Fisheries Science 61(2), 241-244 (1995)

Persistent Relaxation of the Adductor Muscle of Oyster Crassostrea gigas Induced by

Magnesium Ion

Kenji Namba,*1 Makoto Kobayashi,*2 Satoshi Aida,*3 Kazumasa Uematsu,*1 Masayuki Yoshida,*1 Yukie Kondo,*1 and Yuji Miyata*1*1Faculty of Applied Biological Science, Hiroshima University, Kagamiyama, Higashi-Hiroshima, Hiroshima 724, Japan*2Faculty of Integrated Arts and Sciences

, Hiroshima University, Kagamiyama, Higashi-Hiroshima, Hiroshima 724, Japan*3Hiroshima Fisheries Experimental Station

, Ondo, Hiroshima 737-12, Japan

(Received August 24, 1994)

We developed a physiological method to open oyster shells persistently for use in an automated oys

ter shucking machine. When oysters (shell height, 10-13cm; shell length, 5-7cm; molluscous body

weight, 9-22g) were submerged in 0.369M MgCl2 solution, their adductor muscle began to relax persis

tently after 17}5.9min (mean}standard deviation) at a water temperature of 5 and after 18}11.5

min at 15. The muscle did not contract when the molluscous body was stimulated with a needle. Af

ter the oysters were transferred into artificial sea water, the muscle began to contract and the oysters re

sumed shell movement in 78}44.2min at 5 and 24}5.4min at 15. When 120 individuals were

submerged in MgC12 solutions at 17-18, cumulative persistent shell opening rates in the solutions of

0.369M and 0.738M were 14.2% and 63.3% for 30min submersion, 35.0% and 72.5% for 60min sub

mersion, 64.2% and 83.3% for 120min submersion, and 76.7% and 85.8% for 180min submersion, re

spectively. This method for opening oyster shells persistently with the use of Mg2+ will be applicable to

other bivalves for opening shells by adjusting the Mg2+ concentration and submersion time for each spe

cies.

Key words: persistent shell opening, oyster, magnesium ion, oyster shucking

Cultured oysters are, for the most part, sold as so-called shucked shellfish in Japan. This oyster shucking operation

requires experienced workers. The supply of experienced workers began to decline about twenty years ago, and the

consequent rise of wages has become a serious problem for

the oyster aquaculture industry. The industry, therefore, requires rationalization or mechanization of the oyster

shucking operation. Although a number of oyster shuck

ing machines have been built and tested, none of them withstands the conditions of commercial use.

The biggest problem in the development of an oyster shucking machine is how to open oyster shells. Physical or mechanical force has been used to open shells in most of

past trials. The purpose of this study is to develop a method for opening shells and keeping them open by taking advantage of the physiological properties of the adductor muscle of oysters without applying such force.

Bivalves close their shells by contracting their adductor muscle, which are generally composed of striated muscle and smooth muscle. Striated muscles move fast, while smooth muscles move slowly. Furthermore, the smooth muscle of bivalves performs a peculiar contraction called catch contraction. After activation has ended, the catch contraction applies a large tensile force which continues for a long time, typically several days, consuming very small amounts of ATP. Bivalves, including oysters, remain close for a long time by time by this catch contraction. 1,2)

This study aims to find a way to keep oyster shells open even when they are given tactile stimuli, which ordinarily

induce them to close. The shells will remain open if the two kinds of adductor muscles are prevented from contracting and remain relaxed. Magnesium ions (Mg2+) have been reported to block the chemical synapses in most animals; in the African giant snail Achatina fulica, it effectively blocks synaptic transmission in the central nervous system3) and in Aplysia californica, it was used to anesthetize the animal.4) In this study, therefore, Mg2+ was applied to oysters to observe whether it blocks contraction of the adductor muscle and keeps the muscle relaxed.

Materials and Methods

OustersCrassostrea gigas used in this experiment were 10-13cm

in shell height, 5-7cm in shell length, and 9-22g in molluscous body weight. They were seeded and cultured in Hiroshima Bay for about one year, then transferred into baskets hung from an experimental raft of the Hiroshima Fisheries Experimental Station. They were reared under natural environmental conditions from mid-November till mid-January of the following year.

Effects of Mg2+ on Shell MovementOysters cultured in the baskets were taken out and

brushed to remove materials adhering to the surface. They

were then transferred to a 60 l tank, filled with aerated ar

tificial sea water with the composition (Zaroogian et al.5))

shown in Table 1 (water temperature 5 and 15), and

242 Namba et al.

Table 1. Formula for artificial sea water

Fig. 1. A schematic diagram of the apparatus for recording the oyste

shell movement.

were taken out of the tank after 24hours. The shell movement of oysters was recorded by modified Galtsoff method6) (Fig. 1). The left shell was fixed to a glass plate (10 x 15 x 0.5cm) with a polycarboxylate cement (Shofu Carbo Cement) and a string (cotton thread) was attached to the right shell. With the glass plate and the string attached, the oyster was placed in a 10 l constant-temperature bath, which was filled with artificial sea water of the same temperature as the tank. The string attached to the right shell was connected to a mechano-electrical transducer (Nihon Kohden TB-652T or Kyowa KFG-10-120C1-IL-3M2R) to convert the tension resulting from shell movement into an electric signal, which was then recorded

Fig. 2. Effect of Mg2+ on shell movement of an oyster (shell height, 10.5cm; shell length, 6.5cm).

Artificial sea water (15) was replaced with 0.369M MgCl2 solu

tion (15) at the point indicated by the first arrow (Mg2+). Tactile

stimuli were applied to the mantle at the points indicated by ar

rowheads. The oyster closed its shells in response to the first stimulus

but no longer responded to the second and third stimuli. After the

MgCl2 solution was replaced with artificial sea water at the point indi

cated by the second arrow (ASW), the oyster resumed its shell move

ment after about 22min.

continuously. After recording the shell movement for 60min, the artificial sea water was then replaced with 0.369M magnesium chloride (MgCl2) solution or 0.369M calcium chloride (CaCl2) solution, both of which were nearly equivalent to the artificial sea water in temperature and osmotic pressure, for observation of shell movement, i.e., the contraction and relaxation of the adductor muscle. Shell movement was further observed after replacing the solutions with the artificial sea water. When oyster shells opened, tactile stimuli were given to the adductor muscle and the mantle with a needle to see the reactions of the adductor muscle.

Cumulative Persistent Shell Opening Rate of Oysters Submerged in MgCl2 Solution

Oysters grown in the baskets were taken out, brushed to

remove deposits on the surface, and used for the experi

ment immediately. Test containers (60 x 38 x 15cm) were

filled with 20 l of 0.369M or 0.738M MgCl2 solution (17.0

18.0) and the solutions were aerated. Forty oysters were

placed in a single layer in the container and the process of

shell opening was observed for 180min. A total of 240 oys

ters were used to obtain a cumulative persistent shell open

ing rate in solutions at each concentration of MgCl2. In

dividuals which opened their shells were transferred to

Fig. 3. Photograph of oysters opening their shells persistently (arrows) in 0.369M MgCl2 solution (17).

Persistent Shell Opening of Oyster 243

natural sea water and recovery of shell movement was monitored for 24h.

Results

Effects of Mg2+ on Shell MovementThe oysters which exhibited periodical shell movement

in the artificial sea water opened their shells in 0.369M

MgCl2 solution and remained open with the adductor mus

cle relaxed, even when the adductor muscle or mantle was

stimulated. An example of the data is shown in Fig. 2. The

persistent shell opening, i.e., the persistent relaxation of

the adductor muscle, started after 17}5.9min (mean }

standard deviation, n=10) at the water temperature of

5 and 18}11.5min (n=8) at 15. The mean values

were close, but the variance was greater at higher tempera

ture. When the test containers were filled again with the ar

tificial sea water in place of 0.369M MgCl2 solution, the

oysters contracted their adductor muscles and resumed

shell movement after 78 }44.2min at 5 (n=10) and

24}5.4min at 15 (n=8). The time required for recov

ery of shell movement tended to be longer at the lower

water temperature. In 0.369M CaCl2 solution, oysters

(n = 5) kept their shells closed for more than 60min.

Cumulative Persistent Shell Opening Rate of Oysters Submerged in MgCl2 Solutions

Figure 3 shows oysters opening their shells persistently in 0.369M MgCl2 solution. Shells opened to almost the same extent in each solution of two different concentrations (0.369M and 0.738M) of MgCl2.

Cumulative persistent shell opening rates in MgCl2 solution of 0.369M and 0.738M were 14.2% and 63.3% for 30min submersion, 35.0% and 72.5% for 60min submersion, 52.5% and 80.0% for 90min submersion, 64.2% and 83.3% for 120min submersion, and 76.7% and 85.8% for 180min submersion, respectively (Fig. 4). These rates were greater at the higher concentration of Mg2+, and a larger difference existed in the initial phase of submersion.

Concerning the individuals which opened their shells in

MgCl2 solutions for 180min submersion, the time re

quired before the start of persistent relaxation of adductor

muscle was 77}44.0min (n=92) at 0.369M and 38}30.2

Fig. 4. Cumulative persistent shell opening rates of oysters submerged

in MgCl2 solution (17.0-18.0) of 0.369M or 0.738M for 180min.

min (n=103) at U.736M. It was significantly longer at 0.369M (p

244 Namba et al.

was 76.7% for 180min submersion at the Mg2+ concentration of 0.369M, this would be improved by developing a pretreatment for oysters to induce quick spontaneous shell opening in MgCl2 solution.

In this study, we developed a persistent shell opening method that takes advantage of the physiological effects of Mg2+. This persistent shell opening method for oysters will be widely applicable to a number of bivalve species, e.g.,

pearl oyster, which close their shells by catch contraction.

References

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