WHAT IS BIOLOGY AND ITS LIFE HISTORY (E-book)

SEABREAM GILTHEAD

The gilthead seabream Sparus aurata (Linnaeus, 1758) is a perciform fish, belonging to the family Sparidae and to the genus Sparus.

LOCAL NAMES	Albanian	koceja, spalc
	Arabic	denis (Egypt), ourata, warka (Tunisia), zerika (Morocco)
	English	sea bream, gilthead seabream
	French	dorade, daurade
	German	goldbrasse
	Greek	tsipoúra
	Italian	orata
	Portuguese	dourada, doirada
	Spanish	dorada
	Turkish	çipura

Fig. 6 Adult specimen of gilthead seabream

Family Description

Family Sparidae

Percoid fish with body oblong, usually deep and compressed. Scales cycloid or weakly ctenoid. Head large, often with a steep upper profile. Snout and supraorbital area scaleless. Mouth small, with the upper jaw reaching no further than the middle part of the eye. Preoperculum scaled, without spines on margin. Jaw teeth usually differentiated into conical, incisor or canine teeth in front and molar-like teeth behind. Palatines bones usually toothless.
One single unnotched dorsal fin with 10 to 15 stout spines and 9 to 17 rays. The spiny anterior part has the same length of the posterior part with rays. Anal fins with 3 stout spines and 7 to 16 rays; pectoral fins usually long and pointed; ventral fins with axillary scales; caudal fin emarginate or forked. A single continuous lateral line. Colours vary greatly, from silver to reddish to almost black.
Almost all Sparidae are demersal, and are found in relatively shallow waters, often in rocky areas; the young fish generally live in shallower waters than the adults; fry and fingerling school together, while adults usually show a solitary behaviour.
The eggs are pelagic, spherical (with a diameter of around 1 mm) and have an oil drop.
Many sparids are hermaphroditic: when reaching sexual maturity there could be a majority of males (protandric hermaphroditism) or of females (protogynic hermaphroditism). Sparids are carnivorous fish and feed mainly on molluscs and other benthic organisms, which they break with their strong teeth.
Due to their excellent meat, many representatives of this family have a high commercial value. Sparids are divided amongst many genera and a large number of species living in all tropical and temperate seas, including exceptionally cold and brackish waters. In the Mediterranean eleven genera represent the family: Dentex, Sparus, Diplodus, Pagellus, Pagrus, Lithognatus, Spondyliosoma, Oblada, Crenidens, Boops and Sarpa.
DISTINCTIVE CHARACTERISTICS OF THE SPECIES
The genus Sparus is characterized by molariform teeth and 75-85 scales along the lateral line. Sparus aurata is its only species of this genus in the Mediterranean.

Biology

Morphology
The gilthead seabream presents a body with an oval shape, very high and laterally compressed. The head profile is convex with small eyes. The cheeks are covered with scales and the pre-opercular bone is scaleless. The mouth has the mandible shorter than the maxilla. Both jaws show canine (4-6) and molariform teeth, in 2-4 series in the upper jaw and 3-4 series, of which 1-2 are notably bigger, in the lower jaw.
The gill rakers are short, 11-13 on the first branchial arch and 7-8 on the lower part. The lateral line has 75-85 scales. The dorsal fin presents 11 hard and 13 soft rays, the anal fin 3 hard and 11-12 soft rays. The pectoral fins are long and pointed, while the ventral ones are shorter. The caudal fin has pointed lobes. All the vertebra present parapophises and sessile ribs are absent.
The gilthead seabream colour is silver-grey with a big dark spot at the beginning of the lateral line that covers also the upper part of the opercular bone. A gold and a black band is found between the eyes, the golden one always narrow in the central part. The dorsal fin is blue-grey with a median black line. The caudal fin is grey-greenish white with black tips.

Fig. 7 - Gilthead seabream, teeth and jaws

Geographic distribution

S. aurata is common in the Mediterranean Sea, it is present along the Eastern Atlantic coasts from Great Britain to Senegal, and is rare in the Black Sea. Due to its euryhaline and eurythermal habits, the
species is found in both marine and brackishwater environments such as coastal lagoons and estuarine areas, in particular during the initial stages of its life cycle. Born in the sea during wintertime, the fingerlings typically migrate in early spring towards protected coastal waters in search for abundant food and milder temperatures (trophic migration). Very sensitive to low temperatures (lower lethal limit is 4°C), in late autumn they return to the open sea, where the adult fish breed.
The gilthead seabream is usually found on rocky and seaweed bottoms, but it is also frequently caught on sandy grounds. Young fish remain at low depth (up to 30 m), whereas adults can reach deeper waters (maximum depth of 150 m).

Fig. 8 - Gilthead seabream geographic distribution

Reproduction

The gilthead seabream is a protandric hermaphrodite with a breeding season ranging from October to December. The gilthead seabream is a functional male in the first two years and at sizes over 30 cm become females. After spawning, the eggs, which are spherical and transparent, have a diameter of slightly less than one mm. and present a single large oil droplet.

Fishery

The gilthead seabream is fished with traditional and sporting equipment, and sometimes with semi-professional systems (Spain, Sicily, Egypt and Cyprus), trawl nets, bottom set longlines, hand lines are also commonly used. The gilthead seabream is regularly present on the markets in Adriatic, Greece, Turkey and Maghreb. It is commercialised fresh, refrigerated and frozen.

2.2- THE EUROPEAN SEABASS

The seabass, Dicentrarchus labrax, Linnaeus 1758, is a Perciform fish, belonging to the Moronidae family and to the genus Dicentrarchus.

LOCAL NAMES	Albanian	lavraku; levreku
	Arabic	karous
	English	European seabass; bass; sea perch;
	French	bar; loup; loubine; perche de mer; barreau
	Greek	lavraki
	German	wolfsbarsch; seebarsch; meerbarsch
	Italian	spigola; branzino
	Spanish	lubina; robaliza; róbalo; magallón
	Turkish	levrek

Fig. 9 - Adult specimen of seabass

Family Description

Family Moronidae

Perciform fish of elongated body. Operculum with two flat spines; terminal mouth slightly protractile; end of the maxillary visible, not gliding under the sub-orbital bone; small teeth on the jaws and vomer, without canine teeth. Two dorsal fins separated, the first one with 8-10 spines, the second with one spine and 11-14 soft rays; anal fin with three spines and 10-12 soft rays; base of the pelvic fins without scales; caudal fin moderately forked. Large caudal peduncle. Lateral line complete, not continuing on the caudal fin. Small scales, around 55-80 on the lateral line in the Mediterranean species. Colour generally silvery; one species with small black spots; lower fins sometimes yellowish when fish are alive.
Medium to large size fish (till one meter total length) of temperated and cold regions. The two species of the Mediterranean inhabit coastal and brakish waters and are occasionally found in rivers.
Two species occur in the Mediterranean Sea: D. labrax (L.) and D. punctatus (Bloch), the latter with only a marginal interest in artisanal fishery along the southern Mediterranean coasts. They can be identified on the following characters:

presence on the back and sides of dark, permanent spots in adult	D. punctatus
vomerine teeth extend over the vomer like an arrow point	D. punctatus
vomerine teeth only on the anterior part, like a V	D. labrax
diameter of the eye is smaller than the interorbital space	D. labrax
while larger	D. punctatus

 

Biology

Morphology
The seabass, Dicentrarchus labrax has a silvery elongated body, with two clearly differentiated dorsal fins and a rather high tail. The opercular bone has two flat spines and a range of spines are visible in the lower part of the preopercular bone, pointing in the direction of the mouth. The vomer presents teeth with a crescent shape. This species has cycloid scales in the interorbital region. The lateral line is visible as a dark line with 62-80 cycloid scales.
The first dorsal fin has 8-10 spiny rays, and the second dorsal fin 12-13 rays of which the first is spiny. The anal fin has 3 spiny rays and 10-12 soft rays.
The colour is dark grey on the back, passing to grey-silver on the sides, while it is white-silver on the abdomen. Specimens from the sea show a much clearer colour than fish from lagoons and estuarine environments. On the opercular bone there is a dark spot. The juveniles show a livery with little dark spots, mainly on the front or only on the head, which disappear with age. The maximum size is over 1 m with a weight of over 12 kg.
The seabass is a eurythermal and euryhaline species and can survive at temperatures between 2 and 32°C, with a limited territoriality related to their search for food and to reproduction. Outside the spawning period, the seabass can be found anywhere food is available. Maturation and spawning need more specific environmental conditions (temperature, photoperiod, salinity) which determine variation in spawning period.

Feeding

Seabass is a predator consuming small fish and a large variety of invertebrates. In spite of variations associated with differences in latitude, bass hunts at any time of the day. The feeding behaviour is related to size. Juveniles feed mainly on small Crustaceans (Amphipoda, Mysidacea, Isopoda) and small fish (about 1/4 of the diet), like Atherina and Gobius. In fish larger than 20 cm, shrimps and crabs begin to be common preys.

Geographic distribution

D. labrax is common in the Mediterranean Sea, the Black Sea and along the Eastern Atlantic coasts from Great Britain to Senegal. With a tolerance to salinity and temperature fluctuations greater than the gilthead seabream, this species is found in marine to slightly brackish environments such as coastal lagoons and estuarine areas. In particular during the first stages of its life cycle displays the same behaviour of gilthead seabream. Much less sensitive to low temperatures, some fish may overwinter in coastal lagoons instead of returning to the open sea.

Reproduction

In seabass sexes are separate: the female shows a deeper body with a longer pointed head and greater pre-dorsal and pre-anal lengths. Sure sex confirmation is however possible only during the spawning season by checking the presence of sperm by squeezing gently the males and by observing the protrusion of the anus and genital papilla in the females.

Fig. 10 - European seabass geographic
Sexual maturity takes place earlier in males and earlier in Southern populations. There is only one breeding distribution season per year, which takes place in winter in the Mediterranean population (December to March), and up to June in the Atlantic populations. Unlike gilthead seabream, female gonads complete their maturation at the same time and eggs are released all together in a short time, usually at night. For hatchery purposes, spent females have to be replaced by new breeders as soon as new batches of eggs are required.
After being released, the eggs acquire their characteristic spherical shape, with a size that varies according to latitude:

Place	diameter (mm)
Great Britain	1,2 - 1,5
Mediterranean	1,15 - 1,2

 

Fisheries

Seabass is fished both by artisanal and sport fishermen. The quantities caught in the Mediterranean are relatively small which linked to high appreciation for the species in the Mediterranean markets makes it a high value species. The gear used to catch seabass include beach and purse seines, trawl nets, trammel nets and longlines, as well as rod and line. It is regularly present in the Mediterranean markets but it is scarce in the eastern Mediterranean basin and it is rare in the Black Sea.

2.3- LIVE FOOD FOR MARINE FISH LARVAE

The timely and adequate supply of high quality live food is still essential in the rearing of the early larval stages of many marine organisms. As of today the use of some micro-organisms as first feeding of seabass or gilthead seabream larvae is still mandatory.
The rotifer Brachionus plicatilis is an excellent first food for larval stages of marine fish because of its small size, slow swimming speed, its habit of staying suspended in the water column, the possibility to rear it at high densities (a density of 800 - 1,000 rotifers/ml is now quite common in well run hatcheries), its high fertility rate, and the broad tolerance to salinity. Moreover, the rotifer can act as a vector to transfer specific nutritional factors and drugs to fish larvae.
While artemia is immediately (but not cheaply) available on the market as canned dry cysts (their resting eggs), the production of large quantities of rotifers (several billions per day in case of large hatcheries) still requires the production in parallel of marine microalgae on which rotifers feed.
The biology and life history of these organisms, microalgae, rotifers and artemia is briefly explained below, whereas the design of their production units and the description of the production methods are dealt with other sections of the manual.

Fig. 11 - Culture upscaling: small volumes

Microalgae

Microalgae, also called in this manual phytoplankton and algae, are unicellular eukaryotic planktonic algae belonging to several taxonomic groups. Below are quoted the most common microalgae cultured in in the context of marine finfish reproduction. In aquaculture phytoplankton is directly used to feed molluscs and crustacean larvae and indirectly to culture the zooplankton on which the first feeding of marine fish larvae is based.
The benefits of microalgae in the larval rearing of marine fish are not longer limited to their original role to feed rotifers, which could now be largely replaced by artificial diets and baker’s yeast Saccaromyces cerevisiae in their final steps of mass culture. It is now accepted that fish larvae benefit indirectly from the presence of selected phytoplankton species in their tanks during the first rearing days, where they works both as immunological stimulus and as a conditioner of water quality, limiting the development of bacteria and reducing N and P loads.
In past years many microalgae species have been tested in the Mediterranean hatcheries but only a few species are now routinely mass-produced. Their selection was driven by these criteria:

• a high nutritional value for both fish larvae and rotifers;
• absence of toxicity for their predators;
• suitable dimensions and digestibility, to be easily accepted by rotifers;
• a good capacity to adapt to standardised mass rearing conditions;
• a high rate of reproduction in artificial environments;
• reliability and affordability of their mass production.

The most commonly cultured algae in the Mediterranean region are the following:

Bacillariophyceae	Chaetoceros calcitrans
(Diatoms):	Skeletonema costatum
Haptophyceae	Isochrysis galbana
	Isochrysis sp. (Tahitian strain)
	Pavlova (Monochrysis) lutheri
Chrysophyceae	Tetraselmis (Platymonas) suecica
	Tetraselmis (Platymonas) chuii
	Tetraselmis (Platymonas) tetrathele
Chlorophyceae	Dunaliella tertiolecta
	Chlorella sp.
Eustigmatophyceae	Nannochloropsis gaditana
	Nannochloropsis oculata

Due to their high degree of variability, strains from different natural algal populations, although identified under the same species, may differ considerably in their nutritional characteristics and other biological aspects.
The live food production unit of the hatchery usually gets pure microalgae strains of the species selected from either other hatcheries or from specialised institutions such as CCAP (the Culture Collection of Algae and Protozoa of the Dunstaffnage Marine Laboratory, P.O. Box 3, Oban, Argyll, PA34 4AD, Scotland, UK).
Basically, microalgae can be cultures following three different methods: continuous, semi-continuous and batch culture schemes. The hatchery staff must always pay attention to keep algal density, or algal growing rate high enough to reduce chances for development of competitors or of contaminating organisms. Most algal productions in Mediterranean hatcheries centres are based on monospecific batch culture where algae from stock cultures are upscaled in successively larger containers until they are harvested. For commercial hatcheries, this method is considered more reliable than the semi-continuous or continuous culture methods due to the more limited risk of culture crashes, an easier standardisation of culture procedures and a reduced investment if compared with the other two methods.

Fig. 12 - Continuous culture: the bio-fence
Batch culture method:
As a matter of routine, axenic conditions are reserved for pure strains, back-up cultures and small culture volumes up to 20 l carboys, which are kept in air-conditioned, dedicated rooms. Subsequent upscaling to larger volumes takes place in plastic bags, tanks and ponds, placed both indoors and outdoors according to local climatic conditions. The design of a phytoplankton production unit and the standardised culture procedures are described in other sections of the manual.
In principle, culture conditions and the composition of the liquid medium differ according to algal species or even strains. The most commonly used algae, can be cultured well using the same standardised medium and procedures that are described in detail in the chapter on microalgae mass culture.
The most relevant parameters influencing algal growth are: temperature, nutrients, light, pH and turbulence. The standard culture conditions in Mediterranean hatcheries can be summarized as follows:

• temperature: between 18° and 24°C;
• salinity: between 20 and 35 ppt;
• light: from 1 000 to 2 000 lux for smaller volumes up to 10 000 lux for larger tanks, where light penetration is limited;
• pH: normally increased up to 9 by the photosynthetic activity of the algae themselves, it is kept at a lower value (8-8.5) by the addition of acid or carbonic gas (CO₂).

The culture medium is usually prepared with micro-filtered and UV-treated seawater enriched with nitrogen and phosphorous as major elements. Addition of oligoelements includes salts of sodium, calcium, potassium, magnesium and various metals (Zn, Fe, Mn, Cu, Mo; plus Si for diatoms). Vitamins, such as B₁₂, biotin and thiamine, are also added.
The bio-chemical composition of microalgae varies also according to culture conditions. Their content of aminoacids, vitamins and essential fatty acids (PUFA and HUFA) is affected by environmental factors such as water temperature, salinity and light intensity.
Mass production of microalgae requires plenty of space, skilled personnel and time. Cheaper and easier alternatives have been investigated for years. At present, rotifers are mainly fed on artificial diets and few times with baker’s yeast, thus keeping the need for algae to a minimum. Phytoplankton, however, remains a key factor for the batch scale cultures of rotifers in small volumes and, most important, for the larval rearing of gilthead seabream and other species such as molluscs and shrimps.

Biology of rotifers

Taxonomy

PHYLUM: Nemathelminthes or Aschelminthes
CLASS: Rotatoria
ORDER: Monogononta
FAMILY: Brachionidae

Morphology

Rotifers are among the smallest filter-feeding metazoans. Composed of a fixed number of about 1,000 cells, their growth is obtained by plasma increase, not by cellular division. They filter small particles by means of a ciliated annular organ, the corona, located in the anterior part of the body. The corona is also used for its whirling locomotion, hence the name of the Class Rotatoria. Whereas many species spend their life span attached to a substrate by means of their retractile foot, Brachionus plicatilis that is the main species cultured for finfish larval rearing world-wide, is a planktonic, unattached rotifer.
The epidermis contains a densely packed layer of keratin-like proteins called lorica. The rotifer’s body is differentiated into three distinct parts: head, trunk and foot.

• The head carries the ring organ or corona, which is easily identifiable by its ring of cilia. The retractile corona assures locomotion and a whirling water movement which facilitates the ingestion of small food particles (mainly algae and detritus).
• The trunk contains the digestive tract, consisting in a mastax that grinds the ingested particles, the oesophagus, the stomach with gastric glands and the intestine. The excretory system consists of paired protonephridia with terminal cells (cyrtocytes), the duct and the bladder. The protonephridia expel liquid excreta and play an important role in osmotic regulation. The genital organ is unpaired (Monogononta) or paired in the Seisonidea and Bdelloidea. The joint external opening for bladder and oviduct is called the cloaca.
• The foot is a ring-type retractile structure without segmentation, ending in one or four toes bearing pedal glands that secrete an adhesive substance in crawling and sessile rotifers.

Two different morphotypes of B. plicatilis exist: the small (S) type and the large (L) type. They differ in their lorica length: 130 to 340 µm (average 239 µm) for the L-type and 100 to 210 µm (average 160 µm) for the S-type. There are also differences in weight, shape of occipital spines and optimal growth temperatures (L-type rotifers have a wider temperature range while S-type rotifers have a higher temperature resistance). S-type rotifers are suitable as first food for fish larvae with a mouth opening smaller than 100 µm at first feeding, such as gilthead seabream, groupers, and rabbitfish.

Fig. 13 - Brachionus plicatilis: female and male (modified from Koste, 1980)
Two different morphotypes of B. plicatilis exist: the small (S) type and the large (L) type. They differ in their lorica length: 130 to 340 µm (average 239 µm) for the L-type and 100 to 210 µm (average 160 µm) for the S-type. There are also differences in weight, shape of occipital spines and optimal growth temperatures (L-type rotifers have a wider temperature range while S-type rotifers have a higher temperature resistance). S-type rotifers are suitable as first food for fish larvae with a mouth opening smaller than 100 µm at first feeding, such as gilthead seabream, groupers, and rabbitfish.

Life history

The life span of rotifers is measured in days and depends on culture temperature, but in a controlled environment and at 25°C, it has been estimated to range around 7 days. At this temperature, larvae become adults after 0.5 to 1.5 days and then females start to lay eggs approximately every four hours. It is believed that a female can produce ten generations of offspring. The reproductive activity of Brachionus is also influenced by temperature as illustrated below.
Effect of temperature on reproductive activity of Brachionus plicatilis (after Rutner-Kolisko 1972)

Temperature (°C)	15	20	25
Embryonic development (days)	1.3	1.0	0.6
First spawning of females (days)	3.0	1.9	1.3
Interval between spawning (hours)	7.0	5.3	4.0
Life span (days)	15	10	7
Number of eggs pre female during its lifetime	23	23	20

The reproduction of B. plicatilis can be either sexual (called mictic reproduction) or asexual (amictic or parthenogenetic reproduction). Only the latter is adopted for rotifer mass culture due to its faster rate and also due to the absence of males, which are useless as fish feed not having a functional digestive tract.
In the amictic reproduction the offspring are clones genetically identical to their mothers, i.e. all newly born rotifers are diploid females. Such multiplication can go on for months in a population kept in proper rearing conditions. Depending on environmental conditions, each female may produce about 20 amictic eggs.
Males are only produced after a sudden change in the environment, when females produces haploid (n chromosomes) eggs. Males and females breed, and the result is mictic resting eggs, analogous to Artemia cysts, which will hatch amictic females again.

Fig. 14 - Parthenogenetical and sexual cycle of Brachionus plicatilis (modified from Hoff and Snell, 1987)

Food

Rotifers are filter feeders, accepting small particles up to 30 µm in size including bacteria, algae, yeast and protozoa.
Saccaromyces cerevisiae, the common baker’s yeast, is a common staple in the rotifer mass production process. Since in itself has no nutritional value for rotifers, it is believed that the bacteria associated with the yeast represent the true food. To improve rotifer culture and upgrade the nutritional value for fish larvae (in case of yeast-fed rotifers), during mass culture and before their harvest they are fed with special feeds and integrators.

Mass culture parameters and conditions

Rotifer mass culture is critical for larval rearing of gilthead seabream, which strictly depends for the first feeding from live and small preys that should be available in large quantities. What follows in the column marked as preferable range, gives a reasonable example of mass culture conditions to be maintained in order to develop rotifer cultures properly.

Parameter	Acceptable range	Preferable range
Temperature (°C)	20-30	25-27
Salinity (ppt)	1-60	18-25
Dissolved oxygen (ppm)	>4	5-7
NH3/NH4+ (mg/l)	6-10	-
pH	5-10	7.5-8.5
Light (lux)	-	2000

Aeration is a critical element in rotifer culture with yeast and/or artificial diets. A proper balance must be maintained between:

• adequate dissolved oxygen level, i.e., at least 4 ppm;
• sufficient turbulence to keep rotifers and food in suspension;
• a culture medium without excessive turbulence, which causes stress and re-suspends bottom sediments (flocks).

Biology of the brine shrimp, Artemia.

Taxonomy

Taxonomy

PHYLUM: CLASS: ORDER: FAMILY: GENUS:

Arthropoda Crustacea Anostraca Artemidae Artemia

As the mouth size of marine fish larvae increases with age, rotifers are gradually replaced by the larger freshly hatched nauplii of the brine shrimp Artemia salina. Due to the larger size of its mouth at first feeding, seabass fry can accept artemia as a first prey, thus making rotifer supply not compulsory. Later on in the rearing process, and before weaning rations are given, larger Artemia metanauplii stages replace nauplii for larger fry.
The brine shrimp, a small crustacean living in salt ponds, represents an excellent prey for old or large fry due to its nutritional value and mobility in water, which makes it a perfect prey for the young fish. Moreover its easy and short production cycle and the widespread commercial availability of selected batches are added bonuses.
Artemia salina was considered for a long time as the only species belonging to the genus. But because of the substantial differences observed among the various Artemia strains found around the world, a global revision of the taxonomy of the genus is being considered even if it still remains unresolved. The differences between strains concern genetics, reproductive behaviour and physiology.
At present, a solution commonly adopted to simplify the taxonomy of the genus, is to define the species on the basis of its place of origin (Barigozzi, 1980).

Morphology and natural history

Brine shrimp most surprising characteristic is its ability to live in extremely hostile environments, such as salt lakes and man-made brine ponds throughout the world. They show a remarkable capacity to stand severe environmental conditions such as a water salinity values over 200 ppt, thus avoiding predators against which they are defenceless. Its body permeability to salts is very low when compared with other micro-crustacea. Artemia absorbs water from the medium and eliminates salt by defecation. In this way it can keep the correct osmotic blood values in spite of the hyperhaline environment in which it lives.
From an aquacultural point of view, the most important biological characteristic of Artemia is that it produces resistant cysts, containing embryos in diapause (“dormant stage”). Cysts are formed when environmental conditions become intolerable; their function is to protect the embryos against dehydration and against excessively high temperature and salinity values. Cysts preserve the inert embryonic life stage (gastrula stage) as long as they are kept dry or under anoxic conditions.

Fig. 15 - Artemia salina adults, on the right, head of male
After hatching, the Artemia larva goes through about 15 molts with an initial size ranging from 400 to 500 µm. The first two larval stages, nauplii instar I, rich in yolk, and instar II, which has to be enriched with special integrators, are the most commonly used in fish fry feeding.
Under optimal conditions brine shrimp reach the 10-mm long adult stage in eight days and can live for several months, reproducing at rate of up to 300 nauplii or cysts every four days. The adults are characterized by an elongated body with two stalked complex eyes, a linear digestive tract, sensorial antennulae and 11 pairs of functional thoracopods. Males are easily recognizable for their pair of large muscular claspers (the 2nd pair of antennae) in the head region. Females bear the brood pouch or uterus behind the 11th pair of thoracopods.
Parthenogenetic and bisexual Artemia strains exist, where ovoviviparous and oviparous reproduction alternates. Cysts’ hatching gives ovoviviparous offspring; eggs are retained inside the uterus until embryonic development is fully completed (4-5 days) and free swimming nauplii are then released. Change from ovoviviparous to oviparous reproduction seems to be induced by under nourishment or even by an inadequate food quality, rather than by other abiotic factors.
Salt lakes and brine ponds with Artemia populations are found all over the world. At certain times of the year, large quantities of minuscule brown particles (200 to 300 m in diameter) float at the water surface and are brought ashore by wind and waves. This apparently inert brown powder is actually made of dry cysts of brine shrimp, which remain in diapause as long as they are kept dry.

Fig. 16 - Dehydrated artemia cysts

Food

As a non-selective filter feeding, brine shrimp feed on particulate matter of biological origin, bacteria and algae of suitable size. While freshly hatched nauplii of selected batches represent an appropriate food for fish fry; older larval stages have to be given special feeds to upgrade their nutritional value for fish larvae.

Rearing methods

Artemia rearing for aquaculture purposes follows two ways, naupliar production and biomass production. Nauplii and metanauplii are easily produced from hatching cysts in dedicated units. Cyst incubation takes approximately 24 hours at 30°C, in salty water under strong light and with aeration. Fully enriched metanauplii would require an additional 12-24 hours of rearing.

Fig. 17 - Nauplius and metanauplius
Biomass production takes a few days and adult artemia are harvested. These can be either immediately fed to fry, or be frozen to be used later. In recent years, however, this process has been progressively replaced with new and reliable artificial diets.

Artemia use in aquaculture

The use of Artemia nauplii as live food for the rearing of fish and crustacean larval stages, has been one of the most important steps in the development of marine aquaculture. With the exclusion of the rotifer Brachionus plicatilis, Artemia has almost entirely replaced the mass rearing of other live-food zooplankters that were tested earlier on by many researchers and producers but which were discarded for reasons of technical complexity or because they culture was economically unfeasible.

Fig. 18 - Harvesting of floating cysts with special boat
The production of resistant cysts has been the real advantage offered by Artemia. Cysts can be stored for years and live nauplii can be obtained after 24 to 36 h incubation. A reserve of “stored live food” has been the important guarantee needed in hatcheries, where it has simplified the planning of production activities.
The resistance to anoxic conditions of Artemia cysts made possible their storage in sealed containers, simplifying marketing and transport. Moreover, Artemia nauplii were found be an excellent food for fish and crustacean larval stages, even when used alone.
Even if Artemia cysts still remain an expensive product and if their market availability shows fluctuations, important progress has been made during the last years:

• additional sources of Artemia cysts have been found in all the continents and each strain has been classified with regard to quality (in nutritional terms) and hatching efficiency;
• hatching procedures have been standardized and new techniques have been introduced, such as cyst decapsulation, with a consequent improvement of cysts hatching efficiency and of the quality of newly hatched nauplii.

Nutritional value of Artemia

For the larval rearing of marine fish and crustacean species, Artemia nauplii and rotifers are the most commonly used live food preys. The advantages of Artemia are multiple: off the shelf and regularly available dormant cysts, they are easy to produce, are visible as prey and are highly palatable to the larvae
A major constraint, however, in the use of Artemia as a food organism for marine fish larvae is its variable nutritional quality. This problem can be circumvented with the use of enrichment techniques adding essential fatty acids (HUFA’s). As Artemia is a non-selective filter feeder, it filters all particles of suitable size from its surrounding environment and special feeds can be given to improve its nutritional value. Enriched nauplii produce better performances in fish larvae in terms of growth and survival. Besides enrichment with HUFA’s, other nutrients such as vitamins, as well as prophylactic and therapeutic drugs, can also be passed to the fish larvae via the Artemia nauplii acting as carriers.

Fig. 19 - Harvesting of cysts by hand
According to the desired HUFA’s levels in Artemia, different enrichment products and enrichment periods can be used. Maximum levels of approximately 60 mg/g DW are obtained with a 24-h enrichment with Super Selco. In all these cases, the final DHA/EPA ratio in Artemia is less than 1. For more sensitive species that need a higher DHA/EPA ratio, enrichment with DHA Selco is advisable; this results in a total HUFA’s enrichment level similar to that obtained with Selco, though the DHA/EPA ratio exceeds 1.5.