Seiko Tuna is the name given to a range of watches designed for scuba and professional diving. In particular, Seiko made these watches with saturation diving or greater depth diving in mind. All of the watches in the Seiko Tuna range are characterized by protective screwed-on shrouds that have earned them the nickname of “Tuna”.
The Seiko Tuna series covers four main movements: 6159B, 7549A, 7C46, and 5M23. The first was the 6159B movement, which is also called the “Grandfather Tuna.” It was followed by the 7549A and 7C46 movements, which are both called “Golden Tuna” due to the distinct gold tint in their shrouds. Lastly, the youngest in the series, 5M23, is aptly called “Baby Tuna.”
The Seiko Tuna range boasts of many firsts. The range was the first to feature a ceramic-coated titanium shroud, a titanium monocoque case, an L-shaped gasket, and a vented rubber strap. Primarily built for functionality, the Seiko Tuna watches can withstand great depths as tools for professional divers.
History of the Seiko Tuna
Built in 1975, the first Seiko Tuna watch was created in response to a letter the company received in 1968. A saturation diver from Kure City in the Hiroshima prefecture of Japan had sent a complaint regarding Seiko divers watches, which the company had been making since 1965. These pre-Seiko Tuna models were not robust to impact and could not withstand exposure to deep underwater pressure for prolonged periods.
Divers watches must withstand uniquely challenging conditions. Professional divers breathe helium mixed with oxygen at extreme depths where helium gets easily absorbed by a watch’s rubber seals. When divers go through the required decompression, helium is released through special escape valves in divers watches, without which the watch crystal would crack.[1,3]
Ikuo Tokunaga and his team of engineers developed the "perfect professional diver’s watch"
Seiko engineers who read the letter took on the challenge to create the “perfect professional diver’s watch.” These engineers, led by Ikuo Tokunaga, decided to develop the new Seiko Tuna watches for saturation diving from the start. Tokunaga and his team began development in 1968 and released the first Seiko Tuna watch seven years and more than 20 patents later.
The first Seiko Tuna watch (6159-7010) was run by the mechanical 6159B movement. This Hi-Beat movement had been used in the 6159-7001 Professional 300m divers watch released eight years earlier. (Outside of Japan, it was also given the model number 6159-7019, but we will only refer to 6159-7010 in this article.) The Seiko Tuna 6159-7010 is noted for introducing many world’s firsts. It was the first to feature a titanium monocoque case and a titanium shroud coated with ceramic. It was also the first to use an L-shaped gasket and a vented rubber strap.
The Grandfather Tuna, as watch collectors call it, dispensed with the escape valve, a mechanism first introduced by Rolex and Doxa S.A. that allowed helium, hydrogen, and other gases trapped inside the watch case to be released. Instead, the Seiko Tuna 6159-7010 featured a one-piece case and a crystal restrained by a locking ring. It is from this screw-down crystal retaining system that the range earned the nickname “Tuna Can.” Some collectors also call it the “Hockey Puck” after the hockey disk which it resembles.[6,7,8]
The Grandfather Tuna is also notable for surpassing ISO testing standards for divers watches and for its resistance to pressure, shock of up to 10 Gs, extreme temperatures, helium gas intrusion, and magnetic resistance at 60 Gauss.
Another popular model in the Seiko Tuna series is the 7549-7009 (DJM model 7549-7010), which was featured in the 1981 Roger Moore James Bond movie, For Your Eyes Only.[2,3] The watch is barely visible in one underwater shot.
Seiko produced the first Seiko Tuna, 6159-7010, in 1975. Manufactured until 1978, it was replaced by the Golden Tuna, 7549A, which was produced until 1985. Production-run details of the re-issued Golden Tuna, 7C46A, and the Baby Tuna, 5M23A, are obscure.
In addition to the regular models in the Seiko Tuna range, Seiko has released a number of limited editions.
Design and Variations
The Seiko Tuna range was designed by Seiko engineer Ikuo Tokunaga, who is also noted for working on other special timepieces such as pilot, adventure, and railway watches.
Tokunaga and his team specifically designed the series for saturation diving, which[1,3] meant that Seiko Tuna watches needed to be resistant to gas penetration, shock, corrosion, and magnetization.
Seiko achieved its goal by using a special L-shaped gasket and a monocoque case. According to Tokunaga, “Using the special gasket of L type and one-piece case became the conclusive factor in the He-tightness. As the result of the experiment, the penetration of helium gas is suppressed by about 1/100 to the conventional watch case, the inner pressure of the case does not turn into high pressure and the diver’s watch of the ideal saturation diving specification which does not need ‘helium gas escape valve’ is completed.” In essence, the L-shaped gasket made the watch more water resistant and airtight.
Seiko was first company to use titanium in a diver’s watch for the first time in the Seiko Tuna watches. Titanium not only made the Tuna watches highly resistant to corrosion but also lighter. Meanwhile, the Seiko Tuna’s characteristic shroud protected the bezel from turning and the watch from scratches during hard use.
Seiko created a total of 11 variations or executions under the Seiko Tuna series. First came the Grandfather Tuna, followed by the Golden Tunas, which had four variations each. For the 7549A, Seiko issued the 7549-7009 SQ, the 7549-7010 SQ, the 7549-7010 Professional, and the 7549-7000 Professional. For the Golden Tuna Re-issue 7C46, Seiko released the 7C46-7009 SQ, the 7C46-7009 Professional, the 7C46-7008 Professional, the Prospex Marinemaster 7C46-7011, and the 7C46-0AA0. Finally, the Baby Tuna 5M23A had two variations: the 5M23-6A19 (Kinetic Quartz) and the 5M23-6A10 (AGS).
The Grandfather Tuna 6159-7010 introduced the titanium case and the ceramic-coated shroud, two design elements that continued throughout most of the Seiko Tuna watch range. However, for the Golden Tuna 7549-7010, both were made of stainless steel, while for the Baby Tuna 5M23, both were made of plastic.
The Golden Tunas (7549A and 7C46) that followed had their own signature style as well, thanks to their gold-tinted shrouds. This golden color came from nitride-plated titanium.
As its name suggests, the Darth Tuna did away with the gold-tinted shroud in favor of the all-black look of DLC titanium.
The youngest of the series, Baby Tunas (5M23) are also the smallest with a 43-mm diameter. While the first Seiko Tuna (6159B) had a 50-mm diameter, 7549A and 7C46 did not go below 45 mm.
Meanwhile, the Golden Tuna 7549-7010, which is bigger than the 5M23 watches by 4 mm, deviated from most of the Golden Tuna watches due to its unique lugs and curved, domed hardlex crystal.
Details and Specifications
The Seiko Tuna has four movements: 6159B, 7549A, 7C46, and 5M23A. All of these were created in-house.
The 6159B movement running the Grandfather Tuna and the 7549A movement of the first Golden Tuna were both created by the company’s Suwa division.
The 6159B movement, which was derived from the well-respected 61xx movement that ran the Grand Seiko, was confined to professional Seiko dive watches and was never used in any other Seiko model.
It is not clear exactly who manufactured the 7C46 and 5M23A movements of the re-issued Golden Tuna and the Baby Tuna, but it was most likely Suwa  as well.
The 6159B movement contains the most functional jewels in the entire range at 25 jewels. On the other hand, the 7549A movement has the fewest at five jewels, while the 7C46 and 5M23A follow closely at seven jewels and six jewels, respectively. All watches in the range hack and can be hand-wound. Most of them run at 36,000 bph.
The 7549A movement is quartz rather than mechanical, making the watch a little more robust than the mechanical model it replaced.
The 7C46 is a very reliable movement, still used in Tunas, and has a five-year battery life.
The 5M23 is a reliable and accurate kinetic quartz movement. Instead of a battery or winding, it uses the wearer’s arm movement to generate electrical energy which powers the watch.
|Production Run||1975 - 1978||1978-1985|
|Casing Diameter||27 mm||27 mm||27 mm||27 mm|
|Maximum Height||5.65 mm||4.6mm||3.7 mm||4.3 mm|
|No. of Functional Jewels||25 J||5 J||7 J||6 J|
|Vibrations per hour||36, 000 bph||36, 000 bph||36, 000 bph|
|Markings||6159B TWENTY FIVE 25 JEWELS||7549A FIVE 5 JEWELS||7C46A SEVEN JEWELS|
|Instant setting for day and date||Yes||Yes||Yes||Yes|
|Bilingual for day of week||Yes||Yes||Yes||Yes|
The Seiko Tuna range is characterized by black dials and LumiBrite lume. These watches are also known to feature markers instead of numbers to indicate the hours. An upside-down triangle represents the 12-o’clock position, while round dots or circles mark all other hours.
All Seiko Tuna watches have “Seiko” written on their dials, as well as an indication of depth range. When Seiko released the first Tuna watch, 6159-7010, the dial showed the depth range as “600” instead of “600m.” Some believe that this was a misprint, however, as later 1975 6159-7010 models and 6159-7019 models show “600m.”  Others believe otherwise citing watches with “600” instead of “600m” persisted well into 1975.
Of the three models that followed, the 7549-7009 Seiko Quartz (SQ) and the 7549-7000 Professional also have a 600m water resistance, but 7549-7010 SQ Professional has a 300m water resistance. On the other hand, models 7C46-7009, 7C46-7008, and 7C46-0AA0 (Darth Tuna) can withstand depths of up to 1000m. Meanwhile, the Prospex Marinemaster (7C46-7011) and the 7C46-0AC0 have a lower water resistance of up to 300m and the Baby Tuna 5M23 has a water resistance of 200m only.
One of the early models of 6159-7010 with the "600" markings
In addition to the water resistance indication, each model also has a special designation printed on the dial, such as “Professional,” “SQ,” or “Marinemaster.” The Grandfather Tuna 6159-7010 is “Automatic” and “Professional.” In addition, it features the Suwa logo, the only model in the entire series to do so. The Golden Tunas have the “Divers” indication, while the Tuna Can 7C46-0AC0 and the Darth Tuna 7C46-0AA0 both have “Marinemaster” indication. Finally, the Baby Tuna 5M23 has “Kinetic Quartz” in the dial.
The different ranges of Seiko Tuna divers. 200m, 300m, 600m and 1000m dive watches
Each movement has some unique features that differentiate it from the other movements within the range.[16-20] The Grandfather Tuna 6159B features an automatic and auxiliary hand winding with sweep second, a micro adjustor, diashock, and a diafix lubrication device.
Even the Golden Tunas, 7549A and 7C46, have some differences. While the 7549A features a two-pole step motor driving system, the 7C46 has a step motor driving system. The former has a second setting device, while the latter has a train-wheel setting device. Another differentiating feature of the 7549A is its bilingual changeover system for the date, while the 7C46 has an electronic circuit reset switch.
Date and day/date complications
Similar to the Golden Tuna 7C46, the Baby Tuna 5M23 also uses a train wheel setting device and a step motor driving system, which is basically a load-compensated driving pulse type. What sets apart this movement from the rest of the Seiko Tuna watches, however, is its automatic generating system and power reserve indicator.
The Seiko Tuna watches have no subdials, but they do have complications showing the date and/or day, with an instant setting device for the date calendar.
The Seiko Tuna watches have a uniform tuna can-like design from which they derive their nickname. The Seiko Tuna features a round monocoque case that’s mostly made of titanium, except variations under 7549-7010 and 5M23-6A. Both the SQ and Professional executions of model 7549-7010 have cases of stainless steel, while model 5M23-6A19’s Kinetic and AGS variants are made of plastic. Both 7549-7010 and 5M23-6A feature a screwback case.
Another characteristic that sets the Seiko Tuna apart is its shroud. The shroud of the Grandfather Tuna and the Golden Tuna 7549-7009 SQ and 7549-7000 Professional are made of ceramic-coated titanium. The re-issued Golden Tuna, meanwhile, features a ceramic shroud. On the other hand, the shroud of 7549-7010 SQ and Professional and 5M23-6A19 Kinetic and AGS variants are made of the same material as their cases.
Most of the watches in the Seiko Tuna range have a flat hardlex crystal, except for three variants. The company used a domed hardlex for the Golden Tuna 7549-7010 SQ and Professional models, while it used flat AR-coated sapphire for the Darth Tuna 7C46-0AA0 model’s case.
The AR properties can be seen by the blue reflections it gives
All watches in the Seiko Tuna series feature a uniform unidirectional rotating bezel. The bezel works by showing divers how many minutes they have spent underwater. One click means one rotation, and each rotation turns half a minute. There are two access points on the shroud to spin the bezel. Once set, divers simply have to look at the minute which the hand points to determine the elapsed time.
The Seiko Tuna watches were released in limited quantities and with expensive price tags. Due to their limited number, prices of vintage Seiko Tuna watches in early 2015 ranged from $1,200 to more than $2,000. A watch collector might find one of these watches on eBay or other brand-only forums today. For instance, a 6159-7019 sold for $2,200 in March 2015 at the Seiko Citizen Watch Forum.
However, if they do happen to find one, watch collectors should look for signs of water entry, chipped shrouds, stripped crown tubes, and missing or stripped shroud screws before finalizing their purchase decision. Recent prices for early 6159-7010 600 models in Japan have been in the $1,900 to $2,400 range.
Beginner watch collectors or even veteran watch enthusiasts hoping to have their own Seiko Tuna collection can start with any model in good original condition. The shroud and other distinct features should help anyone distinguish a genuine Seiko Tuna model from a fake one.
Those who are looking for something different to add to their collections can search for the Golden Tuna variant 7549-7010 and the Darth Tuna 7C46-0AA0. Both feature different crystals from the rest of the Seiko Tuna range, with the former having a domed hardlex crystal and the latter having a flat AR-coated sapphire crystal.
The 7549-7010 came in both SQ and Non-SQ versions
The 7C46-0AA0 better known as "Darth Tuna"
Watch collectors seeking the ultimate Seiko Tuna timepiece should look no further than the first variant in the series, the Grandfather Tuna 6159-7010. In particular, they should search for the early 1975 execution which had no “m” as it’s rarer than the variant with an “m.” Because of its rarity, watch collectors should expect to pay more for this piece. As a point of reference, Seiko sold the original 6159-7010 for a list price of 89,000 yen.
Notice the "600" without the "m" that came with the later models of the Grandfather Tuna
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