Larval stages of unidentified species, apparently belonging to Hydrachnellae (water mites), heavily infected the gills of Synodontis membranaceus in the Volta lake (in 1963, Paperna, unpublished).
Fig. 7F (p. 175)
Larval stages of the genus Leiperia were reported from tissues of fish of the genera Alestes, Bathybates, Chrysichthys, Labeo and Mastacembalus in the Congo basin (1961), and unidentified in Sarotherodon galilaeus, S. heudelotii and Tilapia zillii in a dam at Nungua, South Ghana (Prah, 1969). Larval stages in fish were also reported from tropical America (Vargas, 1975).
Recent studies regard Pentastomida as affiliated to arthropods (Riley, 1989). Crocodiles are the definitive hosts of the larval stages found in fish (Fain, 1961; Vargas, 1975). Pentastomids settle in the lungs of their definitive hosts and eggs are released via the respiratory passages. The larva, which hatches when ingested by a suitable host, enters the tissues. It has two double hooked appendages, a penetration spine at the anterior end of the cephalothorax and in some (in the neotropic Subtriquetra), a long strongly hooked tail.
Experimental infections (with Subtriquetra) of small fish (30–50 mm) cause fish death even before larval development is completed (around 30–40 days after infection). Larger fish (Aequidens sp., 70 mm long) survived infections with seven, 2.5 mm long larvae, which were already infective (Riley, 1989).
Plate 29 p (p. 201) and Fig. 7 G & H (p. 175).
Species affected and geographical range
Larval stages of freshwater bivalve molluscs of the superfamilies Unionoidea and
Muteloidea are parasitic on fins and gills of fish. Mutelid bivalves are endemic to Africa,
while unionids are distributed world-wide. Unionid larvae were reported from L. Victoria,
L. George in young cichlids and from L. Kinneret, in Israel, also in juvenile or small
cyprinids (Paperna, 1964a & unpublished; Fryer, 1970). Mutelid larvae were found on
Barbus altianalis and juvenile cichlids only in L. Victoria (Fryer, 1970; Paperna,
unpublished).
Description and diagnosis
The larvae of unionids, known as glochidia, have calcareous bivalved shells, often with
little hooks on their inner edge. They clamp on to the gills and fins. The larvae of mutelids
(haustorium larvae) differ distinctly from glochidia by having a non-calcareous pellicle
and a very long tentacle. The larvae attach themselves to the fin and the gills, shed off
the tentacle, and envelop themselves completely in the non-calcareous pellicle. Then
they proceed to thrust two prolongations into the host tissue which serve as an anchor
and an organ for food absorption (Fryer, 1970).
Pathology and epizootiology
Intense epithelial hyperplasia develops in the gills around the attached glochidium, such
proliferative reaction apparently also occurs in skin infections, but on the skin it is less
conspicuous. Mortalities from heavy glochidial infections of the gills have been reported
in farmed salmonids (Davis, 1953). Glochidial infections occurring in farmed fish in
tropical regions (Southeast Asia), however, thus far, have not caused concern (Kabata,
1985). There are no records of infection in farmed fish in Africa and the Near East. Natural
infections occur predominantly in small fish residing in shallow, inshore waters (young
fish and littoral species). In Lake Victoria, infection was more prevalent in Tilapia and
Oreochromis fry than in haplochromid cichlids. Fry 10–19 mm long were already infected
and the highest incidence occurred in 20–50 mm long O. variabilis and T. zillii (50–75%
in various samples). The number of glochidia infecting single fish, however, never
exceeded nine. Infection was less prevalent in the young cichlids of L. George (Paperna,
unpublished). In L. Kinneret, both cyprinids and cichlids hosted glochidia, with
prevalence among the different species ranging from 1 to 33% (in T. zillii) (Paperna,
1964a). Host specificity of glochidia remains unresolved, since in all the habitats
discussed several unionid species are present, while specific diagnosis of glochidia from
these fish has never been attempted. Natural infections by Mutelid larvae are uncommon
and unlikely therefore to be of any epizootiological significance.
Control
Has never been attempted.
REFERENCES
Davis, H.S., 1953. Culture and Diseases of Game Fishes. Berkeley, Ca. Univ. Calif. Press.
Fain, A., 1961. Les pentastomides de l'Afrique Centrale. Ann. Mus. Roy. Afr. Cent., Ser. 8,92: 1–115.
Fryer, G., 1970. Biological aspects of parasitism of freshwater fishes by crustaceans and molluscs. In: Taylor, E.R. & Muller, R. (ed.) Aspects of Fish Parasitology. Symp. Brit. Soc. Parasitol., 8:103–118.
Kabata, Z., 1985. Parasites and Diseases of Fish Cultured in the Tropics. Taylor & Francis, London & Philadelphia.
Paperna, I., 1964. The metazoan parasite fauna of Israel inland water fishes. Bamidgeh (Bull. Fish Cult. Israel), 16: 3–66.
Prah, S.K., 1969. Observation on parasitic infection in freshwater fish in Ghana. In: Obeng, L.E. ed. Man-made Lakes, the Accra Symposium. Accra University Press for Ghana Academy of Sciences. pp. 261–268.
Riley, J., 1989. The biology of pentastomids. Adv. Parasitol., 25: 46–128.
Vargas, M.V., 1975. Descricion del huevecillo, larva y nimfa de Subtriquetra subtriquetra Sambon, 1922 (Pentastomida), y algunas observaciones sobre su ciclo de vida. Revista Biol. Trop. 23: 67–75.
