Specified Skills
Educational Textbook for the Fishing Industry Skills Proficiency Test (Aquaculture)(Feedless Aquaculture)

Japan Fisheries Association(First Edition: February 2020)

Table of Contents
Oysters
1. About Oysters ································································································ 2
2. Life Cycle of Oysters ······················································································· 4
3. Oyster Exterior ······························································································ 7
4. Oyster Interior······························································································· 8
5. Oyster Aquaculture Methods ·········································································· 11
6. Natural Seed Collection ················································································· 12
7. Raising from Ground (Suppressing) ································································· 13
8. Restringing (Main Hanging) ··········································································· 13
9. Aquaculture Management ·············································································· 15
10. Harvesting ································································································· 15
11. Shucked Oysters·························································································· 17
12. Shipment ··································································································· 18
13. Periphyton and Harmful Creatures ································································ 18
14. Hygiene Management ·················································································· 20
15. Shellfish Toxins and Shipping Regulations ······················································ 21
Scallops
1. Japanese Scallops ························································································· 23
2. Scallop Yield ································································································· 24
3. Life Cycle of Scallops ····················································································· 25
4. Scallop Exterior ···························································································· 27
5. Scallop Interior ····························································································· 28
6. Scallop Aquaculture······················································································· 29
7. Natural Seed Collection ················································································· 30
8. Seed Collection and Dispersal ········································································· 31(1) Provisional Dispersal················································································· 31(2) Main Dispersal ························································································· 32
9. Spat Cultivation (Medium-Term Cultivation) ···················································· 32
10. Adult Shellfish Cultivation (Main Cultivation)················································· 33
11. Aquaculture Management ············································································· 34
12. Periphyton and Harmful Creatures ································································ 35
13. Shellfish Toxins and Shipping Regulations ······················································ 37

Oysters-1-1 . About Oyste rs
Oysters a re shellfis h that are eaten all over the world . About half ofthe shellfish aquac ulture in Japan is oyste r aquaculture . In Japan,
approximately 176 ,0 00 tons of oys ters a re cultivated (as o f 2017). The
prefectures with the highest yields are Hiroshima , Miyagi , Okayama ,Hyogo , and I wate , in that order (Fig. 1 ).
Most o f the o ysters c ultivated in Japan a re Paci fic oysters. In addition
to the Paci fic o yster , famous types of oysters cultivated ar ound the world
include the European fla t oyster (in Franc e and Spain), the Eastern oyster(in the Atlantic seab oard o f America), the Portu guese oyster (i n Portugal ,Spain, and France), and the Sydney r ock o yster (in Australia). The
Japanese Pacific oys ter is also gr own and cultiva ted in variou s countries .

-2-

Japan's Oyster Aquaculture Yield(10,000 t)(1200)(2017)
1,040(1000)
(800)
(600)
(400)

253

156

(200)

85

66

Hyogo

Iwate

(0)()
Hiroshima
Hiroshima

Miyagi
Miyagi

OkayamaOkayama

Hyogo

Iwate
Miyagi
Hyogo
Okayama
Hiroshima

Figure 1: Japan's main oyster production
sites and their yields
-3-

Iwate

2. Life Cycle of Oysters
Oysters can be seen on the coasts of Japan (Fig. 3).
It is difficult to distinguish between male and female oysters in winter, but in summer,
the differences become apparent. In summer, female oysters gather their eggs, maleoysters gather their sperm, and they both release them into the seawater at the sametime. The sperm fertilizes the eggs that have been released, and these eggs become
floating larvae 0.1 mm in size that swim around in the seawater. During this stage inthe life cycle, it is impossible for the larvae to escape from predators, so they may beeaten by fish or other animals, decreasing in number.
After 2 to 3 weeks, the floating larvae have grown to 0.3 mm and become adhesionperiod larvae (Fig. 4). Adhesion-period larvae firmly adhere to the surface of a rock or
a shell and then become spat.
Oysters cause a flow of water with their gills, sucking seawater into their shells thenusing their gills to filter out food floating in the seawater, carry it to their mouths, andeat it (Fig. 5).
Farmed oysters normally grow to a size of about 10 cm in 2 to 3 years.

Figure 3: Oysters inhabiting the coast-4-

D-type larvae
Adhesion-period larva

Figure 4: Growth of oyster larvae
A: fertilized egg, C: D-type larva, D/E: umbo-period larva, F: adhesion-period
larva

-5-

Flow of seawater causedby gills

Transportation of foodby action of gills
Pseudofeces (inedible part)

Food

Suspended matter (food,
etc.)
Feces

Figure 5: Action of oyster gills-6-3. Oyster Exterior (Fig. 6)
Oysters have two valves. The shapes of oyster valves are not predetermined. They
grow in accordance with the shape of the location to which they are adhered. The sunken
valve that adheres to a rock or shell is called the left valve, and the flat valve is called
the right valve. The two valves are connected by a hinge and an adductor muscle. Whenthe adductor muscle contracts, the hinge closes, and when the adductor muscle relaxes,
the hinge opens. When oysters die, the hinge opens. When healthy oysters are taken outof the water, they firmly close this hinge and are able to stay alive for some time.
Hinge

Adductor muscle
Seen from flat valve (right valve)
Hinge
Flat valve (right valve

Sunken valve (left valve)

Adductor muscle

Seen from the side
Shell front
Adductor muscle

Sunken valve (left valve)

Flat valve (right valve)

Shell backFigure 6: Oyster exterior-7-4. Oyster Interior (Figs. 7, 8)
Adductor muscle: A muscle for closing the two valves. When the shells are opened andoysters are shucked, this muscle is cut.
Gills: Organs used for breathing and eating feed. A flow of water is caused by fine cilia
invisible to the eye on the surface of the gills, and seawater is sucked into the valves.
Mantle: A layer covering the soft tissue of the valve. From fall to winter, it stores
nutrients and has a white appearance. In summer, when the nutrients become
depleted, it turns transparent.
Digestive diverticulum: This is an organ that digests and absorbs food that has been
eaten. It has a brown appearance.
Gonads: Parts that develop around the digestive diverticulum from spring to summer
and store the eggs and sperm.
Heart: Located next to the adductor muscle. The beating of the heart can be seen in a
shucked oyster that is still alive.

-8-Adductor
muscle

Gills

Digestive
diverticulum
MantleWinter

MantleSpring to summer (before spawning)

Summer to fall (after spawning)

Mizugaki(transparent oyster
after spawning)

Cross section

Cross section

Digestive
diverticulum

Enlargement

Figure 7: Oyster interior-9-

GonadsMantleHeart

Adductor

Gills

muscleFigure 8: Half of an oyster- 10 -

5. Oyster Aquaculture Methods
For oyster aquaculture, oyster spats born in the sea are manually gathered, cultivated,and shipped. Feed is not used in oyster aquaculture. Oysters grow by eating
phytoplankton naturally occurring in the sea.
Main aquaculture methods include the bottom culture method, the stake-suspended
culture method (simple suspended culture method), the raft-suspended culture method,
and the longline culture method (Fig. 9).
The locations and cultivation methods for oyster aquaculture are set by regulations
determined by fishery cooperatives, and these regulations must be followed.

A

B

Stacked bamboo
0.5 m intervals(40 to 42)

C

Weights (4)(Overhead view)

Tide

Anchors (concrete)(10t x 2 to 3) x 2

Wires – 2 to 3 cm in diameter

Buoys (24)(side view)

3 rows of rails (6 rails)

Beam line

Floating barrel

D

Floating barrel

Beam line

Oysters

Oyster seedmaster plate(scallop shells)

Beam line

Tied line 6-7%
Oysterseeds
Hanging line

MasterAnchor plate
30 to 35 per line

Anchor line

Hanging line 14-16%(After cultivation)

Figure 9: Oyster acquaculture methods
A: bottom culture method, B: stake-suspended culture method (simple suspended
culture method), C: raft-suspended culture method, D: longline culture method- 11 -

6. Natural Seed Collection
Securing seeds for cultivation is the most important task in oyster aquaculture. In
Japan, production by natural seed collection is most common. Floating larvae, whichappear in the sea in the summer, are made to adhere to an adhesion tool. In Japan,
scallop shells are used as adhesion tools (Fig. 10).
To collect natural seeds, a seed collection stack, which consists of scallop shells withholes in the middle and wires are passed through them, is hung in the sea (Fig. 11). The
period for hanging it in the sea is determined by using a plankton net to investigate
larvae and conducting a test to observe the number adhered to a test stack.
Artificial seed production, which consists of manual cultivation of larvae in a tank byfertilizing eggs with sperm, is also used.

FigureFigure 10: Seed

11:

collection

collection stacks- 12 -

Conductingseed7. Raising from the Ground (Suppressing)

The oyster seeds that have been collected are first transferred to suppression racks inmudflats, where their growth is suppressed (Fig. 12). The oyster seeds on these racks
are exposed to the air at times due to the ebb and flow of the tide. This slows their growth,
but these environmental changes also make them more resilient. Most seeds used inoyster farming in Japan are created in the Hiroshima Bay and Sendai Bay areas.

Figure 12: Suppression racks (left) and seed boards (right)

8. Restringing (Main Hanging)
The scallop shells to which the oyster spats have adhered are called seed boards ormaster plates.
To give the oysters space to grow large, the seed collection stack is separated, intervals
are created between the seed boards, and they are then restrung on wires or ropes (Fig.
13).

- 13 -

Figure 13: Methods of hanging
seed boards
Wire method (left)
Rope method (right)

The wires or ropes connecting the seed boards are hung from rafts or longlines (Fig. 14).

Figure 14: Oyster aquaculture raft assembled from bamboo(Hiroshima Bay)

- 14 -

9. Aquaculture Management
The growth of oysters differs depending on the season, location, and density of theaquaculture facility, so various modifications must be made to account for thesedifferences. The time of restringing may be changed to suit the harvesting period, andthe depth of hanging at the aquaculture facility may be changed depending on the season.
From summer to fall, typhoons that threaten Japan can destroy farming facilities, so
attention must be paid to weather forecasts.
To avoid the influence of waves and wind, aquaculture facilities may be moved to thelees of an island. If oyster excrement and shells gather at the seabed of farming locations
and contamination becomes severe, fish and shellfish will be unable to live there. In
order to cultivate oysters in a healthy fashion, the number of aquaculture facilities must
be adjusted, and the seabed must be cleaned to prevent contamination from aquaculture
from becoming too severe.

10. Harvesting
Harvesting begins around October or November when the seawater temperature drops
and the oysters begin to gain weight. Machines such as cranes, shucking machines, andwashing machines are used for harvesting the oysters.
When the oysters are landed, a washing machine is used to completely remove mudand deposits that are stuck to the valves. These landed oysters are stored in a pool filledwith seawater, and the insides of the valves are also cleaned (Fig. 15). There is also amethod of landing the oysters, separating them, and transferring them into a cage which
is used to ship the oysters with the valves still attached.

- 15 -

Figure 15: Oyster harvesting using a crane (left) and an on-land pool (right)- 16 -

11. Shucked Oysters
In Japan, oysters are usually shipped shucked with their shells removed.
A tool such as a knife or an oyster opener is used to open the shell and remove theoyster.
The blade of the knife or oyster opener is inserted between the valves, the adductor
muscle is cut, the valves are opened, and the oyster is removed (Fig. 16).

Insert the blade in
the direction of the
adductor muscle

Figure 16: Method for using an oyster opener to remove the shell and thelocation to insert the knife- 17 -

12. Shipment
After oysters have been shucked and removed from the shells, they are thoroughly
cleaned with cool, clean seawater.
After washing, the shucked oysters are packed into various containers and shipped(Fig. 17).
There is also another method of shipping in which the oysters are not shucked andshipped with their shells still attached.

Figure 17: Oyster shucking (left) and cleaning (right)

13. Periphyton and Harmful Creatures
There are harmful creatures who obstruct the growth of farmed oysters and even killthem in some cases.
Various creatures adhere to aquaculture facilities (such as raft buoys (floats), ropes,
glass floats, and cages) as well as to the surfaces of oysters. These creatures are called
periphyton.
- 18 -

Some examples of periphyton are blue mussels, Hydroides elegans, sea squirts, andbarnacles, which surround oysters and adhere in large numbers, depriving them ofoxygen (causing oxygen deficiency) and killing them. In addition, blue mussels, barnacles,
and sea squirts also steal the oysters' food, obstructing their growth (Fig. 18).
There are also cases of fish such as blowfish or bluefin tuna eating oyster spats in theearly stages of the aquaculture process.
Certain types of harmful plankton are known for killing shellfish such as oysters. In
addition, oysters have been known to die in large numbers from unknown causes in thetime period from summer to fall.

Figure 18: The main periphyton and harmful creatures in oyster farming
A: barnacle, B: blue mussel, C: sea squirt, D: Hydroides elegans (related
to lugworms)- 19 -

14. Hygiene Management
Oysters are food. Accordingly, there are rules for hygienic handling that must be
properly observed.

・ Aquaculture must not be conducted in contaminated waters with abundant bacteria.
・ When landing oysters, wash them sufficiently with clean seawater to remove the
mud and deposits.

・ Immediately wash shucked oysters that have been removed from their shells withclean seawater.

・ Washed oysters must be stored at temperatures of 5°C or below.
・ The temperature must be maintained at 5°C or during transport as well.
・ At the time of shipment, the following information must be clearly recorded: whether
the oysters are for raw consumption or for cooking (heating), the expiration date, thename of the producer, the storage method, and the aquaculture region.

・ Cultivating oysters in their shells in clean seawater to reduce bacteria within theoysters and enable them to be consumed raw is called "purification treatment."- 20 -

15. Shellfish Toxins and Shipping Regulations
When oysters eat plankton carrying toxins which live in the sea (shellfish toxin
plankton), these toxins are stored in the oysters. When shellfish such as oysters containtoxins like this, it is referred to as "shellfish toxicity."
Shellfish toxicity does not kill the oysters, but if a person eats them, it may induce
poisoning resulting in paralysis or diarrhea, and in particularly severe cases, it can befatal.
If the shellfish toxicity of oysters exceeds the standards, shipment is cancelled.
"Paralytic shellfish toxins" and "diarrheal shellfish toxins" are common types ofshellfish toxins. The toxin volume is expressed as "mouse units," and safe toxin amounts
are determined for each type.
Harmful plankton surveys and shellfish toxin tests are conducted regularly to ensurethat consumers can safely eat the oysters. When testing detects shellfish toxicity inoysters exceeding safe levels, shipment restrictions are imposed.

- 21 -

Scallops- 22 -

1. Japanese Scallops
The relatives of the scallops that are eaten in Japan are the following four types:
Japanese scallops, Japanese bay scallops, Farrer's scallops, and Noble scallops. However,
the types other than Japanese scallops are only obtained in small numbers. Scallops arebivalve mollusks that live in cold waters, and they are the type of shellfish that grows
both the fastest and the largest. They inhabit most of the waters around Hokkaido, andthere are both farmed and natural Japanese scallops. In the Sea of Okhotsk (Fig. 1), theseabeds are cultivated and scallop seeds are spread in large amounts.
This is called "bottom-culture spreading(propagation),” and 3 years after spreading theseeds (for four-year shellfish), the grown scallops
are pulled up with dredge nets (hasshaku) andHokkaido

landed (Fig. 2).

Sea of Okhotsk

Scallop aquaculture is commonly conducted in
the northern Sea of Japan, Uchiura Bay, andLake Saroma in Hokkaido, as well as Mutsu Bay

Figure 1: Production area for bottomculture scallops

in Aomori Prefecture and along the Sanriku
Coast extending from Iwate Prefecture to Miyagi Prefecture (Fig. 3).
About half of the shellfish aquaculture yield in Japan in 2017 was from scallop
aquaculture.

- 23 -

Rumoi
Lake Saroma
Hokkaido

Uchiura Bay
Mutsu Bay

Sanriku

Figure2:Adredgenet(hasshaku)

Figure 3: Production areas forfarmed scallops

2. Scallop Yield
In 2017, Japan’s nationwide yield for scallops was 174,000 t, and the prefectures withthe highest yields were Hokkaido with 84,900 t and Aomori Prefecture with 84,300 t,
followed by Miyagi Prefecture (Fig. 4).

Scallop yields in Japan (2017)(10,000 t)
849

843

28

Aomori

Hokkaido

MiyagiFigure 4: Japan's main scallop production areas
and their yields- 24 -

3. Life Cycles of Scallops
For scallops, males and females are separate, and when the temperature falls startingin December, gonads develop and they reach sexual maturity.
When spawning season nears, the gonads swell to a large size. For males, they turn acream color (Fig. 5) and produce sperm, and for females, they turn a reddish-pink color(Fig. 6) and produce eggs.

Figure 5: Male scallop (♂)

Figure 6: Female scallop (♀)

A rise in water temperature provides stimulation, and when the temperature of thewater reaches 4 to 8°C, the sperm and eggs are released into the sea. Spawning season
starts around February in earlier regions, and after fertilization in the seawater, theeggs develop into larvae that float in the water after about one week.
After about 40 days, the larvae have grown to about 0.3 mm in size, and they begin toadhere to seed collection tools, ropes, and seaweed using their byssi at this stage (Fig. 8).
During the larva stage, it is impossible for the scallops to escape predators, so they may
be eaten by fish or other animals, decreasing in number.
- 25 -

Figure 7: Floating scallop larvae

Figure 8: Scallops just after adhesion

Next the larvae become spats, and after between 40 to 60 days, when they have grown
to around 8 to 10 mm, their adhesion strength weakens and they naturally fall off andtransition to life on the seabed. Scallops take in phytoplankton and detritus, filtering
these materials out of seawater through their gills and consuming as food in order togrow.
Aquaculture scallops grow to a size of 10 cm within 2 years, and bottom-culture
scallops grow to a size of 10 cm in 3 to 4 years, at which time they can be harvested.

- 26 -

4. Scallop Exterior
Scallops have two shells. The white one is called the right shell, and the brown one iscalled the left shell (Fig. 9). The right valve is rounder and larger than the left valve, andwhen scallops are on the seabed, the right shell is on the bottom.

Right

Left

Fig. 9: Shape of scallop
The size of the shell is measured by the "shell height" or the "shell length." In addition,
growth stops due to cold water temperatures in winter, spawning in spring, and hightemperatures in summer. These stops are visible in the ring formations, and the age ofa scallop can be determined by counting these rings (Fig. 10).

ShellheightGrowthrings

Shell length
Figure 10: Scallop exterior- 27 -

5. Scallop interior (Fig. 11)
Adductor muscles: Scallops have two types of adductor muscles. The large adductor
muscle is used for closing the shell instantaneously when swimming. The otheradductor muscle is small and located to the side of the large adductor muscle, and it
is used to keep the shell tightly closed. The adductor muscles contain glycogen, whichis the basis for the umami of the meat.
Mid-gut gland: Called uro in Japanese, this gland is responsible for actions of the kidneys,
pancreas, and stomach. Food taken in through the mouth (such phytoplankton) is
digested here, and a portion is absorbed.
Gonads: Located to the side of the adductor muscles, these grow from winter to spring.
They turn white in male scallops and reddish pink in females.
Mantle: Called himo in Japanese, this is a thin layer covering the internal organs, andthere are many eyes all around it. The mantle also creates the shell and determines
the direction of swimming.
Gills: These are soft, brown organs to the side of the adductor muscles that are
responsible for breathing, and they also filter and collect food from seawater (such as
phytoplankton).
Mid-gut
gland
Adductormuscles

MantleGills
Gonads
Male(♂)

Figure 11: Scallop interior- 28 -

Female(♀)

6. Scallop Aquaculture
For scallop aquaculture, scallop spats born in the sea are usually gathered, cultivated,and shipped. Cultivated scallops grow by eating food sources such as phytoplankton in
the sea.
In most areas, the aquaculture methods used is the longline hanging method (Fig. 12).
うきだま

かいめん

Sea surface

Glass floats

浮き球

海 面

Glass
floats
うきだま

浮き球
Main
lines
みきづな

幹綱

さいびょうきSeedcollection

採苗器
tools

ぽけっと

さいびょうきSeed collection tools

採苗器

Pockets

ポケッ ト

ぱーるねっとPearl net

まるかご

Round cages丸篭

パールネッ トみみづり

End-hanging

耳吊り

ぶろっく
Blocks

かいてい
Seabedブロック

海底

ず
ほたてがい ようしょくしせつず
Figure12: Scallop
aquaculture facility diagram

図１２ ホタテガイ養殖施設図

However, depending on the waters and the sizes of the scallops to be shipped, theshipping period and the aquaculture methods differ. The locations and aquaculture
methods for scallop cultivation are set by regulations determined by fishery cooperatives,
and these regulations must be followed.

- 29 -

7. Natural Seed Collection
Scallop larvae floating in the sea have a characteristic which causes them to adhere toobjects once they reach a size of about 0.3 mm. Seed collection tools make use of thischaracteristics, and they are lowered into the sea so the floating larvae will adhere tothem. The process of collecting spats which have adhered to these tools and grown iscalled "natural seed collection."
There are two types of these seed collection tools: onion bag, which have Netlon nets or
old nets inside of them, and stick nets, which are old net rolled into stick shapes.
When casting the seed collection tool, floating larvae are collected by using plankton
beforehand, the number and size of the larvae are confirmed using tools such as
microscopes to determine the casting period. Seed collection tool are always cast multiple
times (Fig. 13).

Mutsu Bay, Sea of Japan(onion bag)Uchiura Bay(stick net)

Figure 13: Seed collection tools- 30 -

If sufficient spats cannot be gathered using natural seed collection, this has a major
impact on scallop aquaculture and bottom-culture (propagation) yields, so seed collectionis the most important task. Note that in Japan, there is no use of artificial seedproduction whatsoever for scallops.

8. Seed Collection and Dispersal
Seed collection for scallops starts from around March. The spats that have adhered toseed collection tools grow quickly, so the seed collection tools are hauled out of the waterand the spats are collected.
The spats taken from the seed collection tools are transferred into cages with large
meshes, in accordance with the growth stage.
(1) Provisional Dispersal
In the Uchiura Bay area, where stick net seed collection tools are used, as well as areas
where many starfish, crabs, and other sea life enter the onion bags, these creatures are
removed in order to prevent pest damage to the spats.
In addition, after the sizes of the spats areorganized using a sieve, the following main
dispersal task can be conducted efficiently. This
process is called "provisional dispersal," and it isconducted from around July. The spats that havebeen provisionally dispersed are stored in pearl nets
(cushion cages) and hung in the aquaculture
facilities (Fig. 14).

Figure 14: A pearl net (cushion cage)- 31 -

(2) Main Dispersal
The spats that were provisionally dispersed and hung are landed. Seeds for bottomculture use and half-developed scallops are sorted according to spat size using sieves sothat mature scallops can be shipped. A number of scallops determined according to the
purpose is put into pearl nets, and the scallops are once again hung in the aquaculture
facilities. This process is called "main dispersal," and it is conducted from around August.
In addition, depending on the periods for shipping the scallops as partially or fully
mature scallops, second and third dispersals may also be carried out in some cases.
In areas where provisional dispersal tasks are not conducted, seed collection tools arehauled up from late July to August, and starfish and crabs are removed. Furthermore,
the sizes of the spats are organized by using a sieve, a number of scallops determined byshipping size is inserted into a pearl net, and the scallops are once again hung from thefarming facilities.
In addition, at the time of each dispersal, the sizes of the meshes and the pearl nets
themselves are changed to match the growth of the scallops. It is important toappropriately adjust the number of spats to added to each net in order to avoid impeding
growth.

9. Spat Cultivation (Medium-Term Cultivation)
The spats for main dispersal are hung for a certain time period and cultivated incontainers such as pearl nets until they reach a certain size suitable for aquaculture orbottom-culture spreading.

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10. Adult Shellfish Cultivation (Main Cultivation)
Spats used for medium-term cultivation are transferred into round cages (andon cages)(Fig. 15) or pocket cages (Fig. 16) until they develop into partially and fully-maturescallops. Alternatively, they may be hung in aquaculture facilities by forming a hole in
the end of the shell, directly attaching a rope using a pin with a hook attached or similar
method, and hanging the shell with this rope (this is referred to as "end-hanging
farming") (Fig. 17).
Afterward, the aquaculture process carried out until shipping is referred to as "maincultivation."
During this period as well, it is important to appropriately adjust the number of scallops
in each cage to avoid impeding growth.

Figure 16: A pocket cage

Figure 15: A round cage- 33 -

Double-opening
position×
Right shell side

Single-opening position(white shell)

Figure 17: End-hanging and positions
for forming holes in the shell

11. Aquaculture Management
When handling scallops, aquaculture workers must keep in mind that they are living
creatures. Classification when sorting the spats by size and hole-forming work for endhanging should be conducted early in the morning when possible, before the water and airtemperatures rise. When these operations must be performed during the day, it isimportant to carry them out in a cool work shed and avoid direct sunlight.
Spats are particularly vulnerable to direct sunlight, high temperatures (approx. 22°C
and above), drying (wind), rainwater, and oxygen deficiency, so ample caution must be
taken. Furthermore, shells with a cracked valve (referred to as chipping), shells for which
the interior has become yellowish-brown (referred to as internal coloring), and shells forwhich growth has stopped are referred to as "abnormal shells" (Fig. 18).

When abnormal shells are cultivated at high densities or cages or scallops crash into
each other due to the aquaculture facilities swaying due to factors such as rough seas,
the shells of the scallops may become damaged.

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ChippingInternal

Warpingcoloring

Figure 18: Abnormal shells
12. Periphyton and Harmful Creatures
In certain years, various creatures adhere to aquaculture facilities (including ropes suchas the main lines and hanging lines, pearl nets, round cages, and glass floats) or to thesurfaces of scallop shells. Furthermore, starfish and crabs also feed on scallops directly(Fig. 19).

Starfish that have entered Pugettia quadridens (crab)

Glebocarcinus

seed collection toolsthat has entered a seedamphioetus (crab) thatcollection tool

has entered a pearl net

Figure 19: Harmful creatures
In addition, other examples of periphyton that adhere to farming facilities and shells
include blue mussels, barnacles, sea squirts, sea anemone, and polychaetes (Figs. 20 and
21).

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Blue mussels

Ciona

Clonal plumose
anemoneBlue mussels

StarfishHydrozoa

Hydroides elegans

Figure 20: Periphyton- 36 -

Figure 21: Other periphyton
A: barnacle, B: blue mussel, C: sea squirt, D: Hydroides elegans (related to
lugworms)

13. Shellfish Toxins and Shipping Regulations
Bivalve mollusks such as scallops gather plankton, filter them from seawater, andconsume them as food. However, when they eat plankton carrying toxins which live inthe sea (shellfish toxin plankton), bivalve mollusks which are generally nontoxic store
these toxins in their bodies and become toxic. This process of becoming toxic is called- 37 -

"shellfish toxicity," and it is not fatal to affected bivalve mollusks.
In the case of scallops, shellfish toxicity is known to be primarily stored in the mid-gut
gland. If a person eats scallops which contain toxins, symptoms such as paralysis or
diarrhea may result, and in particularly severe cases, it can be fatal.
"Paralytic shellfish toxins" and "diarrheal shellfish toxins" are common types shellfish
toxins. The toxin volume is expressed in "mouse units," and safe toxin amounts are
determined for each type.
Harmful plankton surveys and shellfish toxin tests are regularly conducted in order toensure that consumers can safely eat the scallops.
When testing detects shellfish toxicity in scallops exceeding safe levels, shipment
restrictions are imposed.

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