Parasitic Crustaceans


          Crustaceans constitute the second-largest class in the Phylum Arthropoda, and it includes the following subclasses:

·        Branchiopoda:  fairy shrimp

·        Ostracoda:  seed shrimp

·        Copepoda:  (e.g. Cyclops)

·        Branchiura:  fish lice, e.g. Argulus

·        Cirrepedia:  barnacles

·        Malacostraca:  wood lice, pill bugs, sand hoppers, crabs, lobster and crayfish.


Parasitic copepods:

Lernaea spp., common name anchor worms, is parasitic among freshwater fish.  The larvae are free-living, and resemble other free-living copepods.  However, the 3rd copepodid stage is parasitic, and will seek freshwater fish.  Males and females mate, the males die, and the females undergo a remarkable metamorphosis to develop a prominent head which they use to burrow into the flesh of the fish.  The penetration site frequently becomes the site for secondary bacterial and/or fungal infections.


The posterior portion of the parasites protrudes from the skin of the fish.  The adult female can survive up to 30 days, during which it produces pairs of egg sacs which can contain 200-250 eggs each.


Life cycle of Lernaea spp.


Adult female removed from host (Anchor to the right, egg sacs to the left)


Adult female in situ



Lernaeocera spp. is a parasitic copepod found in the gills of marine fish.  The life cycle is similar to that of Lernaea spp. mentioned above.  The anchor part is considerably stouter, and the egg masses are larger and more convoluted.


Comparison of adult female Lernaeocera and Lernaea


Female Lernaeocera spp. on gill



Ergasilus is a copepod ectoparasite of freshwater fishes.  It has elongated maxilla which fuse and encircle a gill filament.



Ergasilus sp. is a member of a small group of parasitic crustaceans that prey upon freshwater and marine fishes. I may be found on the skin, fins, and gills of fishes, but is most frequently found on the gills. They can cause significant morbidity and mortality when heavily infesting fish. They have also been implicated as vehicles for other fish diseases.

Ergasilus has a direct life cycle using only the fish as a host. Ergasilus can spend prolonged periods swimming free, and mating takes place while the male and female are swimming. The male then dies. Egg incubation occurs while the egg clusters are attached to the female. 

The offspring hatch and are broadcast into the water. The offspring undergo four molts before becoming adults.

There are several species of Ergasilids and none is too host specific. Ergasilids infect eels, gars, herrings, killifishes, paddlefishes, perches, pirate perch, smelts, sticklebacks, sunfishes, temperate basses and troutperch.


Ergasilus spp. adult female


Ergasilus spp. egg masses



Parasitic Branchiurids   

     Unlike the parasitic copepods mentioned above, Argulus spp. (e.g. A. japonicum), AKA the fish louse, are motile ectoparasites which can detach from one host and swim to another.  Each individual possesses two adhesive disks with which it can adhere to the skin.  There is a sharp stylus which it uses to pierce the skin and extract a blood meal.


Argulus spp. is an ectoparasite of freshwater fishes.


Argulus sp. adult


SEM of Argulus, showing adhesive disks and legs


Argulus sp. on fish



Parasitic Cirripedia        

Sacculina spp. is a most unusual barnacle.  Although the larvae resemble free-living species, the female will attach onto crabs and develop in the gonads of the host crab, much like a tumor.  The hosts gonads are destroyed in the process of the parasite developing, so that infection with Sacculina induces a phenomenon of parasitic castration (See for other examples).



The genus Sacculina is one of the Rhizocephalan barnacles that parasitizes crabs. Similar to other barnacles, Sacculina have a planktonic larval stage, the nauplius, and a settling stage, the cyprids. The adults, however, unlike other typical barnacles, are internal parasites (called the "interna"), cuticular tumors which grow inside their crustacean hosts. These tumors can develop a system of branching roots that ramify throughout their hosts’ bodies and absorb their nutrients. The life cycle of Sacculina, therefore, comprises two stages: the endo- and ecto-parasitic stage.

Sacculina larvae are dioecious. The male larvae are often smaller than those of the females. The life cycle begins with the female cyprid invading the crabs and then developing into a parasite with an internal root system (interna). Once the interna matures, it will develop a reproductive body outside the crabs through the abdominal part called the virgin externa. Male cyprids will then enter the virgin externa, which give rise to a fertilized externa with the eggs brooding inside it. Larvae will then be released via the externa once the eggs became mature.

Investigations of Sacculina infestations are often restricted to temperate species and little is reported from tropical intertidal regions. In Hong Kong, so far there are no detailed investigations of Sacculina infestations, the only records of Sacculina (parasitizing crabs) being those of Morton and Morton (1983) and Morton (1994). In January 2001 we discovered that the intertidal crab Leptodius exaratus was occasionally found bearing externae of Sacculina at Lobster Bay (a sheltered boulder shore) in Cape d’Aguilar. Investigation of the externa transverse sections revealed that this Sacculina species is Sacculina sinensis, which was once found by Baushma in Hong Kong in 1933. The size of the externa of S. sinensis is small with a mean width of 5 mm. By culturing the larvae of S. sinensis, we found that there are 4 naupliar stages and 1 cyprid stage. All the larvae are non-feeding and the limbs (antennules, mandibles and antenna) are structurally simple. Through extensive searches of crabs in a variety of intertidal shores around Hong Kong, only L. exaratus on boulder shores in Cape d’Aguilar and Lan Lai Wan in Tai Tam (Hong Kong Island) were found to have Sacculina infestation.

The infection rate and occurrence of Sacculina spp. are lower in Hong Kong compared to other South-Asian regions. In Taiwan, the infection rate of Sacculina spp. on crabs often reached 30 – 40 % and hosts included the rocky shore crabs Grapsus albolineatus, G. intermedius and G. longitarsis (Liu and Lutzen, 2000). In Japan, the crab Gaetice spp. is also found to be infected by Sacculina. In Hong Kong, Sacculina infestation has only been recorded for the crab Leptodius exaratus (the record of Sacculina confragosa parasitizing the crab Epixanthus frontalis in Morton 1988 is suspected to be a misidentification) and the infection rate is only around 10 % at Lobster Bay and in another boulder shore near Lap Sap Wan. In addition, so far neither Grapsus spp. nor Gaetice depressus in Hong Kong has been observed to have signs of Sacculina infestations. We are now preparing a manuscript describing the adult and larval morphology of S. sinensis and comparing its occurrence with other Asian regions. The differences of infection rate and also the crab species infected between Hong Kong and other Asian region is a potential area for further research.



Feminized male blue crab showing the spherical egg mass of a Sacculina protruding from the abdomen.


Process by which the Sacculina adult differentiates into a tissue mass inside the tissues of the host.