Toxicon 54 (2009) 1054–1064
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Structural diversity, systematics, and evolution of cnidae
Daphne Gail Fautin*
Department of Ecology and Evolutionary Biology, and Natural History Museum and Biodiversity Research Center, University of Kansas, 1200 Sunnyside Drive,Lawrence, KS 66045, USA
a r t i c l e i n f o
Article history: Available online 4 March 2009 Keywords: Cnidocyst Nematocyst Ptychocyst Spirocyst Cnidaria Anthozoa Cubozoa Hydrozoa Medusozoa Scyphozoa Staurozoa
a b s t r a c t
Cnidae are secreted by the Golgi apparatus of all cnidarians and only cnidarians. Of the three categories of cnidae (also called cnidocysts), nematocysts occur in allcnidarians, and are the means by which cnidarians defend themselves and obtain prey; spirocysts and ptychocysts are restricted to a minority of major taxa. A cnida discharges by eversion of its tubule; venom may be associated with the tubule of a nematocyst. About 30 major morphological types of nematocysts are recognized, but no single nomenclature for them is accepted. Function seems not tocorrelate tightly with morphologydnematocysts of at least some types are used both offensively and defensively. Similarly, it is not clear if morphology correlates with toxicity. Some types of nematocysts are taxonomically diagnostic whereas others are widespread. Nonetheless, an inventory of types of cnidae (the cnidom), with their distribution and size, is an essential component of most taxonomicdescriptions. Complicating the taxonomic value of cnidae are the facts that not all members of a species may have the same types of cnidae, even at the same life-cycle stage, and size of nematocysts of a species may vary geographically and with size of individual. The diversity of nematocysts is so great and the features within each major type are so variable that homologies have not been determined.Nematocyst complement, morphology, and size likely reﬂect both phylogeny and biology; the feedback between the two may confound analysis. Although cnidae are valuable in taxonomy of at least some groups, more understanding of the forces that affect them is needed for their systematic and phylogenetic value to be understood and their potential as indicators of evolution to be realized. Ó 2009Elsevier Ltd. All rights reserved.
1. Introduction Nematocysts are the sine qua non of phylum Cnidariadall cnidarians and only cnidarians produce them. At least some types of nematocysts are associated with venom; this is the source of the sting of jellyﬁsh, for example. Nematocysts are the means by which cnidarians protect themselves and capture preydcnidarians are exclusively carnivorous (e.g.Hand and Fautin, 1988) although some that live in shallow water harbor photoendosymbionts from which they may derive ﬁxed carbon
* Corresponding author. E-mail address: email@example.com 0041-0101/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.toxicon.2009.02.024
(e.g. Muscatine, 1961; Muscatine and Cernichiari, 1969). Each microscopic capsule (length range is about20–200 mm) is secreted by the Golgi apparatus of a cell specialized for this function, termed a nematoblast (Watson and Wood, 1988). Thus, despite common usage to the contrary, a nematocyst is not a ‘‘stinging cell’’dit is the capsule made by the cell that delivers the sting. Nematocysts constitute one of three categories of such intracellular secretory products of cnidarians. The others areptychocysts and spirocysts, capsules that occur in only a limited diversity of cnidarians (see Section 3 below). Bozhenova (1988: 71) expressed a distinctly minority, and somewhat outdated, view in declaring, ‘‘The division of cnidae into spirocysts, nematocysts and ptychocysts seems to be groundless.’’ The collective term for these capsules is
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