Sergeant Major (Abudefduf saxatilis)
Just returned from a wonderful week as teaching assistant for a coral reef ecology course that has a field component in Belize on the Meso-American reef. While I love the reefs and seagrass beds, one of my favorite spots to visit in Belize is Twin Cayes- an amazing mangrove caye. Drifting down the channel that separates the two cayes is something I look forward to every trip. Unfortunately my compatriots usually end up pulling me out of the channel as I often take a full hour to explore the same area they cover in maybe 20 minutes, but then that is the secret of enjoying the mangrove channel: minimize any movement as it stirs up silt and organics and go slow, very, very slow, edging along the prop roots at the side of the channel and allow time to take it all in.
As the others go screaming along the channel, I wait well behind, letting the silt clear and the animals recover from their passage. The reward is a wealth of invertebrate and fish life, and one of my favorite marine denizens of Belize: the juvenile Sergeant Major.
In Belize, we find juvenile sergeant majors in the shallows under docks and similar structures as well as in the mangroves. The adults are common in the patch reef and very common in the continuous fore-reef areas. Adults feed on coraline algae, copepods and other small crustaceans, anemones, tunicates, and invertebrate larvae. Juveniles appear to feed on benthic and planktonic algae, invertebrate larvae and small crustaceans (copepods especially?). Matt Wittenrich at Florida Institute of Technology has successfully (though with great difficulty) raised larvae from egg hatch through metamorphosis by feeding them green water and wild caught plankton using a flow through system to clear out whatever the juveniles did not eat. He noted through observations they would eat rotifers and green water (algae) but really grew best when they had plenty of copepods to eat. Below are his images of the larval development from ~1 day post hatch to ~20 days post hatch when metamorphosis is complete.
Juvenile sergeant majors are one of the fish I see most commonly under the prop roots of Twin Cayes. They alternate between flitting nervously around a prop root or patch of algae as I come past and charging out to challenge me, then retreating back to the safety of prop root or algae “nest”, which is a behavior they continue to show out on the patch reef when they are 15cm in size.
- Abudefduf saxatilis