Phoronida
Phoronids are epibenthic (or infaunal) tubiculous marine invertebrates closely related to brachiopods (and perhaps bryozoans; see Nesnidal et al. (2013)) that have oval, U-shaped, or spiraling rings of ciliated tentacles called the lophophore used for fee
- PDF / 1,893,630 Bytes
- 15 Pages / 504.57 x 720 pts Page_size
- 103 Downloads / 155 Views
10
Scott Santagata
Chapter vignette artwork by Brigitte Baldrian. © Brigitte Baldrian and Andreas Wanninger. S. Santagata Biology Department, Long Island University-Post, Greenvale, NY 11548, USA e-mail: [email protected] A. Wanninger (ed.), Evolutionary Developmental Biology of Invertebrates 2: Lophotrochozoa (Spiralia) DOI 10.1007/978-3-7091-1871-9_10, © Springer-Verlag Wien 2015
231
232
INTRODUCTION Phoronids are epibenthic (or infaunal) tubiculous marine invertebrates closely related to brachiopods (and perhaps ectoprocts; see Nesnidal et al. (2013)) that have oval, U-shaped, or spiraling rings of ciliated tentacles called the lophophore used for feeding and respiration (Temereva and Malakhov 2009a). Although phoronids can dominate the density and coverage of some benthic marine habitats (Larson and Stachowicz 2009), very little is known about their ecological role in such habitats. The majority of taxonomic studies of phoronids have been conducted by Emig (1974). Although at least 23 species have been described by various authors indicative of wide morphological diversity in adult forms, the majority of phoronid morphotypes have been synonymized under 11 cosmopolitan species and two genera, Phoronis (Wright 1856) and Phoronopsis (Gilchrist 1907). Adult phoronid bodies have generally been considered to be tripartite, divided by transverse septa into the epistome, tentacle crown, and trunk regions. Each of these body regions was originally described as having a true coelomic cavity lined by mesoderm and were interpreted as the protocoel (epistome), mesocoel (lophophore), and metacoel (trunk), linking this arrangement to the tripartite coeloms found in echinoderms (Masterman 1898). The epithelial lining of the epistome in two species of Phoronis was found instead to be myoepithelial cells lacking adherent junctions surrounding a gel-like extracellular matrix, thus not exhibiting the features of a true epithelial layer (Bartolomaeus 2001; Gruhl et al. 2005). Further complicating these findings are ultrastructural observations of the epistome lining of Phoronopsis harmeri, which does contain adherent junctions (Temereva and Malakhov 2011). This cavity can be derived from the protocoel formed in Phoronopsis harmeri larvae but may collapse and break down during development among various other species (Zimmer 1978). Alternatively, this larval cavity may be lost during metamorphosis and reform during post-metamorphic growth (Santagata 2002). Regardless of whether adult phoronids have two
S. Santagata
or three true coelomic compartments, this distinction does not clarify their phylogenetic position relative to other lophophore-bearing animals (ectoprocts and brachiopods) or to annelids, nemerteans, and mollusks. Similar to ectoprocts, the adult phoronid gut is U-shaped and the anus is positioned outside of the tentacles. Unlike ectoprocts, each of the tentacles has a blind capillary with nucleated red blood cells containing a form of hemoglobin (Garlick et al. 1979). These capillaries are connected to loph
