Tải bản đầy đủ - 0 (trang)
3 Otomo-ura: A Restored Vacant Tidal Flat

3 Otomo-ura: A Restored Vacant Tidal Flat

Tải bản đầy đủ - 0trang

9 Colonization of Restored and Newly Created Tidal Flats by Benthic Animals



121



building during the summer of 2014, and about one third of the intertidal area has

been lost.



9.4

9.4.1



Benthic Animals in Otomo-ura

Sampling Methods



To find benthic animals of a variety of mobility types and life forms (epifauna and

infauna), we carried out both quantitative and qualitative samplings as a part of

research programs by Biodiversity Center of Japan (2014, 2015). In 2013 and 2014,

we quantitatively sampled the bottom sediment (to 20 cm) using a core sampler

(diameter: 15.4 cm) and collected small epifaunal and infaunal animals using a 1 mm

mesh sieve. Two points were placed in the central area, and one point was placed in

each peripheral area of the northern and southern ends of the tidal flat. Then, three

cores were taken for each point (Fig. 9.1). In the qualitative method, two persons

observed for 15 min the bottom surface around the point where core samples had

been taken and recorded sessile species on and mobile animals under cobbles and

boulders. Observers also sometimes dug up the bottom using trowels to find deeper

burrowers and infauna such as mud shrimp, Upogebia species, and a bivalve, M.

oonogai. The observations and samplings were conducted during a low tide of the

spring tide in August of 2013 and 2014. Similarly, we made observations and samplings during a low tide of the spring tide in August of 2012. However, in 2012 we

observed benthic animals in 12 quadrats (25 × 25 cm2) that were randomly placed for

each point. For species that were difficult to identify in the field, we took photos of

the quadrat surface and then collected the animals for species identification in the

laboratory. In addition, we were unable to obtain any quantitative samples for the

central area in 2012 because the bottom surface was covered with cobbles. However,

many juveniles (shell length: ca. 2–5 cm) of the pacific oyster C. gigas were observed

on those cobbles (Matsumasa et al. 2013, 2014), so we counted the oysters in eight

quadrats (20 × 20 cm2) during daytime low tide on 15 September 2012.



9.4.2



Changes in Species Richness



Eighty-five species of benthic animals were recorded using both quantitative and

qualitative methods in the Otomo-ura tidal flat. The number of species observed in

2012, 2013, and 2014 was 27, 51, and 59, respectively. Our quantitative investigation in 2012 was performed in only the peripheral area because the bottom surface

of the central area was obstructed by cobbles, as mentioned previously. In any event,

the use of such a method in 2012 would have overlooked animals living under cobbles and boulders, in addition to deeper burrowers, that might not have been



122

50

40



Number of species



Fig 9.2 Yearly changes in

the number of benthic

animal species on the tidal

flats of the peripheral and

central areas of Otomo-ura



M. Matsumasa and K. Kinoshita



30

20



10

0



n.d.

2012



2013



2014



Year

Peripheral area



Central area



overlooked in 2013 and 2014 (see above). However, species richness increased in

both the peripheral and central areas from 2013 to 2014 (Fig. 9.2), showing that the

colonization by benthic animals was in progress in the restored tidal area. Since the

number of species collected by the quantitative method for the peripheral area

changed little (21, 20, 17 in 2012, 2013, and 2014, respectively), the increase in the

total number of species there was mainly due to the increase of benthic animals collected by only the qualitative method, such as a chiton Acanthochitona sp. and gastropod species Omphalius rusticus and Batillaria attramentaria. In contrast, the

number of species in the central area increased from 38 in 2013 to 48 in 2014, and

both results included ten species collected by only the qualitative methods. The

increase in the total number of species in the central area was mainly due to the

increase of infaunal animals collected by the quantitative method, such as polychaetes Glycera nicobarica and Pseudopolydora spp. and small peracarid crustaceans Ampithoe sp. and Melita sp.

Figure 9.3 shows species discovery curves depicting the cumulative number of

species recorded in ascending order for six cores. In the central area, the increase in

species richness estimated by the quantitative method was obvious. The three curves

for the peripheral area and the one curve for the central area in 2013 increased

almost linearly with the increase in relative area, but only the curve for the central

area in 2014 had an inflection point and leached 33 species, which was the highest

value among the curves. This change in shape of curves for the central area is probably because the habitat heterogeneity of the area increased from 2013 to 2014. The

bottom substrate in the peripheral area was clayey sand, while that of the central

area consists mainly of cobbles associated with patchy deposition of fine sand.

Sessile organisms, such as the oyster C. gigas, a barnacle Amphibalanus amphitrite,

and algae, were observed on the cobbles. In particular, the oyster was conspicuous



9 Colonization of Restored and Newly Created Tidal Flats by Benthic Animals

40



Number of species



Fig 9.3 The relationship

between species richness

and relative area in

quantitative samples from

the peripheral and central

areas of Otomo-ura



123



2014, Central



30



2013, Central

2013, Peripheral



20



2012, Peripheral

2014, Peripheral



10

0



1



3



2



4



5



6



Relative area (number of cores)



Number of species / core



15



20

2014



2013

15



10



10

5

5

0



Peripheral Central



0



Peripheral Central



Fig 9.4 Number of benthic animal species collected using the quantitative method in the peripheral and central areas of Otomo-ura



in the lower part of the central area, and the estimated density (mean ± SE) was

540.75 ± 37.475 individuals per m2 (n = 8) on 15 September 2012. The growth of

sessile organisms, including C. gigas, could provide heterogeneous habitat by causing the deposition of sand for small animals, which responded specifically to the

microhabitats provided by the primary substrates (i.e., cobbles and deposited sand)

and secondary substrates (i.e., sessile organisms) (Matsumasa 1994). This might

also affect the species richness on a finer scale since the number of species per core

was significantly higher in the central area than in the peripheral area in both 2013

and 2014 (U-test, P < 0.05; Fig. 9.4).



9.4.3



Species Composition



Although species richness in the peripheral area was lower than that in the central

area, species composition indicates that the peripheral area supplied a unique habitat to benthic animals that had not been found in the central area. The reed-marsh



124



M. Matsumasa and K. Kinoshita



mud crab Helice tridens inhabited the upper part of the peripheral area. The estuarine ragworm Hediste sp. and the acorn barnacle Fistulobalanus albicostatus were

also typical inhabitants of the peripheral area. These species are brackish water

representatives, so the freshwater inflows in the peripheral area might have been an

important factor shaping the species composition there. On the other hand, major

species that were typical to the central area, such as the mud snail Nassarius (Hima)

hypolius, the tellin clam Macoma incongrua, the caridean shrimp Crangon sp., and

the pebble crab Gaetice depressus, are more polyhaline. Of the 85 species that were

found in the Otomo-ura tidal flat in this study, 17 were observed only in the peripheral area, and 23 were observed only in the central area. The peripheral and central

areas are both therefore considered to be important in supporting the biodiversity of

tidal flats in this region. However, as mentioned above, almost all of the peripheral

area was reclaimed for road building during the summer of 2014.



9.4.4



Abundance of Major Infaunal Animals



The reclaimed peripheral area had included the burrowing area of the reed-marsh

mud crab H. tridens. The reclamation might have some negative effects on this crab

population. We adopted the qualitative method to find animals under cobbles and

boulders, highly mobile animals, and deeper burrowing animals, including the mud

crab, which is the important component linking the upper intertidal burrowing area

with the lower intertidal flat (Takeda and Kurihara 1987; Takeda et al. 1988;

Kurihara et al. 1988), but our method could not evaluate the population densities of

those species. Our methods also could not clarify the change in abundance of one of

the most dominant species, the Pacific oyster C. gigas, which is an important fishery

resource (Matsumasa et al. 2013, 2014, 2015). Although our sampling methods had

these limitations, the abundances of some benthic animals that had been collected

by the quantitative method using a core sampler could be compared among years

and between areas. The Manila clam Ruditapes philippinarum is one of five such

dominant species. Although the effect of “year” on its abundance was marginal

[one-way ANOVA for log (N+1) transformed data; P = 0.0575], it still increased

from 2012 to 2013 and remained at a similar level in 2014 in the peripheral area

(Fig. 9.5). Its abundance in the central area (Fig. 9.6) also did not differ significantly

(alpha level: 0.05) between 2013 and 2014, and comparisons of its abundance

between the peripheral and the central areas for each year did not show any significant difference. However, the density of the polychaete Heteromastus species

clearly increased from 2012 to 2014. The effect of year was significant [one-way

ANOVA for log (N+1); P = 0.0103], and the Bonferroni test indicated that the difference between its abundance in 2012 and 2014 was significant (P = 0.0089) in the

peripheral area. Also, in the central area (Fig. 9.6), the abundance of the polychaete

species showed a significant difference between years (t-test, P = 0.0253) and was

not significantly different from those in the peripheral area. Another polychaete

Perinereis species did not show any significant differences in its abundance among



125



9 Colonization of Restored and Newly Created Tidal Flats by Benthic Animals

30



2



Ruditapes



20



Number of individuals (N / core)



Mya



1.5

1



10



0.5



0

10

8



0

30



Perinereis



2



Upogebia



1.5

20



6



1



4



10



2

0



Heteromastus



2012 2013 2014



0



0.5

2012 2013 2014



0



2012 2013 2014



Year

Fig 9.5 Changes in density for five dominant species collected using the quantitative method in

the peripheral area of Otomo-ura



15



2



Ruditapes



10



Number of individuals (N / core)



Mya



1.5

1



5



0.5



0



0



8



80



Perinereis



6



60



4



40



2



20



0



2013



2014



0



Heteromastus



3



Upogebia



2

1



2013



2014



0



2013



2014



Year

Fig 9.6 Changes in density for five dominant species collected using the quantitative method in

the central area of Otomo-ura



126



M. Matsumasa and K. Kinoshita



the 3 years or between the two areas and appeared relatively constant in core samples for every year and area. We could not find any significant differences in the

abundances of the bivalve Mya (Arenomya) arenaria oonogai and the decapod crustacean Upogebia species among the years or between the areas, but their abundances tended to decrease from 2013 to 2014 (Figs. 9.5 and 9.6). Since the adults of

the relatively large infaunal bivalve and the crustacean deep burrower are difficult to

catch using our core sampler (see Cross et al. 2012 for Mya; Kinoshita 2002,

Kinoshita and Itani 2005 for Upogebia), more specific investigations are needed for

these large benthic animals in the future.

On the whole, the bottom sediment of the restored tidal flat, which initially would

have been nearly lifeless substrate, has been inhabited by infaunal animals, and

some dominants increased in number after 2012. No significant spatial and temporal differences in abundance were detected for the five dominant species, indicating

that both peripheral and central areas would have been equally suitable for colonization by infaunal animals had adequate substrates been present.



9.5



Colonization by the Direct-Developing Gastropod

Batillaria attramentaria



As mentioned earlier, colonization by direct-developing species such as the marine

mud snail B. attramentaria and the predatory sea snail Euspira fortunei would

require more time than colonization by species that have larval stages in their life

cycles. Actually, neither the mud snail nor the predatory snail were observed until

August 2013 in the tidal flat of Otomo-ura or in another newly created tidal flat in

the estuary of the Unosumai River, Otsuchi Bay (39°20′19.79″ N, 141°53′45.53″),

which is situated about 40 km north of Otomo-ura (see Chap. 10). However, the

mud snail B. attramentaria was first found in August 2014 at both the peripheral

and central areas in Otomo-ura (Matsumasa et al. 2015). To inquire into the source

of the snail, we tried genotyping ten individuals of this species collected at Otomoura with 14 microsatellite loci (Itoh et al. 2013; Miura et al. 2014) from nuclear

DNA and the nucleotide sequences of a 1020 bp region of the mitochondrial gene

for cytochrome c oxidase subunit I (COI) (Matsumasa et al. 2015). The result for the

nucleotide sequence of the COI gene showed that all of the individuals from Otomoura exhibited the haplotypes of the “Tsushima group,” which are commonly found

in northern Japan (Kojima et al. 2004). A phylogenetic tree by the neighbor-joining

method using Nei’s DA with 15 populations from various places in northern Japan

indicated that the population of Otomo-ura was differentiated from the other populations but was closest to that from Ofunato Bay, north of Hirota Bay. The results of

assignment tests based on microsatellite data also showed that the genetic distinctness of the Otomo-ura population from the other populations was very clear except

for the two populations of Ofunato Bay and Kesen-numa Bay, north and south of

Hirota Bay, respectively. Therefore, the Otomo-ura mud snails might be immigrants



9 Colonization of Restored and Newly Created Tidal Flats by Benthic Animals



127



from the source populations in Hirota Bay or the neighboring bays. Since no data

was available for the populations in these areas before the tsunami, more detailed

investigations into the populations in these areas are needed to help clarify where

the direct-developing mud snail came from and how they reached the restored tidal

flat.

The alien predatory snail E. fortunei, another direct-developing species, has been

introduced with its prey Manila clam R. philippinarum, which is an important fishery resource, to northern Japan (Okoshi 2004) and has damaged the clam populations. Fortunately, this predator, which was unintentionally introduced to the Sanriku

region, has been found neither in the restored tidal flat of Otomo-ura nor the newly

created flat in the Unosumai River estuary. On these tidal flats, the populations of

the prey Manila clam are free from predation by the snail E. fortunei. It is important

not to transport the clam to these tidal flats to keep these habitats free from the alien

predator.



9.6



Colonization by the Protozoa Perkinsus Parasitizing

the Manila Clam



Just like in the above predator-prey relationships between the predatory snail E.

fortunei and the prey clam R. philippinarum, parasite-free populations are also

likely to develop at tidal flats that have been restored and newly created by the tsunami if only larvae of the clam reach those vacant habitats. The host-parasite relationship between the clam R. philippinarum and the protozoan parasite Perkinsus is

one of the suitable subjects of researches to examine this possibility. Since the prevalence of the protozoan parasite is thought to be caused mainly by transporting

infected clams (Hamaguchi et al. 2002), Perkinsus-free populations are likely to

develop at restored and newly created tidal flats when only larvae of the clam reach

such habitats. Perkinsus parasites infect the clam after its settlement, not larva in the

planktonic stage. Therefore, from April to June 2014, we investigated the prevalence and infection intensity of the protozoan parasite in the Manila clam on nine

tidal flats including the flat in Otomo-ura and the other two tidal flats newly created

by the tsunami and subsidence using Ray’s fluid thioglycollate medium assay (Choi

et al. 1989; Umeda and Yoshinaga 2012, Kinoshita and Matsumasa submitted). The

prevalence was less than 30 % in both the restored Otomo-ura flat and the two newly

created tidal flats, and the values were much lower than those in the previously

existing tidal flats (>95 %) where clam seeds had been released before the disaster.

However, the Perkinsus infection in clams was confirmed for all of the restored and

newly created tidal flats. The results indicate that the zoospores of Perkinsus could

reach the restored and newly created tidal flats from the neighboring preexisting

tidal flats because Perkinsus is thought to be transmitted mainly via zoospores,

which are released from the dead host to seawater (Auzoux-Bordenave et al. 1995).

Park and Choi (2001) reported that the infection intensity was much lower in

small clams than in large clams in Korean waters. In contrast, Kinoshita and



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

3 Otomo-ura: A Restored Vacant Tidal Flat

Tải bản đầy đủ ngay(0 tr)

×