Larval Rearing

 

Culture Requirements
Thirty-six reef fish families reproduce by releasing eggs directly into the water column. These include popular aquarium fish groups such as the tangs and surgeonfishes, butterflyfishes, wrasses, hawkfishes, and all marine angelfishes. Unfortunately, the long and complex larval phase has kept most pelagic spawners, like the Centropyge, from being cultured. Compared to the larvae of commercially propagated species (clownfishes, dottybacks and most gobies), Centropyge larvae demand a smaller, more nutritious and easily digested food source at hatching; are more sensitive to environmental changes (water quality, lighting, temperature) and require optimal nutrition throughout development; and take much longer to both reach and complete metamorphosis. Our main objective has been to resolve these problems.

Breakthrough
On November 3rd, 2002 Reef Culture Technologies LLC closed the life cycle for the Fisher’s Angelfish (Centropyge fisheri). To the best of our knowledge this was the first pygmy angelfish species ever raised in captivity. The company has since developed a reliable rearing method for a number of pygmy angelfishes, raising an additional five species.

First closing the Centropyge life cycle and then developing a reliable rearing technology was accomplished through a number of small advances, both in the diet and the environment, which helped the larvae survive a little longer each time. The first of these was the discovery of suitable copepod species.

Copepods
Copepods are the largest class of Crustacea inhabiting nearly all aquatic ecosystems on this earth. They can be pelagic, benthic or even parasitic. Of the over 4,500 copepod species that exist, most inhabit the oceans, where they form an important link between algae and higher trophic levels. The typical copepod body is cylindrical and segmented and, like other Crustaceans, is divided into a head, thorax and abdomen.

Copepods reproduce sexually. Upon fertilization eggs are secreted into ovisacs, which are attached to the female’s first abdominal segment. Each ovisac can contain anywhere from a few to 50 fertilized eggs. The eggs hatch into nauplii that go through 4-6 stages and then become cepepodites. After 5 copepodite stages they metamorphose into an adult, which is final molt.

Marine copepods are an important food source within natural zooplankton communities, especially for marine fish larvae. First off, they are widespread throughout all oceans and can occur at high densities, blooming seasonally. Furthermore, most species have a very good nutritional profile. All ontogenic stages are rich in proteins, highly unsaturated fatty acids, most amino acids and pigments. In addition, they are rich in digestive enzymes. This helps digestion and assimilation of essential nutrients in species where the early larvae have poorly developed digestive systems. With adult species ranging anywhere from 0.5 mm to 5 mm in length and some nauplius species hatching as small as 30 um in diameter, they also have an ideal size range as prey. Finally, their jerking and gliding motion elicits a feeding response and makes them easy to capture.

The natural qualities of copepods make them a model prey organism for rearing marine fish larvae. Consequently, there has been considerable interest to bring them into mass culture. Unfortunately, this has not been economically feasible for many suitable species particularly those that produce the appropriate nauplii preyed on by smaller first-feeding larvae. The primary biological attribute that makes such species unsuitable for aquaculture is their long reproductive cycle, which being sexual, can take anywhere from one to four weeks to complete under optimal conditions. By comparison, rotifers, commonly used to grow marine fish larvae through the first few weeks, reproduce asexually in less than 24 hours. Furthermore, desired copepod species are more difficult to maintain in captivity than rotifers, demanding better water quality, cleaner conditions, more space, and more nutritious feeds. The main copepod suborders (and genera) of interest to aquaculture are:

  • Calanoid (Acatia sp., Clausocalnaus sp., Eurytemora sp., Calanus sp. Pseudocalanus sp., Gladioferens sp.)
  • Cyclopoid (Apocyclops sp., Oithona sp.)
  • Harpacatcoid (Tisbe sp., Tigriopus sp., Tisbenta sp., Schizopera sp., Euterpina sp.)

A First Food Organism
Over the years we attempted to raise Centropyge larvae with a number of food organisms. Among them, small strain rotifers and several ciliate species. Success was limited until 2001 when we began experimenting with copepod nauplii. At the time we were working with Fisher’s angelfish eggs. The superiority of the new food over ciliates and rotifers was immediately apparent. First off, the larvae were more active at first feeding and could frequently be observed attacking the prey, something we had never witnessed before. A larva would swim up to the nauplius, wind up its tiny body into the characteristic S-shaped attack posture and strike. Their swimming and hunting abilities perceivable improved each day. Second, the body of nauplii-fed larvae increasingly reddened starting on day 5. Upon closer examination we found this to be the build up of tiny vascular vessels filled with, what was obviously, blood. In contract, the larvae reared on rotifers and ciliates would gradually become darker until dying in near black color on day 9. Third, larvae feeding on nauplii were growing, increasing close to 5 mm in length from day 4 through day 10. Those fed rotifer and ciliates died at the size they had on day 5, when their yolk is completely exhausted.

Copepods nauplii used to feed early stage Centropyge larvae

Copepods nauplii used to feed early stage Centropyge larvae

Copepods nauplii used to feed early stage Centropyge larvae

Larval Development
The six species have a very similar course of development (ontogeny) up to metamorphosis. The eggs are tiny, averaging 0.7 mm in diameter, and hatch after only 16-18 hours at 27-28 ºC. At this time the larvae are nearly transparent, about 2 mm in length and very primitive, lacking eyes, a mouth, a digestive tract and functional fins. As the yolk sac gets depleted these develop. Three to four days after hatching the larvae are able to start feeding. Properly nourished larvae will undergo noticeable vascularization (blood vessel formation) during the first 2 weeks of development. This “reddening” only occurs if they are growing and healthy. 15 and 25 days after hatching the larvae start to laterally compress. They then take on silver coloration and develop strong pigmentation along the dorsal area.

Metamorphosis starts 45-50 days after hatching and, depending on the species, can last up to 50 days. A darkening of the soft dorsal and anal fins marks the beginning of this transitional period. As juvenile coloration gradually fills in, the larvae become more stationary and behave less erratically. At this time they can be transferred to a grow-out tank. Tank-raised fishes are known for their captive hardiness, disease-resistance and readiness to accept conventional fish foods. Our Centropyge juveniles developed those same qualities. In fact, our first generation Fisher’s angels are spawning regularly after just 230 days.

Larval Stages of Centropyge flavissimus

Larval Stages of Centropyge flavissimus
Larval Stages of Centropyge flavissimus
Larval Stages of Centropyge flavissimus
Larval Stages of Centropyge flavissimus
Larval Stages of Centropyge flavissimus
Larval Stages of Centropyge flavissimus
Larval Stages of Centropyge flavissimus
Larval Stages of Centropyge flavissimus
Larval Stages of Centropyge flavissimus
 

 

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