Evolution and systematics

Pentastomida once was classified as a minor phylum, a fact reflected in most modern textbooks of parasitology. The classification is being changed, however. The evolutionary history of tongue worms is unique among parasites. The fossil record apparently extends to the late Cambrian period (500 million years ago [mya]), exceeding that of the next oldest parasites, certain copepods, by some 370 million years. Tiny fossil tongue worms have been etched from ancient fine-grained, deep-water limestone when the rock has been dissolved with dilute acid. In most respects these 0.04 to 0.08 in long (1-2 mm) larvae are indistinguishable from modern tongue worms. Therein lies a conundrum, because extant adult tongue worms are parasitic in terrestrial vertebrates, which do not appear in the fossil record until the late Devonian, approximately 350 mya. Nonetheless, fossil agnathan fishes are known from the Upper Cambrian, and it is noteworthy that all of the fossil tongue worms discovered so far have been collected from limestone containing diverse conodont fauna. Conodonts are primitive fish-like chordates. Although a marine ancestry for tongue worms seems assured, the exact nature of the ancestral host may never be known. In addition to having anterior pairs of claws, some of these ancient tongue worms have two pairs of vestigial nonsegmented trunk limbs. These structures are lacking in other fossil specimens and in all modern forms. A compounding problem is that the larvae of modern tongue worms are highly modified for tissue migration. Clawed limbs, together with other limb-like outgrowths evident in early larval development within the egg, have so far proved impossible to homologize with euarthropod head and trunk appendages.

A relation between tongue worms and branchiuran crustaceans (class Maxillopoda, subclass Branchiura) is supported by results of studies of sperm of other species and by comparison of ribosomal 18S recombinant RNA sequences in the two groups. In numerous extant parasitic maxillopods representative of several subclasses, the degree of cephalization together with the development of the trunk and anchoring devices is highly variable, to the extent that many adults bear no resemblance to their free-living counterparts. Always, however, the first larval instar, the nauplius, is easily recognized. The highly modified first instar of tongue worms, the so-called primary larva, is very different from its putative naupliar forebear because it has evolved to penetrate and traverse tissues in (mostly) terrestrial hosts. These larvae hatch with only two limb-bearing head somites and three trunk somites. Subsequent growth of the trunk is by a form of pseudometamerism, without the addition of further somites. The development of some copepods, relatives of branchiurans, also fits this pattern.

The present, still unresolved debate hinges heavily on the relative merits of the fossil evidence versus that of ribosomal RNA and sperm morphology. In this entry, tongue worms are considered a class of crustaceans. The class Pentastomida contains eight families and approximately 110 species assorted between two orders—the primitive Cephalobaenida and the advanced Porocephalida.

Cephalobaenida

• Cephalobaenidae. Three genera. Cephalobaena, one species found in snakes; Raillietiella, more than 35 species found in amphibians, snakes, lizards, amphisbaenans, and birds; Rileyiella, one species found in mammals.

• Reighardiidae. One genus. Reighardia, two species found in marine birds.

Porocephalida

• Sebekidae. Seven genera. Alofia, five species; Leiperia, two species; Selfia, one species; Agema, one species, all found in crocodiles; Sebekia, 12 species, found in

Part of the spleen of an infected mouse showing at least four encysted nymphs of Porocephalus crotali. These nymphs are infective to the snake definitive host, and each one is enclosed in the cuticle of the previous instar, which constrains the nymph to a C-shape. Photo by John Riley. Reproduced by permission.)

Part of the spleen of an infected mouse showing at least four encysted nymphs of Porocephalus crotali. These nymphs are infective to the snake definitive host, and each one is enclosed in the cuticle of the previous instar, which constrains the nymph to a C-shape. Photo by John Riley. Reproduced by permission.)

crocodiles and chelonians (one species); Pelonia, one species; Diesingia, one species, found in chelonians.

Subtriquetridae. One genus. Subtriquetra, three species, found in crocodilians.

Sambonidae. Four genera. Sambonia, four species; Elenia, two species, found in monitor lizards; Wad-dycephalus, 10 species; Parasambonia, two species, found in snakes.

Porocephalidae. Two genera. Porocephalus, eight species; Kiricephalus, five species, found in snakes.

Armilliferidae. Three genera. Armillifer, seven species; Cubirea, two species; Gigliolella, one species, found in snakes.

Linguatulidae. One genus. Linguatula, more than six species, found in mammals.

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