Morphology of the Lateral Line System and of the Skin of Diplomystid and Certain Primitive Loricarioid Catfishes and Systematic and Ecological Considerations

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Language: English

Lateralis systems, neuromast lines, and skins of the freshwater benthic siluroids Diplomystes, Nematogenys, Trichomycterus areolalus, "Trichomycterus" mendozensis and Hatcheria macraei were examined by light and scanning microscopy, The lateralis system comprises the main lateral line, postotic, otic, supraorbital, infraorbital, preopercular, and mandibular canals in primitive catfishes (e.g., diplomystids). The absence of the supratemporal canal is a siluroid synapomorphy. An interrupted lateralis system loosing parts of the supraorbital, infraorbital, and preopereulo-mandibular canals is found in other catfishes (eg. trichomycterids). A rudimentary preopercular canal or its absence is a synapomorphy of trichomycterids. The lateralis system is formed by narrow simple tubulcs in primitive catfishes (e.g., diplomystids), whereas they are highly ramified in other catfishes (e.g,. Arius felis and Parapimelodus). Variation in the structure of the sensory canals and in the presence of certain sensory tubules and pores seem to be diagnostic for different hierarchic levels of siluroids.

A few large superficial neuromasts may be aligned in rows in siluroids such as diplomystids (e.g., Diplomystes chilensis), Arius, Ictalurus, Galeichthys, Bagre, Rhamdia, Nematogenys, and certain trichomycterids (e.g., Trichomycterus areolatus); if so, they occupy the same position as the rostral, anterior, middle, and supratemporal accessory pit lines in other teleosts and they are interpreted as homologues. In addition, accessory supraorbital, antorbital, infraorbital, and opereular pit lines are present in Nematogenys. Trunk lines (dorsal, subdorsal. middle, ventral, and accessories) of large, elongate superficial neuromasts occur between the occiput and caudal fin in adult catfishes such as Nematogenys, Ictalurus, and occasionally in Diplomystes. The pit lines of primitive siluroids are characterized by one or a few elongate superficial neuromasts unlike those in gonorynchiforms, cypriniforms, and characiforms, which consist of fields of numerous, small superficial neuromasts.

There are important differences in the presence, distribution and size of epidermal papillae. goblet cells, club cells, and uniccllular unculi; in the distribution and types of taste buds; and in the presence or absence of ampullary organs, superficial neuromasts, and solitary cells in the skin of these genera. These differences may be useful in systematics; evaluation of them in a phylogenetic context is not possible at present, due to scarce information in most siluroid families and in other ostariophysans.

Unicellular unculi are present in most ostariophysans, with the exception of gymnotoids and non-ostariophysan teleosts. They are known from at least eight catfish families, including Nematogenyidae and Trichomycteridae. They are unknown in diplomystids. Differences in structure and distribution of the unculi among catfish groups may be of taxonomic and phylogenetic value. The presence of alarm substance cells and fright reaction are ostariophysan synapomorphies. Diplomystids are characterized by numerous club cells on the whole epidermis of the body. Siluroids and gymnotoids possess electroreceptive capabilities; specialized tuberous receptors are found only in gymnotoids. Siluroids may have ampullary organs deeply sunk in the epidermis (eg., Trichomycterus areolatus) or near to the epidermal surface (e.g., Diplomystes).

The siluroids examined (Diplomystes, Nematogenys,  trichomycterines, and "Trichomycterus" mendozensis) are carnivorous, and their diets appear similar. Thus the differences in type and distribution of epidermal gustatory structures may function in ways other than for discrimination of particular prey species. Adult  Diplomystes and Nematogenys are typical habitants of the Potamon of Andean torrential rivers; however, their skin differs strongly in the presence of gustatory receptors, ampullary organs, and mechanoreceptors. The skin also differs among diplomystid species. Trichomycterus areolatus and "Trichomycterus" mendozensis are typical habitants of the "dead zone" of the Rhitron and they differ strongly in the lateralis system and pit-organs. The adaptive and phylogenetic significance of these differences needs to be explored (or explained).

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