Lattice organs and newly characterized submarginal pore-plates and pore-fields of the carapace in Ascothoracida (Crustacea: Thecostraca)

Abstract

Lattice organs on the dorsal part of the carapace were examined by scanning electron microscopy (SEM) in females, males, and/or cypridiform ascothoracid-larvae (in the ascothoracid-larva I stage, for the first time ever) of six species of Ascothoracida representing four genera and three families: Waginella sandersi (Newman, 1974), W. ?metacrinicola (Okada, 1926), and Gorgonolaureus muzikae Grygier, 1981 (family Synagogidae); Baccalaureus Broch 1929, unidentified species (Lauridae); and Ascothorax gigas Wagin, 1968 and A. synagogoides (Wagin, 1964) (Ascothoracidae). All were of the "keel in a trough" or "tube in a trough" type, but they varied even more than those of previously studied ascothoracidans in number, form, orientation, and terminal pore position. Such extensive variability, summarized graphically herein, limits the potential utility of Ascothoracida (parasites of anthozoans and echinoderms) as an out-group for polarizing lattice organ character-state variation in Cirripedia (free-living and parasitic barnacles). While the ground-pattern of lattice organs in Thecostraca (comprising Ascothoracida, Cirripedia, and Facetotecta, or "y-larvae") includes two anterior and three posterior pairs, ascothoracid-larvae and males of Ascothorax Djakonov, 1914 and Dendrogaster Knipovich, 1890 (family Dendrogastridae) have only two posterior pairs; evidence as to which pair is missing is discussed. The hypothesis that dorsal setae in thecostracan nauplii are the precursors of lattice organs in later developmental stages is reexamined; one-to-one positional matching of such setae to lattice organs is difficult in Ascothoracida. Newly characterized structures of unknown function, termed "reticulated pore-plates", exist along the hinge line in a juvenile male of G. muzikae. The "pits" reported earlier along the anterior valve margin in ascothoracid-larva II of A. synagogoides are actually clusters of pores that may be homologous to these pore-plates. Potentially homologous pore-fields in other ascothoracidans are reviewed from the literature or described anew using SEM.