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past fishes of the month

FISHES OF THE MONTH - MAY 2005

FLAGTAILS of HAWAI`I
Kuhlia xenura and Kuhlia sandvicensis

 


Kuhlia xenura? - Honolua Bay, Maui

Kuhlia
xenura? Palea Point, O`ahu

FLAGTAILS (Kuhliidae)
     Flagtails are a small family of silvery, perchlike fishes with a single dorsal fin. The tail fin is banded in many flagtail species, hence the common name. In Hawaiian species, however, only juveniles have banded tails.

A Tale of Two Species
     Until recently only one flagtail species was believed to occur in Hawai`i. In the late 1990s, however, aquarium collector Darrell Takaoka noticed that there seem to be two types of juveniles in tide pools. Some have black reticulations on their heads, whereas others have plain heads. He took specimens to Dr. John E. Randall at the Bishop Museum, who determined that the "zebra-head" flagtails represent a species not previously thought to occur in Hawai`i. Investigating further, Randall and his wife found that the scientific names of both species had been mixed up. The correct scientific name of the less-common "zebra-head" species--Kuhlia sandvicensis-- was the name in current use for the more-common "plain-head" species. The correct name for the common "plain-head" flagtail turned out to be Kuhlia xenura, a name long forgotten in the annals of ichthyology. Further confusing the issue, the zebra-headed sandvicensis (which might expected on the basis of name to be endemic to Hawai`i) was found to be widely distributed in the Central Pacific, while K. xenura was found to be endemic.
     Meanwhile, a biology student at Louisiana State University, Lori Benson, was hot on the same story. In 1999, while doing research in Hawai`i for her PhD thesis on flagtails, she was tipped off by local fishermen that there were two types of äholehole, one with bigger eyes than the other. Suspecting two species, she procured specimens and submitted tissue samples for DNA sequencing. In 2001, however, while Benson's studies were being completed, Randall and Randall published their results in the journal Pacific Science. Benson's PhD Thesis was submitted in 2002. Both studies were completely independent, and each contains useful ID information not present in the other.

How can I tell adults apart? 
      It's very difficult, unless you want to count scales, fin rays and gill rakers. Even then, the differences are slight. In the field, adults of the two species appear virtually identical, although under certain lighting conditions faint dark reticulations can sometimes be seen on adult Zebra-Heads.
 I tentatively assigned the name "xenura" to the flagtails in the two photos above on the basis of probability: Hawaiian Flagtails (Kuhlia xenura) are reported to be more abundant than Zebra-Head Flagtails (K. sandvicensis).
      Theoretically, adults of the two species can also be differentiated by the characters below, but in practice these are almost impossible to see, even in photos:
1) eye size: The Hawaiian Flagtail has a slightly larger eye in relation to the length of the head.
2)
dorsal head profile: the Zebra-Head Flagtail has a straight dorsal head profile whereas the Hawaiian Flagtail has a slightly concave dorsal head profile.
3) color: The Hawaiian Flagtail often has an olive or bronze tinge along the back, and sometimes bronze patches on the side, whereas the Zebra-Head is more silvery on the back and whitish below. Also, in the Hawaiian Flagtail the upper edge of the eye reflects a reddish color in strong light. (See photo at bottom of page.)
4) habitat? As adults, both species seem to prefer turbulent areas, where they form dense resting schools by day. Some observers suggest that the Zebra-Head might prefer resting in dark caves by day while the Hawaiian might prefer resting in the open, but this has yet to be proved.


HAWAIIAN FLAGTAIL · äholehole
Kuhlia xenura (Jordan & Gilbert, 1882)
     By day, these silvery fish form dense stationary schools, usually near the tops of reefs or along dropoffs in areas of heavy surge where turbulence and constant fine bubbles screen them from predators. Subadults will also school in very shallow water along protected sandy beaches and in stream mouths. At Honolua Bay, Maui, they aggregate near submarine freshwater springs. These resting schools often occupy the same location year after year, although they sometimes shift locations temporarily. At night the fish disperse to feed on plankton. The young have banded tails, but are otherwise plain (in contrast to young Zebra-Head Flagtails which have black reticulations on the head). Juveniles are abundant in tide pools; they enter brackish water, and will even penetrate some distance up streams. In old Hawai`i only the young were called äholehole; adults were simply ähole. To 12 in. Endemic.

ZEBRA-HEAD FLAGTAIL · äholehole
Kuhlia sandvicensis (Steindachner, 1876)
      This flagtail was only recently identified in Hawai`i because it is so similar to the Hawaiian Flagtail (above). However, juveniles in tide pools are easy to identify because they have a whitish head with wide black reticulations on the upper surface which extend rearward as two black lines on either side of the dorsal fin. Also, the tip of the soft dorsal fin is white. These markings fade on adults, although the dark reticulations are sometimes visible under certain lighting conditions.
      There must be some differences in behavior and habitat between Zebra-Head and Hawaiian Flagtails, but if so they remain for the most part undiscovered. Here is some of what we know so far, according to a PhD Thesis on these two species by Lori Benson of Louisiana State University: 1) Juvenile Zebra-Heads sometimes occur near stream mouths, but they do not actually enter freshwater streams, whereas juvenile Hawaiian Flagtails are sometimes found well upstream. 2) Juvenile Zebra-Heads avoid pools that are closed off from the ocean at low tide, whereas juvenile Hawaiian Flagtails will live in such pools. 3) Juvenile Zebra-Heads will not seek shelter in cracks and crevices although juvenile Hawaiian Flagtails will. Benson also mentions that while adults of both species feed nocturnally, "adult K. sandvicensis were observed and collected actively feeding during the day on at least one occasion."
     If you have additional information on the differences between these two species, or good photos of flagtails in caves, I'd love to hear from you.


The photos of juvenile flagtails below are used courtesy of Danene Warnock

Juvenile Zebra-Head Flagtails in tide pool- Danene Warnock, Kihei, Maui.

Juvenile Zebra-Head Flagtail (above) and juv.Hawaiian Flagtails (below) in tide pool - Danene Warnock, Kihei, Maui

Hawaiian Flagtails - identified on basis of olive patches and reddish color on upper edge of eye.



Is this the end of the flagtail story? Possibly not. Giant äholehole, up to 17 in. long, occasionally turn up in fish markets. Perhaps these represent a third Hawaiian species.


 

  

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  Text and photos copyright John P. Hoover