Abstract
Ctenophores in the class Tentaculata are distinct from Cnidarians in that they use sticky, not stinging, tentacles to capture and subdue their prey. The structures that make these tentacles sticky are colloblasts, specialized multicellular adhesive structures for predation. Located on the tentacles, tentacle side-branches (tentilla), or oral tentilla, colloblasts are only found in comb jellies (phylum Ctenophora). To perform comparative anatomy of the diversity of ctenophore colloblasts, specimens were collected from the epi- to bathypelagic zones near the coasts of central California and the Hawaiian Islands using blue-water divers and remotely operated vehicles. Tentacle samples were immediately fixed in a 4% formalin solution at sea, and then prepared in the lab via secondary fixation in 2% OsO4 for scanning electron microscopy (SEM). Diversity of ultrastructural characteristics was observed using SEM, and the morphometrics of the collosphere, external secretion granules, and spiral filament were recorded for 20 species, within 9 families and 9 genera, including 13 undescribed species. Morphometry of colloblasts reveals that the shape of the collosphere (the organizational unit of sticky granules) falls into 3 classifications: spherical, ellipsoidal, or non-uniform. External secretion granule deposition falls into 2 categories: clustered or patterned; the cap cell membrane was either present or absent. This morphological variation is summarized graphically and will be useful to describe the functional diversity and feeding ecology of the interesting and controversial phylum Ctenophora.
Highlights
Ctenophora is a diverse phylum of gelata consisting of about 190 described species, with estimates for the number of additional undescribed species being roughly equivalent (Appeltans et al 2012)
Collosphere and external secretion granule measurements were successfully made on all 20 species; spiral filament morphometrics were only recorded for 13 species due to no visible extended colloblasts, or unsuccessful dry-fracturing (Table 1)
External secretion granules were deposited by the cap cell in either a single cluster covering the surface of the collosphere or in 1 of 3 patterned granule deposition forms
Summary
Ctenophora is a diverse phylum of gelata consisting of about 190 described species, with estimates for the number of additional undescribed species being roughly equivalent (Appeltans et al 2012). Molecular and phylogenomic analyses of ctenophores have indicated that they may be the sister group to all other animals (Dunn et al 2008, Whelan et al 2017, but see Simion et al 2017). Through the use of electron microscopy, morphometrics of colloblasts have been recorded for some species (Bargmann et al 1972; Benwitz 1978; Franc 1978; Mackie et al 1988; Emson and Whitfield 1991; Carré and Carré 1993a, b; Eeckhaut et al 1997), but only one study has compared variation in colloblasts across a small portion (4 species from 4 families) of the phylum (Franc 1978). The goal of the present study is to survey the diversity of colloblast characteristics across the phylum through the use of scanning electron microscopy (SEM) and morphometric analyses
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