Raising Sergeant Majors (Full Article)Author: Matthew L. Wittenrich
I have bred and raised many species of marine aquarium animals, and I am frequently asked, “What is the most difficult species you have raised?” My answer, without hesitation, is the sergeant major Abudefduf saxatilis, a type of damselfish.
Damselfish have always been $5 fish—those pretty, cast-iron fish you seed your new system with. Although damselfish are extremely popular and among the top imports of marine aquarium fishes, we know little about their reproduction and larval development in captivity.
It would be nice to say they are as easy to raise as they are inexpensive. Unfortunately, most damselfish are among the most difficult species of coral-reef fishes to raise in aquariums. The reasons for this are not completely understood, but damselfish larvae are small, highly selective in choosing their tiny zooplankton meals, very sensitive to water quality issues, and exhibit marked developmental shifts that warrant different approaches to larval rearing.
Commercial growers have long been interested in the mass culture of popular damsels, like the domino and blue damsels. Several institutions attempted to raise a few species in the early 1990s, but never achieved great success. I think the vast majority of growers never really took an interest because of their price.
After the reports from pioneers like Martin Moe and Frank Hoff about how hard it was to get a decent dollar for clowns and even French angelfish, it is unsurprising that more emphasis has not been placed on this group of fishes. How could researchers spend years going after a damselfish that would fetch no more than a few cents in the wholesale market? More recently, however, researchers and commercial growers are realizing the need for captive-bred fishes, and this has led to renewed interest in damsel culture. To truly contribute to a sustainable marine aquarium trade, we must place emphasis on those species most exploited for the trade.
That’s where I come in. Each year, as the water temperature slowly rises from the cooling of winter, I patiently polish the glass of my diving mask in anxious preparation of the sergeant major spawning season. If conditions are right, late May marks the beginning of the season and dozens of spawning fish can be seen gathering in the shallow waters of central Florida.
Brooding males can be seen a good distance away underwater, their bright blue nuptial colors being a dead giveaway to the location of their nests. Male sergeant majors are highly energetic, defending their clutches of eggs and attracting new mates. When I get closer, the bright violet color of the eggs becomes overwhelmingly obvious. The sizes of the nests are sometimes enormous and contain eggs from multiple females at varying developmental stages.
Once I find males defending nests in a sheltered area away from ripping currents, I place 12-inch ceramic tiles in a variety of locations near the brooding males. Eventually one of the males will recognize this as a novel spawning site to attract mates, and a pair will spawn on the tile.
Checking the tiles every few days, I wait until the eyes of the developing embryos are developed but have not yet turned silver. After placing a new tile in place of the old, the tile containing the spawn is placed in a bucket and driven an hour down the road to be incubated artificially in round, black 50-gallon tanks.
Here I turn the tile upside down, suspended by a rack constructed of bent rigid airline tubing, and supply a steady stream of air from a diffuser directly under the eggs. Collecting eggs and hatching larvae is non-problematic, and I can usually get over 1000 healthy larvae from a single spawn.
My initial goal with raising sergeant majors was to collect some data on larval development and feeding ability for my dissertation research. I thought it would be easy at first, but getting the data turned into a three-year battle. During the first year, the hurdle was getting the larvae to survive beyond five days after hatching. For the second year, it was getting the larvae to survive to 15 days—and even still today, the 15-day mark is a huge hurdle.
Not All Tanks Are Created Equal
I first attempted to raise sergeant majors five years ago. Initially I hatched the larvae in 15-gallon laundry sinks that were painted black on the inside. I fed them rotifers and watched them die. Five days after hatching, there was massive mortality.
I added some wild plankton to the mix and routinely raised them to day 15. There was still a big drop in surviving numbers at day 5, but I did manage to get some to survive to day 15. Then I tried raising some with live microalgae, some with paste, some without rotifers, and several different densities and sizes of plankton.
I was beginning to see progress. Numbers would slowly dwindle until there were two or three left in the tanks by day 17. Flexion (bending upward of the notochord tip as part of the process of caudal-fin formation) was just barely evident, and I kept thinking I was going to raise at least one. Then the next day they would all be dead. By the time I thought I was getting somewhere, the spawning season would be over and I was out of eggs until the next year.
Three years after collecting my first batch of sergeant major eggs, I finally succeeded. I set up three separate systems to get closer to my goal of raising sergeants to metamorphosis and collecting my much-needed data. Three different sizes and shapes of round, black tanks were used, and it quickly became apparent that not all black tanks were created equal. All systems were maintained on recirculation systems with protein skimmers, UV sterilizers, and oversized bio-towers, but only the larvae in 50-gallon tanks survived to metamorphosis.
Food is a major constraint to rearing sergeant majors. I spent hours watching tiny larvae interact with their environment and potential zooplankton meals. Even though they were swimming in a soup of potential prey, the larvae wanted nothing to do with the vast majority of them. While they did consume rotifers, dinoflagellates, and a myriad of other organisms from the plankton, they definitely did not survive on it.
It is copepods they will hold out for. They inspect everything around them, but unless it jerks, jostles, or tries to escape, they don't seem to be interested. I watched hundreds of larvae at 10-minute intervals inspecting and casting aside roughly 50 potential meals before finding a copepod nauplius. At the sight of a copepod, the larva would stop, curl into an S pattern and lunge forward.
The flow pattern in the tank is established in such a way to create a slow, horizontal flow field across the surface of the tank. In this way the larvae remain stationary in the currents, frantically turning their head from side to side searching for suitable prey.
When they identify a potential prey organism, the larvae relax their body and allow the current to carry them at the same pace as the prey. From this position they assume the S pattern and strike before repositioning themselves in the current field. If unsuccessful in their first attack they will often stalk a single copepod for several minutes, making a series of unsuccessful attacks before finally capturing it.
With adequate densities of copepod nauplii, growth is quick and flexion is achieved around day 15 and metamorphosis is complete by day 20, usually. Diet has extreme effects on growth and metamorphosis. In earlier trials, no larvae hit flexion until day 17 and the tip of the notochord was barely flexed up. I establish strong upwelling flows and the larvae sit in the current and wait for the prey to pass by. The larvae never consumed a single Artemia and fed on limited organisms in the plankton. I find that my only means of success is to keep the larval tank on a flow-through system to constantly flush the plankton from the tank. This is important to get rid of what they will not eat.
While sergeant majors may never become as popular as blue damselfish or flame angels in the aquarium trade, they do represent an important model to help us overcome some of the challenges faced by marine fish aquaculturists. While breeding marine animals in captivity has come a long way in recent years, we still understand little about the life history of many marine fishes. The larvae of marine fishes are vastly different, and our approach to rearing them must be flexible enough to accommodate such diversity. Damselfish culture should occupy a dominant theme in today’s hobby.For more information, please visit the Marine Ornamental Fish and Invert Breeders site at www.marinebreeder.org.
See the full article on TFH Digital http://www.tfhdigital.com/tfh/201101#pg91