Sea lice characteristics, habitat, reproduction, nutrition

Abraham McLaughlin
Sea lice characteristics, habitat, reproduction, nutrition

The sea ​​lice (Anilocra physodes) are parasitic crustaceans of the Cymothoidae family. These isopods are characterized by parasitizing a wide variety of fish species, being able to be located on the external surfaces between the scales, in the oral cavity and in the gill cavities..

Despite being parasitic organisms, they have undergone few notable body modifications. The only truly distinctive body modification focuses on the ends of most of the legs, which were transformed into formidable grappling hooks for attachment..

Anilocra physodes By Parent Géry [CC0]

The hooks of the first pair of legs are directed in such a way that they prevent these crustaceans from shedding despite the movements of the fish, the speed of their movement and their erratic movements.

Anilocra physodes generally it fixes in the cephalic region of the host fish as a rider. However, they can also be attached to the dorsolateral region of the body and less frequently to the gill cavities or in the mouth of the fish..

Females remain sessile in a single host while males and juveniles can change hosts frequently. Once a parasitized fish is removed from the water, the sea lice shed the host very quickly. The same happens when the host dies.

The youth stadiums of A. physodes They are very agile swimmers, which allows them to make very daring movements to achieve fixation on the cephalic region of a host fish.

Anilocra physodes it is a species that can change sex. These fish are sequential hermaphrodites, that is, the male sex organs mature first and then the female sex organs develop. Both structures are operational together.

Article index

  • 1 General characteristics
    • 1.1 Sex differentiation
    • 1.2 Coloring
  • 2 Habitat and distribution
  • 3 Taxonomy
  • 4 Playback
  • 5 Nutrition
    • 5.1 Natural predators
  • 6 References

General characteristics

Sea lice are crustaceans that can reach a size of up to four centimeters. Like other isopods, its body is flattened dorso-ventrally and consists of a series of plates or tergites superimposed to form a kind of armor. The back does not present lateral projections as in other groups.

The head is fused with the first thoracic segment. Due to their parasitic habits, some sensory organs such as antennae have been reduced, being rounded apically and not very evident in terms of their length..

The anterior end of the cephalic region is truncated. The endopodites of the uropods slightly exceed the distal region of the pleotelson. The uropodial branches are strongly flattened so the distal fan is closed.

Additionally, the mouthparts have undergone functional modifications to bite and remain fixed on the host. In this sense, the seven pairs of legs they have have also been modified to reinforce the grip on the fish..

The legs are curved and have well developed terminal claws. Sea lice lack functional ambulatory legs. In the following video you can see this species:

Sex differentiation

Males are generally smaller than females, of which they are further distinguished by the presence of copulatory structures in the eighth segment of the pereion. The body length of the male is usually between 2 and 3 cm.

On the other hand, females are characterized by having a kind of ovigerous sac or pouch in which the eggs are stored until they hatch. The females do not leave the eggs free directly in the sea, but carry them in a kind of brood bag until their development.

Ventral view of a sea louse (Anilocra physodes) By Marco Vinci [CC BY-SA 3.0 (]


Females have an asymmetric coloration while individuals that frequently change host (males and juveniles) have a uniform coloration. The coloration of sea lice depends on the position and coloration of the bottom (fish scales) where they fixate..

The head region of the sea lice is usually darker than the rest of the body which tends to be light in color and even translucent. This change in coloration is mainly due to the arrangement and morphology of the chromatophores and has camouflage implications on the skin of the host..

This particular coloration shows up as an optical flattening that has great adaptive value against predators..

Habitat and distribution

This species of crustacean is distributed in the northeastern region of the Atlantic Ocean, in the Mediterranean Sea and the Adriatic Sea. It has been reported in countries such as Spain, Turkey, France, Senegal, North Africa (Mediterranean) and the United Kingdom. In addition, parasitized fish have been observed in the archipelago of the Canary Islands.

Additionally, several studies of ectoparasites in fish report the presence of Anilocra physodes in the Aegean Sea and the Black Sea and intermediate areas such as the Marmara Sea. The habitat of these small crustaceans is varied and closely related to the habitats of parasitized fish species in temperate waters..

This crustacean has been recorded in fish with pelagic habits, demersal (fish that live near the bottom of brackish seas and lakes), benthopelagic, neritic, species associated with reefs and oceanic fish

The larvae remain close to the surface where they actively wait to attach to a host. Apparently they prefer to fixate on fish whose coloration is shaded in such a way as to facilitate the parasite's camouflage process. These fish are generally carnivorous and live near the bottom of the sea..


The gender Anilocra It is made up of approximately 51 species throughout the world. As within the family Cymothoidae, comprehensive morphological and molecular assessments are necessary to understand well the relationships within the genus and within this family of crustaceans..

The morphology of the mouthparts and the selection of hosts are characteristics of interest that can contribute to the relationships between the species of Anilocra.

Various groups of species have been established based on shared characteristics, such as body shape, legs, and anténules. However, there is no phylogenetic evidence to validate them..


The females of Anilocra physodes are permanently sessile and always attach to a host species of fish.

On the other hand, males are free-living, so reproduction depends on the encounter of a swimming male with a female attached to a fish. Once the reproductive event occurs, the eggs hatch directly into a speckled larva..

Once the male fertilizes the female through two gonopores present at the base of the pereiopods of the sixth segment of the pereion, the eggs are transferred to large plate-shaped lamellae or oostegites. These lamellae project onto the bases of the anterior pereiopods, covering almost the entire ventral surface of the female..

This species of pouch protects the eggs until they develop into “speckled” larvae that are then released into the surrounding water. This development period can last for about three weeks before hatching..

These larvae have characteristics that are very similar to those of the adults, however, they lack the last pair of pereiopods and are smaller in size..

The larvae of A. physodes they are photopositive, so they move in the direction of the areas with the greatest availability of light.


The sea lice feed exclusively on the host's blood. Once attached to the fish A. physodes sucks blood thanks to its modified mouthparts to pierce. This species, due to its habits, may be involved in the transmission of some protozoa to the host..

The parasitism of this species extends to a great diversity of hosts including more than 57 species of bony fish Actinopterygii and cartilaginous fish Elasmobranchii.

The most commonly parasitized families are Sparidae, Carangidae, Mugilidae, Centrachantidae, Sciaenidae, Mullidae, Scorpaenidae and 25 other families to a lesser extent.

Fish parasitized by Anilocra physodes By Parent Géry [CC0]

Several species of fish are parasitized by this small crustacean. Among these are listed: Spicara smaris, S. maena, Scomber japonicum, Sparus auratus, Dicentrachus labrax, Boops boops, Diplodus annularis, D. vulgaris, D. sargus, Pagellus erythrinus, Spondyliosoma cantharus Y Oblado melanura.

All these observations come from the Aegean Sea and the Black Sea. On the other hand, this parasitic crustacean has also been recorded perching in some species of cephalopods in the western Mediterranean..

Parasitized fish generally carry a single individual of these crustaceans. In several cases the presence of a fixed male and female on the lateral-caudal surface is reported, probably reproducing.

Natural predators

These crustaceans are also part of the diet of several species of fish. For example, Hoplostethus mediterraneus a benthopelagic fish that feeds primarily on crustaceans such as Meganyctiphanes norvegica Y A. physodes.

Several deep-sea species of the Aegean Sea also frequently consume these small crustaceans..

Other species of bony fish frequently catch specimens of A. physodes when they are free and in search of a host. The larvae are frequent prey of fish species that feed on the surface of the water.

Some species of cleaner fish or specialized in feeding on ectoparasites are capable of removing these isopods from other fish. Healthy and highly mobile fish that are parasitized can get rid of the parasite by rubbing the affected surface against the bottom or some coral to later ingest it..


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