fishes of the month
FISH OF THE MONTH - APRIL
more than you want to know about the
Stegastes fasciolatus (Ogilby, 1889)
the next few months we'll be taking a look at some of common Hawaiian
reef fishes which tend to fall under the radar of most snorkelers
and divers. Although "beneath notice," they can be quite
interesting once you know a little about them.
Stegastes fasciolatus (Ogilby, 1889)
This drab, blackish or brownish gray
damselfish has bright lemon-yellow eyes and a patchy unkempt appearance
due to some scales being lighter than others. Common in Hawai`i, it
prefers areas with moderate to low wave activity and feeds primarily
on green filamentous algae. Each Pacific Gregory maintains an all-purpose
territory (containing shelter and a nesting site as well as food)
which it boldly defends against all other algae-eating fishes. The
territory extends about 2-4 ft. in all directions from the shelter
hole and is usually defined by natural formations such as coral heads
or clumps of weed. (Containing more algae than surrounding areas,
Pacific Gregory territories are sometimes, but not always, discernible
to humans.) Like similar Indo-Pacific and Caribbean damsels, the Pacific
Gregory "farms" its patch of filamentous algae by removing
undesirable coralline algae. It also eats small invertebrates living
in its algal farm.
As anyone who has watched a Pacific
Gregory for a few minutes can attest, this pugnacious little fish
will unfailingly attempt to drive any other herbivore from its algae
patch. It does not waste energy by attacking carnivores such as wrasses,
but experiments show it can learn to recognize and attack algae-eating
fish species it has never seen before. A male Pacific Gregory will
also defend its territory against males of its own species. When displaying
aggression to another male, it darkens its yellow eyes (see "Oh
Those Dark Eyes" below). Juveniles, bluish black with a pale
yellow tail, appear during the summer months, often on wave-scoured
reef flats. Those under 1 in. have an iridescent blue-purple streak
along the top of the head and margin of the dorsal fin. The species
name means "banded." To about 6 in. Indo-Pacific, but with
a slightly different color pattern in Hawai`i (see "Three
Color Patterns" below). Pomacentrus jenkinsi is a
synonym. Photo: Pupukea, O`ahu. 15 ft.
The feisty Pacific Gregory is a surprisingly
important member of the reef community. Where abundant, this small
fish alters the local behavior of herbivores such as Brown Surgeonfish,
Convict Tangs, Yellow Tangs, forcing them to feed in schools instead
of individually. (Only by schooling can the other herbivores overwhelm
the Gregory's defenses.) It can also cause the Fourspot Butterflyfish
to feed at night instead of during the day.
importantly, however, Pacific Gregories increase both the biodiversity
and algal productivity of the reef. Experiments off O`ahu show that
some algae and coral species, normally prevented from growing by
intense parrotfish and surgeonfish grazing, thrive within Pacific
Gregory territories. (Their territories may also be almost the only
places where certain rare corals grow!) Another study in Australia
and Papua New Guinea showed that algal communities inside Pacific
Gregory territories, and those of similar damselfishes, were up
to 3.4 times as productive as those outside. Finally, a study in
Käne`ohe Bay, O`ahu, showed that young coral growth was highest
on the windward edge of a patch reef where territories of S.
fasciolatus are common.
Organisms with a disproportionate
influence on an ecosystem--such as these territorial damselfishes--are
sometimes called "keystone" species. Their removal would
probably produce many unanticipated changes.
THOSE DARK EYES
many territorial fishes, Pacific Gregories must defend their turf
against rival members of their own species. This is true of juveniles
as well as adults. A 1968 study at the Hawaii Institute of Marine
Biology showed that rival juvenile Gregories often play "chicken"
using a combination of two signals: raising the dorsal fin to indicate
fright, and darkening the yellow eyes to gray to indicate aggression.
"Fights" can take place without any physical contact,
the winner being the one who displays to the other with the highest
aggression indicator (the darkest eyes) and the lowest fright indicator
(lowest dorsal fin). In adulthood, however, when reproductive success
is at stake, dark eyes are not enough--rival males actually chase
and bite one another. They also use a nasty behavior called head-scraping
wherein two fish approach each other head on, position themselves
side-by-side, flare out their gill covers, then swim backwards rapidly.
The object is to scrape the opponent's eye with a row of short sharp
spines on the edge of the gill cover. In this case, darkening the
bright yellow eyes might make it harder for the rival fish to orient
itself properly, lessening its chance for successful attack. Interestingly,
when attacking other species of fish, Gregories do not darken their
Like many Hawaiian
fishes, local Pacific Gregories have a slightly different color pattern
than Pacific Gregories elsewhere. Although the basic blackish color
remains the same, outside Hawai`i these fish have pale blue to lavender
spots on the head, breast, and at the base of the anal fin. In the
Western Indian Ocean they also exhibit yellow on the rear of the dorsal,
anal and tail fins. By contrast, adult Hawaiian specimens lack blue
spots and yellow fins, having instead a distinctive black mark between
the 1st and 3rd dorsal spines (see arrow). Differences such as these
might in the past have provided justification for naming three distinct
Pacific Gregory subspecies. Today, the subspecies concept is out of
favor. If consistent physical differences other than color could be
demonstrated between the three populations, they would likely be declared
separate species. If not, they would be left under a single name,
as has happened here. DNA sequencing, however, is quickly adding other
layers of complication to this never-ending question of species vs.
subspecies vs geographical variants. Stay tuned.
Garfield of San Diego writes: " I was freediving around
Black Rock in Ka'anapali watching Pacific Gregories at a depth of
about 10 ft. when I noticed a male swimming in acrobatic loops within
his territory--a behavior called "dipping." I immediately
stopped, as he then appeared to be carrying out what looked like spawning
behavior. He rubbed his entire underside in a wiggling motion on the
coral rock area. Was he fertilizing eggs? I saw no female. I moved
in to see if I could see eggs or any evidence of a nest. Nothing.
Furthermore, I did not hear any clicking or other sounds during his
loopy swimming or as a warning for me to keep my distance. I retreated
to a respectful distance and watched.
I was rewarded. Shortly thereafter, a female
swooped in and rubbed her underside, also in a wiggling motion, against
the same area as the male. She must be laying eggs. During this time
of several seconds, the male
nervously swam around, guarding his clutch and looking for interlopers.
The female then dashed out of the nest and, without delay, the male
fertilized the eggs as described above.
Because these tiny eggs are transparent and
the coral is white, I could see no evidence of an egg mass. However,
because of my experience observing the nesting California damselfish
rubicundus) and its bright-yellow eggs, I noticed the distinctive
behaviors of the two Hawaiian damselfishes immediately. (Check
out Judith's books!)
ALL EVENS OUT IN THE END
A recent Taiwan study showed that Pacific
Gregories whose permanent territories were temporarily taken over
by larger Indo-Pacific Sergeants during their nesting season "got
even" by eating the Sergeants' eggs, 1,100 daily on average!
The eggs are probably an important food item for the Pacific Gregory--at
least in Taiwan. No one has yet investigated whether a similar phenomenon
occurs between Pacific Gregories and Hawaiian Sergeants. I personally
cannot recall ever seeing Gregories attacking Hawaiian Sergeant egg
patches. If you see this, let me know!.