, such as Sagami and Suruga
Bays, and those with depths of less than 100 m. The latter are classified into two further types:
1) Bays deeper than 50 m with steep shores and less developed tidal flats and deltas, such as Aomori
Bay and the Inland Sea, and 2) Those that have depths of less than 50 m with shoals and well
developed tidal flats and deltas. Examples of these include Tokyo, Osaka, Ise, and Mikawa Bays,
the last two being the objects of this research.
Ise and Mikawa Bays have recently been noted for primary and secondary water pollution
caused by waste water discharged from Nagoya, a huge industrial centre which is located behind
these two bays. Among the various environmental factors, the distribution of sediment materials is
of particular importance, since it strongly affects the abundance and faunal composition of osttracod associations. Attention is also paid to seasonal changes in the environment which, through
changes in the constituents of the sediments, affect the ostracod fauna in each station investigated.
OF THE AREAS STUDIED
Ise and Mikawa Bays, like Tokyo, Osaka, and Hakata Bays, are typical examples of the inner
bays of Japan in which eutrophication has been notable in recent years. One thing that all these inner
bays have in common is that large cities and industrial areas are located in their catchment areas,
and thus the discharge load of waste water from these areas is extremely large.
Ise and Mikawa Bays are also typical inner bays of Japan from the geographical point of view
in that: 1) They have relatively large areas (Ise Bay has an area of 1,620 km2 and that of Mikawa
Bay is 510 km2).2) They are not very deep and the centres of these bays form a furrow (The average
depth of Ise Bay is 19 m, while that of Mikawa Bay is 9 m). 3) Large amounts of fresh water from
rivers flow into these bays. The total amount of fresh water flowing in annually reaches 80% of the
volume of each bay (Saijo, 1984). 4) The mouths of these bays are scattered with islands, and the
channels are narrow. Thus the bays are half-closed, and the exchange of water with the open sea is
limited (Text-fig. 1).
Sediments: Mud dominates the bottom sediments in all parts of the bays except the mouths and
coastal parts. Coarse materials increase towards the mouth of both bays, and gravel is distributed
around the basement which is composed of Tertiary and Pre-Tertiary rocks and is exposed in the
channel part. Distribution of the bottom surface sediments based on grain size analysis is shown
in Text-fig. 2. In this research we also classified the bottom sediments into seven types based on the
particles which were left on a 200 mesh sieve (see the sediments types IV and Text-figs. 2 and 3).
Currents: Two kinds of water current exist in these bays; namely, the tidal current which changes periodically twice a day and the residual current, which is the mean of all the currents. The tidal
Recent Ostracoda in Ise and Mikawa Bays, Japan 415
current, which flows in from the open sea through Irako Channel reaches speeds of as much as
2 kts around the channel. It then branches into Ise and Mikawa Bays, and the current speed decreases to 0.4 kt and 0.2 kt, respectively, towards the head of each bay. The main part of the tidal
current in Ise Bay runs along the Chita Peninsula, and the western part of this bay forms a counter
current area. Another branch, which passes through the Morozaki and Nakayama Channels,
enters Mikawa Bay, and then further divides into two streams, one of which runs towards the north
and the other towards the east. The residual current, although its velocity (about 0.1 kt) and constancy are not great, is important because of its effect on the formation of water masses and distribution of substances in the bay. This current is evident in summer, but not in winter. Roughly
speaking, a counterclockwise circular current is seen in the southern half of Ise Bay, and a clockwise current in the northern half; whereas in Mikawa Bay a counterclockwisecurrent is found in the
central and inner parts, and an outward stream near the mouth of the bay. In addition, a vertical
circulatiw has been observed near the mouth which flows inward in the bottom layer and outward
in the upper layer of water (Unoki, 1978).
Temperature and Chlorinity: The two bays resemble each other with respect to seasonal changes
in the water temperature and the chlorinity. In winter the upper and lower water layers in these
bays are well-mixed and thus the water temperature is slightly higher in the lower layer than in the
upper, i.e., inversion of water temperature takes place. However, inversion is not observed in the
water density. Isopleths representing 17 %O and 19 %O chlorinity are used as indices of the limits of
open sea influence and land drainage, respectively. In spring stratification develops slowly in the
water iqthe bays; in May the isopleth representing 17 %O chlorinity moves to the mouths of the
2-Distribution of the bottom surface sediments based on grain size analysis (after Aichi Fisheries
Experimental Station, 1970).
3-Distribution of the bottom surface sediments based on the materials and grain size of the particles
which were left on a 200-mesh sieve (1 : Diatom facies, 2: Diatom and pellet facies, 3: Pellet facies, 4: Fine
biogenic facies, 5 : Coarse biogenic facies, 6: Fine to medium sand facies, 7: Coarse sand facies).
Circles indicate the tot31 number of ostracod specimens per unit sample (lcc).
Recent Ostracoda in Ise and Mikawa Bays, Japan 417
bays. In summer stratification reaches its peak. Both the isopleths representing the water temperature and those representing the chlorinity become parallel to the water surface in the bay. The
top layers down to 10 m deep are all occupied by water with a high temperature and low chlorinity
(17 %o). In autumn, stratification is promptly destroyed, an event which takes place in September
Transparency: The transparency is used as an index of marine pollution, i.e.. it is closely related
to the amount of chlorophyll produced by phytoplankton which causes secondary pollution of the
sea water. In these areas the transparency increases in winter and decreases in summer, and it
decreases towards the head of the bay. The isopleths of transparency are similar to those of chlorinity. Transparency in these areas has decreased radically from 6 m to 3 m over the decade from
1961 to 1971. In 1980 as many as 98 occurrences of red tide blooms were observed, lasting for 297
days in total. Occurrences of red tide blooms have been increasing in the last decade, and are now 2.7
to 4 times as common as a decade ago. Sekiguchi (1975) reported that species of the genus Conodiscus dominate among the diatoms which cause red tide blooms. Concentrations of more than 0.1
ppm inorganic nitrogen and more than 0.015 ppm inorganic phosphorous are used as indicators of
the occurrence of red tides (Unoki, 1984).
Low oxygen water mass: In summer the oxygen concentration decreases, not only since oxygen
transport from the upper to the lower layer is prevented by the stratification, but also because
organic substances dropping from the upper layer decompose. This phenomenon has been particularly notable in Mikawa Bay where the size of the low oxygen water mass has been increasing
year after year, sometimes covering even the whole bay.
During the “84-R-5” Cruise by the research vessel “Seisui-maru”, a Mie University, Faculty
of Fishery, on June 2nd (Stations 1-13) and 3rd (Stations 14-35) 1984, 35 sediment samples were
collected in Ise and Mikawa Bays. The sediment samples were collected by a Smith-McIntyre grab
sampler (32 x 35 cm in surface; 18 cm in maximum depth) at stations disposed all over the bays at
intervals of 3 to 10 km, so that they represent practically all parts of the bays. In this study we
examined 23 samples from Ise Bay and 5 from Mikawa Bay (Text-fig. 1). About 500 cc was
taken from the surface part of each muddy or -sandy sediment sample, stained with Rose
Bengal, washed immediately through a 200-mesh sieve (74 pm openings), and then dried. We
tried to collect at least 200 specimens of ostracod individuals at each station, though this object
was not achieved at all the stations. Living individuals were easily distinguished by the reddish coloured soft parts when stained with Rose Bengal. The volume of each surface sediment
sample was measured in order to calculate the numbers of individuals and species per cubic centimetre. It should be noted that the volumes measured are those of the dried samples. The volumes
of the raw samples were not measured exactly. It is expected, however, that the dried sample
volumes, in combination with the sediment types in these sea areas which we studied before, will
be of use in estimating the population density of each ostracod species.
All the ostracod specimens used in this study are deposited in the Institute of Geosciences,
Faculty of Science, Shizuoka University.
To henthonic organisms the bottom surface sediment characteristics, which form their habitats,
418 A.-M. BODERGAT
AND N. IKEYA
are of particular importance. In most ecological studies the sediments are classified on the basis of
particle size. Sediment grain size distribution in a particular sea area is the net result of various
physical agencies and is one of the fundamental parameters of the environment which reflect certain
aspects of the hydrographic conditions. However, the sediment composition is no less important
to the benthonic organisms than the grain size distribution.
In this research we identified the constituent components of the particles retained on a 200mesh sieve (74 pm opening), and recognized seven sediment types on the basis of their component
substances and grain size distribution (Text-fig. 3). We also discuss the relationship between
the sediment types classified by this method and the ostracod species distributions.
The component substances of the sediments and their approximate amounts are summarized
in Table 1. Below are brief descriptions of the seven sediment types.
I) Coarse biogenic facies (Stations 4,8, 13 and 27): This facies is distributed along the eastern
coast of Ise Bay and the eastern part of Mikawa Bay. The main components of this facies include
coarse grains, molluscan shells and shell fragments, fragments of fish bone, spines and plates of
echinoderms. Pellets and rock fragments are also found, although they are not abundant; the
former increase slightly towards the head of the bay, and the latter towards the mouth. The foraminifers inhabiting this facies include the agglutinated Trochammina hadai which increases towards the head of the bay, and the calcareous Miliolid species and Nonionella stella which increase
towards the mouth. At station 13, which is close to the mouth of the bay, a small number of
planktonic foraminifers are also found.
11) Fine biogenic facies (Stations 1, 2 and 3): This facies is distributed in the innermost parts
of Ise Bay. Its components, like those of the previous facies, include minute fragments of molluscs, fish, and echinoderms. Pellets and diatom frustules are commonly found as well. Only
two species of foraminifers, Trochammina hadai and Haplophragmoides sp., both agglutinating
forms, inhabit this facies.
111) Diatom facies (Stations 7, 20, 22, 25, 28 and 29): This facies is distributed along the northern coast of Mikawa Bay and in the northern part of Ise Bay. Pelagic diatom frustules, mainly
Coscinodiscus spp., are its major component, while minute fragments of echinoderms, gastropods,
pelecypods, and fish bone are minor constituents. Pellets and rock fragments are almost never
found; instead mica and plant fragments, which easily become suspended in water, occur in large
quantities. Among the foraminifers, Trochammina hadai is found at all the stations except station
20. Miliolid species, although in very small numbers, are present at all the stations.
IV) Pellet facies (Stations 11, 15, 23 and 24): This facies is found in the deep central parts of
Ise and Mikawa Bays. The major components are rugby-football-shaped particles of three different sizes, the first large (0.8-0.4 mm in major axis, 0.5-0.2 mm in minor axis), the second
medium (0.6-0.2 mm in major axis, 0.3-0.1 mm in minor axis), and the third small (0.1-0.2 mm
in major axis, 0.05-0.1 mm in minor axis). Although the pellet sources are not identified, the first
two forms are probably attributable to polychaetes or gastropods (Schafer, 1953), whereas the
last one, which is domi?ant in station 15, is likely to be from copepods because of its size (Sekiguchi, 1975). Fragments of echinoderms, gastropods, pelecypods, and fish bone are common,
although much less in quantity than the pellets. Diatoms, plant fragments, mica, and other
pelagic materials are almost non-existent. Medium grained quartz and schist fragments are also
present at station 15. Of the foraminifers, Trochammina hadai is the only species found at station l l , whereas Miliolid species and Nonionella sp. are present at station 15.
V) Diatom and pellet facies (Stations 6, 10, 12 and 14): This facies is widely distributed on
the outer side of the pellet facies in Ise Bay. Pellets produced on the bottom and diatom frustules
produced in the water column occur in almost equal amounts. Calcareous biogenic materials are
not present in significazt amounts. Mica and rock fragments are found in small amounts at station
OF THE SAMPLING LoCATiONS, THE
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I: Macroscopicobservations.M: Mud, gM: Gravellymud, sM: Sandy mud, fs: Fine sand, G: Gravel. *1: Living
polychaetes were abundant. *2: Living echinoderms were abundant. *3: Living gastropods and pelecypods wete
abundant. *4: Living ophiuroidswere abundant. * 5 : The transparency of water was less than lm because of red tide
blooms. 11-XI: Microscopicobservations. 11: Fragments of spines and plates ofechinoderms; 111: Shellsof molluscs (gastropods, scaphopods, and pelecypods), fragments of them, andjuveniles; IV: Fish bones,vertebral columns
and parts of skeletons of small fish, and scales of fish; V: Fragments of thoracics; VI: Spicules of cornacuspongids;
VII: Foraminifers (p: planktonic, a: agglutinated, c: calcareous test); VIII: Diatom frustules; IX: Pellets excreted probably by invertebrates, such as polychaetes, gastropods, and copepods; X: Fragments of plants; XI:
Rock fragments. (M: Mica fragments, R: Quartz particles and fragments of gneiss, schists, etc.)
420 A.-M. BODERGATAND N. IKEYA
14, which is close to the coast, but do not occur at any other station. Of the foraminifers, the agglutinated Trochammina hadai is dominant at all the stations. Calcareous forms are very rare and
are only seen in Nonionella sp. and Miliolid species at stations 6 and 12.
VI) Fine to medium sand facies (Stations 9, 16, 17, 18, 19, 21, 26, 33, 34 and 35): This facies
is widely distributed in the coastal part of Mikawa Bay along the Atsumi and Chita Peninsulas,
around the mouth of this bay (Morozaki and Nakayama Channels), the eastern part of the mouth
and the western part of Ise Bay. Calcareous biogenic materials are rare. Fine to medium grained
sands made of rock fragments are the major component together with abundant mica fragments.
Of the agglutinated foraminifers, Trochammina hadai, is rare at stations 17, 19 and 21 although at
station 26 it is abundant. Of the calcareous foraminifers, the Miliolidae-Nonionella assemblage is
dominant at station 19, whereas the Ammonia-Elphidium assemblage dominatesthe other locations
near the mouth of the bay. Tests of calcareous foraminifers are particularly abundant at stations
34, 35 and 36, which are located in the channel area.
VII) Coarse sand facies (Stations 5, 30, 31 and 32): This facies is distributed along the western coast of the inner part of Ise Bay and in the western part of the mouth of the bay. Rock fragments are present in large amounts but mica is not abundant. Calcareous biogenic materials are
extremely rare at station 5, whereas molluscan shell fragments, although not abundant, are commonly found at the mouth of the bay. Sponge spicules occur at three stations near the mouth of
the bay. Large quantities of Balanus fragments are found at station 30. Fragments of echinoderms,
gastropods, pelecypods, and fish bones are all abundant at station 32. Foraminifers are found
only at stations near the mouth of the bay. A number of species are identified including Miliolid
species, Ammonia beccarii, and some oceanic species such as Cibicides lobatulus, Hanzawaia
nipponica, and Rosalina sp.
OF OSTRACODS IN RELATION TO ENVIRONMENTAL
Ostracods were examined at 23 stations in Ise Bay and 5 in Mikawa Bay, out of a total of 35
sampling stations in these bays. Station 9, in the western part of Ise Bay, was barren of ostracods
and only one ostracod was found at stations 10 and 14 respectively, in spite of the large quantity of
sediment examined. At all other stations significant numbers of ostracod individuals were collected.
A total of 63 genera and 127 species were recognized, and living and dead specimens were
distinguished as well. The total number of specimens per unit volume (lcc) from each station are
shown in Text-fig. 3. Distribution maps of 12 species that are characteristic of these bays are
shown in Text-figs. 4 and 5. It is apparent from Text-figs. 4 and 5 that some ostracod species
are distributed all over the bays, while others are restricted to either the inner parts, deep areas
in the centre, shallow aoastal regions, or the mouth of the bay. It is interesting to see that the
occurrences of some taxa are characteristically concentrated in particular areas.
Among the various environmental parameters, the grain size distribution and composition of
the bottom sediment, salinity of water, spreading of terrestrial materials, etc. have often been
referred to by some researchers (Ishizaki, 1968-1971) as major elements that determine or influence
ostracod distribution. We must remember, however, that the distribution of carapaces of any
particular species ultimately preserved in the bottom sediments may not necessarily reflect
the distribution of the individuals of that species during life. Clarification, by reference to the
hydrographic conditions, is needed on whether carapaces in the sediments belonged to autochthonous individuals or were transported from neighbouring areas.
TEXT-FIO.&Distribution of the six species characteristic of the bays. 4a: Cytheromorphu ucupunctuta (Brady,
1880), left valve male (St. 6), IGSU-0-486. 4b: Cytherois spp. 4c: Bicornucythere bisunensis (Okubo, 1975),
right valve female (St. l), IGSU-0-487. 4d: Ambtoniu Obui (Ishizaki, 1971). left valve female (St. ll), IGSU0-488.4e: Nipponocythere bicurinutu (Brady, 1880),left valve male (St. 8). IGSU-0-489. 4f: Spinileberis quudriuculeutu (Brady, 1880), left valve female (St. 26). IGSU-0-490. (Bars below the figures of species indicate 100
Circles indicate the percentage of individual specimens of the species for all ostracod specimens obtained
from the station.
TEXSFIQ. 5-Distribution of the six speciescharacteristicof the channel area. 5a: Ponrocytherejuponicu (Hanai,
1959). left valve male (St. 34), IGSU-0-491. 5b: Munseyellujuponicu (Hanai, 1957), right valve female (St.
32), IGSU-0-492. 5c: Semicytheruru miurensis (Hanai, 1957), right valve male (St. 31), IGSU-0-493. 5d: Schizocythere kishinouyei (Kajiyama, 1913), right valve female (St. 36), IGSU-0-494. 5e: Hemicytheruru cuneutu
(Hanai, 1957), right valve female (St. 32), IGSU-0-495. 5f: Puuenborchellu triunguluris (Hanai, 1970), left
valve male (St. 34), IGSU-90496. (Bars below the figures of species indicate 100 microns respectivelly.)
Circles indicate the percentage of individual specimens of the species for all ostracod specimens obtained
from the station.
Recent Ostracoda in Ise and Mikawa Bays, Japan 423
Substrate: According to observations by Ishizaki (1968-1971), the distribution of some ostracod
species is not restricted by the bottom surface sediment types; it even seems to be independent of
the distribution of the sediment types. Some examples of such ubiquitous species were in fact found
in this research as well.
One must remember, however, that both Ishizaki’s work and this research describe the distribution of ostracod species in terms of the total number of ostracod individuals of each species, including both living and dead specimens, which may not necessarily reflect the distribution of the
living individuals of that species. Carapaces of dead individuals of certain species inhabiting seaweeds on rocky shores and sandy shallow bottoms are transported by water currents, sorted with
the sediment materials, and then deposited at various places. Carapaces of dead individuals are
thus dispersed over wide areas extending far beyond those of living individuals, although their
occurrence becomes sparser as distance increases from the living habitat. In the area studied
Cytherois uranouchiensis, C. nakanoumiensis, and many juvenile valves of Loxoconcha modesta
are among the species that show such a tendency; they are distributed over wide areas without being
restricted by the bottom sediments. However, attention must be drawn to the numerous valves of
Cytheroisspp. concentratedat station 7 (Text-fig.4b). These species inhabit the seaweedzone on rocky
shores but not the muddy bottoms of central areas of the bay. Since their valves are thin and light
they were probably carried in suspension like diatom frustules and deposited by currents in the
central area of the bay. If so, they also fit into the postulated relationship with the sediment types
in our classification. Transportation of ostracod valves has already been pointed out by Kontrovitz (1975) and Kontrovitz and Nicolich (1979).
Some individuals of certain benthonic species that are found in great abundance at particular
stations, such as Bicornucythere bisanensis, Cytheromorpha acupunctata, Ambtonia obai, Nipponocythere bicarinata, and Spinileberis quadriaculeata, prove to have been transported from their
native habitats giving them an extended distribution area which is seemingly independent of sediment type. In such cases one can specify the living habitats of these species and their characteristic sediment types to some extent, since individuals, particularly living ones, are abundant in some
places and rare at others to which they have been transported. The five species listed here are all
characteristic of muddy-fine sediments of inner bays.
Bicornucythere bisanensis, as pointed out by Abe (I 983), inhabits mainly mud-rich sediments in
inner bays where the bottom currents are weak enough to allow deposition of a flocculent layer on
the bottom. In fact, this species is most densely concentrated in the innermost parts of these bays
(Text-fig. 4c). Cytheromorpha acupunctata and Ambtonia obai are distributed in deeper areas in the
centre of the bay, where the mud content is higher. Nipponocythere bicarinata is distributed all over
these bays, whereas Spinileberis quadriaculeata is concentrated in shallow areas along the coast,
where the mud content is not so high. The former, in comparison with the latter, tends to be more
densely distributed in areas where the sediment grain size is slightly larger. The latter is restricted
to the inner parts of both bays which are exposed to land drainage flowing in, suggesting that this
species is more adapted to a brackish environment.
Both Pistocythereis bradyi and P. bradyformis are found on muddy sand but have different distribution areas, the former occurring in the inner parts of the bay, the latter near the mouth. No
living specimens of these two species have been found and the living habitats are not yet known.
Pontocythere spp. and Callistocythere spp. are distributed at stations near the mouth and the
channels of the bays, where the sand content is high. This agrees with the results of previous research conducted in other areas.
Semicytherura miurensis, Schizocythere kishinouyei, Hemicytherura cuneata, Paijenborchella
triangularis, Munseyella japonica, etc. are also found exclusively at places near the mouth of the
bay where the sand content is high.
424 A.-M. BODERGAT
AND N. IKEYA
The numbers of living and dead specimens taken as a whole, indicate the net results of the
environmental parameters at that place. It is expected that this will provide information which
will be useful in dealing with fossil ostracod assemblages.
Marine and terrestrial influences: The influence of water flowing in from rivers is particularly
significant in these bays, compared with that of marine water which is introduced only through
the narrow mouths of the bays. Fresh water and various other substances carried in by rivers are
mixed with sea water and cause changes in the water and bottom sediments. The extent to which
these changes take place is not constant, but varies from one place to another in the bay. Distribution of ostracod species is also diverse, of course, in correspondence with the variations in the
environment. Occurrence of both marine and brackish species at any one place is considered to
indicate that the place is under the influence of both marine water and land drainage. This has
already been suggested by Wagner (1957), Rosenfeld (1979), Bodergat (1983), and Maddocks and
The amount of land drainage introduced is particularly large near the head of the bay and the
mouths of rivers along the coast, and its influence is assumed to be most significant. However, almost
no fresh water ostracod species have been found which would have been transported by rivers. The
brackish water species inhabiting these areas include Bicornucythere bisanensis and Spinileberis
quadriaculeata. The maximum numbers of Cytheromorpha acupunctata and Ambtonia obai, which
have been regarded as brackish water species, were found at stations 11 and 12, respectively. These
stations are all located in the deep area in the centre of the bay which is not under the direct influence of river water. The fact that no living specimens were found at these stations might imply
that the carapaces found there were transported from the neighbouring areas and deposited there.
The distribution patterns of Nipponocythere bicarinata, Pistocythereis bradyi, and Trachyleberis
niitsumai are similar to each other. They are concentrated near the head of the bay, but found all
over the bay as well although extremely rare near the mouth of the bay. We assume that these
species are characteristic of the inner bays where the salinity is lower than in normal marine water.
According to Hanai (personal communication), Krithe sp. has a natatory life style. This species
is densely distributed along the eastern coast of Ise Bay, which forms the route of marine water
flowing into the bay. This species is smaller in size than those from offshore bottom sediments, and
is likely to be a new species that has adapted to shallow sea environments.
As for Trachyleberis scabrocuneata and Pistocythereis bradyformis, Ishizaki (1968 and 1971)
has found a large number of individuals in muddy bottom sediments in bays. In the areas we studied, however, their distribution is restricted to the mouth of the bay which is under the influence
of marine water, and does not include the inner parts of the bay. The species listed in Text-figure.
5 are considered to be those under strong influence of marine water.
Industrialization: Water pollution has been particularly notable in recent years because of
rapid urbanization and industrialization. With the increase in water pollution, the number of
species (S) and population density (D), as well as the number of individuals (I) over the number of
species (S) of the benthonic organisms, increase moderately until the amount of precipitation of the
polluting substances OD the sea bottom reaches some limit. Once this limit is reached, however,
(S) decreases first, followed by (D), while the ratio (I)/@) increases. Further increase in precipitation of the polluting substances causes a radical decrease in both (S) and (D), as well as some decrease in the ratio (I)@). Still further increase in pollution makes habitation by living organisms
totally impossible (Kitamori, 1984) (Text-fig. 6).
The benthonic organisms include polychaetes, molluscs, crustaceans, echinoderms, and so forth;
among these, the first two are dominant on the muddy bottom in the inner bay. It is generally
assumed that the polychaetes are quite resistant to pollution, while the crustaceans are rather
sensitive and have lowtolerance. Iwaki et al. (1976) referred to the polychaete Paraprionospio pi-