Seiko Prospex Street Series Green "Baby Solar Tuna" Watch Review - SUT421

Seiko produces a number of iconic dive watches, but possibly the most distinctive are those nicknamed "Tuna." I recently discovered that there is a midsize Solar Tuna and purchased the green Seiko Prospex SUT421 variant as a gift.

This variant is styled similarly to the SRPE29 Street Series "Safari Tuna" that I reviewed a few years ago, but swaps the automatic movement for solar, and shrinks in every dimension. I recommend reading that review if you'd like to learn more about the history of the Seiko Tuna.

Seiko Baby Solar Tuna SUT421 First Impressions

The chunky tuna can case works well shrunken down. Combined with a light dial and mint green strap it reduces the harsh tuna looks. Seiko also sells a red and pink variant of this model called SUT409.

Seiko Baby Solar Tuna SUT421 Details

The baby tuna is just under 39mm in diameter and only 10.4mm thick. This gives it an entirely different look than the "real" Tuna, or even compared to the 43mm Safari. The Tuna design language also dictates compact lugs, so it fits comfortably on even tiny wrists.

Despite its smaller size and cheeky appearance, SUT421 is still branded as  Professional Specifications (Prospex). It packs 200m of water resistance, is anti-magnetic rated to 4,800 A/m, and of course, has a metal shroud protecting its bezel from impacts.

The shroud is nearly fully brushed silver. While the Safari Tuna had a translucent bezel insert with lume, the 120-click bezel on SUT421 has a green insert with a traditional pip insert. The insert is machined similarly to Seiko's SPB143, although with much deeper grooves.

Seiko describes the SUT421 dial as "white...with a subtle pattern." The pattern consists of subtle, slightly darker polka dots on a milky white translucent dial (necessary to allow light to pass through to the solar panel). Black hands and markers use a similar design to the Safari Tuna.

As mentioned in the review comparing SPB143 to SPB453, most new Seiko divers have been revamped with a date window at 4:30, allowing lume markers at all hour positions required by the latest ISO dive standard. SUT421 avoids this concern by omitting a date display altogether.

Wearing the Seiko Baby Solar Tuna SUT421

The Safari Tuna was already quite wearable for a 43mm watch, but shrinking it down by over 4 mm makes it even easier to wear.

Moving to quartz also allowed Seiko to thin out the case, avoiding it becoming top-heavy (and living up to the Tuna Can name).

I love that Seiko put black hands on this model, as one might expect silver on this more feminine design. The black hands (and marker outlines) make legibility much better against its white dial. I also like this simple rectangular hand shape over the funkier ones on other models.

Since it's still a Prospex, Seiko also applied its always-great LumiBrite liberally to the hands and markers. The lume is thick, bright, and long-lasting.

Bracelet and Replacement Straps for Seiko Baby Solar Tuna SUT421

While Seikos silicone straps are soft and comfortable, I've long voiced that they are not my preference, especially those with metal keepers. At least this lighter-colored one will avoid showing dust as much as a black one (like on the MM300 Black Series).

SUT has 18mm wide lugs. Strap choices will be slightly limited, but StrapHabit has some good options. I especially like a dark green or white rubber strap with this model.

Here is a list of some of the straps that would fit and we think look great with this watch. You can also view our Guide to Choosing a Watch Strap. At the end of the article, you'll also find photos of some of our favorite combinations.

-Tropical Rubber
-Smooth Rubber
-Beads of Rice Bracelet
-TQ18 straps also fit!

Seiko Prospex Baby Solar Tuna SUT421 Movement

A solar movement is a great choice for a slimmer, daily-use Seiko tool watch. Caliber V131 provides up to 6 months of operation with no light exposure. It even has two jewels.

It also has an end-of-life indicator (EOL). When the charge becomes low, the second hand will operate at two-second intervals to indicate that it requires exposure to light.

As mentioned, it is a time-only caliber (no date display). It's rated at +/- 15 seconds per month of accuracy.

Conclusion

The Tuna design language transitions well to a smaller size and retains the charm of the original. Seiko SUT421 opens up this iconic style to smaller wrists. Unfortunately, it's not inexpensive. Browsing the Seiko website revealed that even entry-level automatic divers with the 4R movement are now $500-600. This must have allowed Seiko to also increase the prices of quartz models.

While $450 seems like a lot for a quartz watch with a silicone strap, you are still getting Seiko's reliability and design language. With the watch market as a whole creeping up in price, I think that we just have to get used to it.

You can learn more about it at the Seiko Website.

Seiko Prospex Baby Solar Tuna SUT421 Specifications:

Name: Seiko Prospex Solar Diver
Reference Number: SUT421
MSRP: $450
Diameter: 38.7mm
Height: 40.4mm
Thickness: 10.4mm
Lug Width: 18mm
Movement: Seiko V131
Power Reserve: 6 months without light exposure
Water Resistance: 200m
Crystal: Hardlex (mineral)
Crown: Screw-down
Bezel: Unidirectional rotating, 120 click
Bracelet: Silicone

Shop Other Straps for the Baby Solar Tuna SUT421:

Tropical FKM Rubber

Beads of Rice Bracelet

TQ18 FKM Rubber:

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TQ18 Sailcloth and Leather Hybrid:

Smooth FKM Rubber