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.
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 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
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