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2 An Experiment Before the Great East Japan Earthquake

2 An Experiment Before the Great East Japan Earthquake

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14



R. Kado and N. Nanba



Jetty

A



10 cm



B



C

40 cm

Below

M.S.L.



25 cm



10 m



Fig. 2.2 Diagram showing the schematic arrangement of the test plates set on the wall of the jetty

for assessing the roles of the barnacle Semibalanus cariosus and sea urchin Strongylocentrotus

nudus in community assemblage processes. Three test plates (25 cm2 on each side) were set inside

three stainless frames. Plates A and B with the barnacles already settled on the plates before this

experiment were placed 10 cm apart from the wall (Plate A) using the stainless frame guard and

placed directly on the wall (Plates B), respectively. Plates C without barnacles on the plates was

placed directly on the wall



when the experiment started. The third set, set C, was used as a control without any

sessile animals on the plates. Set A was placed 10 cm apart from the jetty wall surface to decrease chance of predation by sea urchins, and set B and set C were placed

directly on the wall where sea urchins, S. nudus, were distributed. All of these plates

were installed at 40 cm below MSL during June of 2004 (Fig. 2.2) when the experiment was initiated. Then, the number of species and total weight including the plate

itself were investigated every 2 months over 2 years.

An outline of the results is as follows. In the autumn, many mussels M. galloprovincialis had settled only on the plates with the barnacles and not on the plates

without barnacles, i.e., control plates (set C). In May 2005, i.e., 11 months later

from the start of experiment, small numbers of surf barnacles and green and red

macroalgae had colonized on the set C (Fig. 2.3C). Contrastingly, set A and B plates

showed a clear two-layered structure with many grown mussels that covered over

the barnacles. The barnacles lived and even grew to large individuals underneath the

upper layer of mussels. Green and red macroalgae grew moderately on the set A

plates but were limited on set B plates (Fig. 2.3A, B). Between the space with barnacles and the mussel canopy, some interstitial organisms such as small snails were

found on the plates of all treatments.

In May 2006, 2 years after the start of the experiment, a few C. challengeri and

C. gigas and small numbers of green and red algae were found on the set C plates.

On the set A plates, mussels had continually grown on the barnacles which were still



2



Normality of Succession of an Intertidal Community…



15



Fig. 2.3 Succession of the community assemblages on the test plates manipulated with the three

different conditions: (A) plates started with barnacle Semibalanus cariosus and placed them 10 cm

apart from the wall that inhibited access of sea urchins; (B, C) plates started with or without S.

cariosus, respectively, and were placed directly on the wall enabling access of sea urchins



alive but grew little, and green and red algae flourished on the mussels. On the set

B plates, number of the mussels and barnacles decreased and bare spaces appeared

with a few oysters and green algae. In December 2006, 30 months after the start of

experiment, communities on the set C and B plates showed almost the same structure as that found on the natural jetty walls where only C. challengeri and C. gigas

dominated. On the A set plates, however, a number of large mussels dominated

externally; the reddish barnacle Megabalanus rosa settled on the mussel shells and

S. cariosus attached underneath. In addition, a number of small snails, annelids,

flatworms, and amphipods were found interstitially on the A set plates.

These differences in communities between the A and B set plates and the A and

C set plates showed that the sea urchin S. nudus and barnacle S. cariosus are key

species in the littoral community. Excess density of the former resulted in a “sea

urchin barren,” and recruitment of S. cariosus contributed to the formation of a littoral community with a high biodiversity. Inside the community, the growth of

S. cariosus was inhibited by overgrowth of the mussel M. galloprovincialis.

However, the barnacle seemed to benefit from the cover of the mussels because they

were apparently protected from predation by sea urchins and predatory gastropods.

On the other hand, M. galloprovincialis received an advantage from the barnacle

since the barnacles form a substrate for the settlement of this mussel although they



16



R. Kado and N. Nanba



had a risk to be preyed on by sea urchins. Thus, there may be a trade-off effect

between these two species. Based on these observations. We hypothesized that as

far as sea urchins were abundant, the community would be continually dominated

by oysters and serpulids and would not return to the previous status dominated by S.

cariosus (Fig. 2.3C).



2.3

2.3.1



Succession of Intertidal Community After the Great

East Japan Earthquake

Methods



The post-tsunami research was performed at the former experimental site mentioned above. The experiment began on July 2, 2011, using an underwater camera

(WTW-WA7000H, Wireless Tsukamoto Co., Ltd.) which was fixed on the top of a

stainless steel frame-shaped pyramid (Fig. 2.4). The camera, focused on the vertical

wall surface, was brought down slowly using a rope attached to a frame from the

uppermost part of the jetty to the bottom. Video images were recorded on a portable

hard disk (PoliceNote 2400S, Sun-Mechatronics) with a monitor screen. Depth and

density of organisms were measured with a measure scale tape that was lowered

from the top of the jetty and was displayed on each image. The top of the jetty was

32 cm higher than M.S.L. at that time and about 11 m above the seabed. Surveys

were carried every 2 months during the first year and four times in the following

years. In addition, to compare the communities developed on the jetty wall between

before and after the earthquake, monthly monitoring on the abundance of sedentary

Fig. 2.4 A diagram

showing the underwater

camera system used in this

study to observe the littoral

and sublittoral

communities on

the jetty wall



video

cable



attachment

rope



underwater

camera

stainless

steel frame



2



Normality of Succession of an Intertidal Community…



17



organisms on two PVC test plates (25 × 25 cm) has been investigated. The test plates

were set at 40 cm below M.S.L., kept for a month on the Sakihama jetty wall, and

then changed each month to new ones.



2.3.2



Results and Discussion



2.3.2.1



Pioneer Species into the New Habitats



At the jetty where this research was made in July 2011, the extent of the subsidence

was 1.3 m (Fig. 2.5b), which was relatively more than other adjacent land areas, due

to difference in substrata hardness and that the site stands out about 200 m apart

from the land (Fig. 2.1). The observed scale of the subsidence means that almost

two thirds of the sessile littoral organisms in this site sank into the subtidal zone.

The first settlers were the barnacle S. cariosus. They were abundant and completely covered the bare area from the mid-intertidal to about 6 m below M.S. L. not

only on the wall but also on rocky shores around the bay (Fig. 2.5e). They survived

with a high rate except loss due to detachment by their own overcrowded settlement. According to our previous study (Kado et al. 2002), this barnacle releases its

nauplii when the surface chlorophyll a concentration exceeds 1 mg/m3 at the beginning of the spring bloom. The nauplii settled on the shore 3–5 weeks later when the

chlorophyll a concentration remained at approximately the same level (Kado et al.

2002). Although the chlorophyll a concentrations were unknown during March to

June 2011, it was most likely that the concentration had been kept over 1 mg/m3 at

least for more than a month after the earthquake. This high recruitment of S. cariosus right after the earthquake might have been attributed to two reasons. One of the

reasons may be the decrease in filter feeders. Tsunamis caused by the Great East

Japan Earthquake in 2011 washed away a number of organisms including cultured

species such as oysters C. gigas, scallop Mizuhopecten yessoensis, and sea squirt

Halocynthia roretzi and the fouling organisms such as barnacles M. rosa, mussels

M. galloprovincialis, and other sedentary animals that settled on the culture rafts,

buoys, and ropes. All of these filter feeders fed on phytoplankton. Accordingly,

nauplii of S. cariosus could have sufficiently fed on phytoplankton during the algal

spring bloom and maintained a high survival rate throughout the larval stages. The

heavy settlement of this barnacle was, however, not the first case in this area. Indeed,

the same phenomenon had been observed in May 2004. However, at that time, the

abundance of the sea urchin, Strongylocentrotus nudus, reached more than ten individuals/m2 and almost all of the settled S. cariosus had been consumed by the sea

urchin within the next 3 months (Kado personal observations). This high sea urchin

abundance had been continued until February in 2011. However, the sea urchin

abundance decreased due to the tsunami. Kawamura et al. (2014) also reported that

abundance of sea decreased in Otsuchi Bay, Iwate Prefecture, by the backwash of

tsunamis caused by the earthquake. Thus, the second reason of high settlement rate

of S. cariosus right after the tsunamis was likely due to low sea urchin abundance

(<1 individual/m2) (Fig. 2.6).



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