Further ultrastructural observations on the epidermis of phoronids: Phoronis australis and Phoronis hippocrepia

Abstract

The epidermis of two species of phoronids, Phoronis australis and Phoronis hippocrepia, was studied using transmission electron microscopy and histochemical techniques. Results were compared with information available for Phoronis psammophila. The epidermis of each of the three species is similar with respect to certain cell types, namely supporting cells, fine-granule-containing cells, and four types of gland cells. These species differ, however, in abundance and distribution of each cell type. Differences may be related to different degrees of lubrication produced in various kinds of habitats (epifaunal, rocky, and sandy habitats). On the other hand, these differences may be related to burrowing activities in the three kinds of substrates. We conclude that possible correlations among structural patterns of the epidermis and various habitats primarily involve the relative abundance and distribution of gland-cell types along the trunk and ampulla. Phoronids are marine tube-dwelling invertebrates. The two species used for the present work, Phoronis australis Haswell, 1883, and Phoronis hippocrepia Wright, 1856, and a third species, Phoronis psammophila Cori, 1889, to which they are compared, build tubes in different substrates. P. psammophila lives in the intertidal and shallow subtidal zones (0-18 m depth) in sandy or muddy sediments, and its tube is covered by aggregated sediment particles (Emig, 1971; Fernandez et al., 1991); P. hippocrepia is found from the intertidal zone to 55 m depth, burrowing in rocks or empty shells or incrusting hard substrates; its tube is sinuous, without aggregated particles, except on the portion of the tube outside of the substrate (Emig, 1971; Emig & Bailey-Brock, 1987). P. australis occurs between the low-tide mark to 36 m depth and builds its tube in the tube wall of cerianthids, which may be considered a hard substrate (Emig, 1971; Emig et al., 1972). A detailed description of the epidermal cell types and their distributions along the trunk of P. psammophila was provided by Fernandez et al. (1991), who confirmed the statements of Pourreau (1979) relative to the role played by epidermal gland cells in tube construction. We have studied the ultrastructure of the epidermis of the trunk and ampulla of P. australis and P. hippocrepia in order to compare these species with P. psammophila and to describe possible relationships between the structure of the epidermis and habitat. I We thank Dr. C. C. Emig (Station Marine d'Endoume, Marseille) for the facilities needed to collect specimens of Phoronis hippocrepia and Drs. G. San Martin and E. Garcia Raso for kindly providing the specimens of Phoronis australis used in this study. This work was supported by a research grant (PB 860010) of the Comisi6n Interministerial de Ciencia y Tecnologia (CICYT). TRANS. AM. MICROSC. SOC., 112(4): 280-291. 1993. ? Copyright, 1993, by the American Microscopical Society, Inc. This content downloaded from 207.46.13.57 on Thu, 08 Sep 2016 06:10:38 UTC All use subject to http://about.jstor.org/terms VOL. 112, NO. 4, OCTOBER 1993 MATERIALS AND METHODS Individuals of both species studied were collected on the Mediterranean coast; Phoronis hippocrepia was found in a submarine cave near Marseille (France) at 10 m depth, and Phoronis australis was collected at 2-3 m depth in Almeria, SW coast of Spain, within tubes of Cerianthus sp. (Anthozoa). Specimens were fixed in 4% glutaraldehyde buffered with 0.1 M cacodylate in filtered seawater and postfixed in 1% osmium tetroxide in the same buffer. Tissue pieces then were dehydrated in an acetone series, stained en bloc in 2% uranyl acetate during dehydration, and embedded in Araldite via propylene oxide. Ultrathin sections were obtained with an LKB III ultramicrotome, stained with lead citrate, and examined with a Philips EM 201 electron microscope. For histochemical tests, specimens were fixed in neutral buffered formalin or Bouin's fluid and embedded in paraffin wax. Blocks were sectioned at 5-7 ,m. The following histochemical techniques were applied (details in Gabe, 1968; Lison, 1960; Pearse, 1968): 1. For characterization of mucopolysaccharides: periodic acid-Shiff (PAS), Alcian blue 8GX (AB) at pH 2.5 and 1.5; AB-PAS, and Toluidine blue (TB) at pH 4.2-1.5. 2. For proteins: Danielli's tetrazo reaction with or without previous treatment by DNFB (dinitrofluorobenzene), performic oxidation, benzoylation, and deamination.