You’re on a dive, minding your own business when suddenly, your mask is being bombarded by a frantic and extremely angry creature. In shock you reverse, exiting the firing line when you realise that the monster in question is none other than the Sergeant Major fish. If your biggest fear in the ocean is the ‘dreaded shark’, think again, it’s the Sergeant Major you really have to worry about!! In disbelief you take a second glance and notice that the creatures you once thought were small and peaceful are all behaving in this manner to anything crossing or invading their path.
Why do they behave like this? And why have you never noticed this before?
Abudefduf is from the Arabic; abu “the one with”, def “side", and duf “prominent”. Its name, referring to the vertical black bars on its side, roughly translating to "the one with prominent sides". This is also where its gets its common name “Sergeant Major,” as the stripes resemble the bars on a military uniform. Despite the general common name Sergeant Major there are up to 20 species of this damselfish, part of the family Pomacentridae, that you can be found worldwide. Generally, their bodies are deep, oval and compressed with a continuous dorsal fin and a forked caudal fin, but each species varies in size and the number of dorsal fins, dorsal soft rays, anal spines and anal soft rays.
Here in the UAE we have the Indo-pacific Sergeant Abudefduf vaigiensis. Widely distributed throughout the Indo-Pacific: Red Sea, Indian Ocean to eastern Africa, north to southern Japan, and south to Australia. In 1991, it was even discovered in the Hawaiian Islands, possibly floating marine debris such as abandoned fishing nets, where it now coexists with the Hawaiian Sergeant Abudefduf abdominalis. They form large aggregations while feeding during the day and play hide and seek by themselves at night on coral reefs, tidal pools and rocky reefs at depths between 1 to 15 m. The Indo-pacific Sergeant can grow up to 20 centimeters long, feeding on zooplankton, benthic algae and small invertebrates.
The Interesting Part!
So now you have the Sergeant Majors background check. Here’s the fascinating part!
Between March and August, the Sergeant Majors congregate in large numbers to reproduce. To start the process, the males prepare temporary nesting sites by coating patches of rocks, shipwrecks and corals with an adhesive substrate. The females then wander around looking for that special partner to mate with. When a female is near, the male will try to attract the female by preforming a unique and intriguing dance which involves moving up and down erratically, like the local dance club 😊. If she’s digging his moves she will deposit up to 20,000 eggs. Once the eggs have dispersed, they attach themselves to the substrate the males prepared and will then be fertilized.
After all the fun, the females eventually take off leaving the males with all the responsibility of protecting his little patch. The male is extremely attentive by continuously aerating his eggs stopping fungus build up. He will also turn a darker bluish colour which most likely acts as a warning colour to predators or even possible camouflage. Any fish that comes too close are aggressively chased away and divers are left in bewilderment.
The eggs start as solid red ovals around 0.5-0.9mm. Once they are fertilised they start to turn a green colour with a deep red yolk leaving the surfaces as a patchwork. After 4-5 days the eggs will hatch an hour after sunset. If you are able to see the hatching, it can be amazing to watching as they wiggle out from the substrate and break free. Once hatched they are around 2.4mm in size and set off to tackle the big blue ocean.
I have been very privileged over the last few weeks to witness parts of this process and they are amazing to watch. The reef has literally been covered in patches of varying shades of red and green. The determination of the male Sergeant Major whilst hilarious to watch is also impressing for such a small creature to display.
With my new camera I was able to catch some of these processes, but my next goal is to capture the courtship ritual and hatching (I may need a bigger camera though).
Next time you are out diving, make sure you don’t overlook the little guys, and look out for the patchwork of the ocean created by the Sergeant Major Fish.