Order HOMOSCLEROPHORIDA Dendy, 1905

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Introduction

Scope
The order Homosclerophorida (= Microsclerophora Sollas, 1887) was originally created with three families, Plakinidae, Corticidae, and Thrombidae (Sollas 1888; Dendy 1905), with the latter moved to the Astrophorida Demospongiae. Placinidae Topsent, 1890d is a misspelling of Plakinidae. Two families Plakinidae and Oscarellidae were subsequently recognized, then merged into Plakinidae (Solé-Cava et al. 1992; Diaz & Van Soest 1994; Boury-Esnauly et al. 1995; Muricy et al. 1996a; Muricy 1999), and more recently split again into Plakinidae (with spicules) and Oscarellidae (without spicules), based on substantial molecular and histological evidence (Gazave et al. 2010).

Over 12 nominal genera have been included in these families, but only seven are considered to be valid and now included: Plakortis Schulze, 1880, Plakinastrella Schulze, 1880, Plakina Schulze, 1880, Placinolopha Topsent, 1897a, Corticium Schmidt, 1862, Oscarella Vosmaer, 1887 and Pseudocorticium Boury-Esnault et al., 1995. The two genera placed in incertae sedis — Corticellopsis and Astroplakina — are excluded based on the clear astrophorid nature of their spiculation. Less than 100 species of sponges from these genera have been published worldwide of which only about 80 are considered valid. The number of species described in the literature is clearly geographically biased towards areas that have been better studied such as the Mediterranean, the West Pacific and the Caribbean. It is expected than when the least studied biogeographic regions are surveyed for this group the species numbers will increase considerably. Species of various genera such as Oscarella, Plakina, Plakortis and Corticium with relatively simple morphologies are found in distinct biogeographic regions (e.g. Plakortis spp. in the Caribbean and in the West Pacific), and several species are considered cosmopolitan (e.g. Oscarella lobularis, Plakortis simplex, Plakina monolopha). Such disjunct populations, although morphologically similar, are expected to be reproductively isolated and to have diverged genetically enough in time to deserve a status of distinct species.

Taxonomy
A generic classification of the Plakinidae and Oscarellidae is centered around four principal diagnostic features: presence/absence of siliceous skeleton (Plakinidae vs. Oscarellidae); presence/absence of a cortex associated with the architecture of the aquiferous system and type of choanocyte chambers; number of spicule size classes; and presence and type of ramifications in the actines of calthrops, with at least three distinct general morphologies recognized: small homolophose calthrops (Plakina); large lophodiods, lophotriods and lophocalthrops (Placinolopha); and small heterolophose calthrops (Corticium).

Database Notes

Synonymy.
Order: Homosclerophorida Dendy, 1905: 64. Microsclerophora Sollas, 1887 (in part).
Family: Plakinidae Schulze, 1880: 447. Corticiidae Vosmaer, 1887: 324. Placinidae Topsent, 1890d: 231.
Family Oscarellidae Lendenfeld, 1887a: 582.

Diagnosis

Homoscleromorpha with flagellated exo- and endopinacocytes, a basement membrane lining both choanoderm and pinacoderm, oval to spherical choanocyte chambers with a sylleibid-like or leuconoid organization, and a unique incubated cinctoblastula type larvae; spicules, when present, are peculiar tetractines (calthrops) and derivatives through reduction (diods and triods) or through ramification of one to all four actines (lophose calthrops).

ID Keys

KEY TO GENERA
(1) With inorganic (spicular) skeletal complement ----------------------------------------------------------------------------- 2
Without inorganic (spicular) skeletal complement ----------------------------------------------------------------------------- 6

(2) Skeleton mainly composed of diods, triods, and/or calthrops in one size class --------------------------------------- 3
Skeleton mainly composed of diods, triods and/or calthrops with a large size variation --------------------------------- 5

(3) Lophose diods, triods, or calthrops complement the main skeleton of non-lophose spicules ----------------------- 4
Lophose spicules absent, diactinal “microscleres” (microrhabs) present in some species---------------------Plakortis

(4) Heterolophose calthrops (candelabra) complement the main skeleton of non-lophose spicules, which might be absent or rare; choanocyte chambers usually aphodal---------------------------------------------------------- Corticium
Lophocalthrops with one to four homogeneously ramified actines complement the main skeleton of non-lophose spicules; candelabra absent; choanocyte chambers usually eurypylous ----------------------------------------- Plakina

(5) Skeleton composed of non-lophose diods, triods and/or calthrops in three size classes ---------- Plakinastrella
Skeleton formed by diods, triods, and/or calthrops with large size variation; the larger spicular category presents terminally-branching rays---------------------------------------------------------------------------------------- Placinolopha

(6) With sylleibid-like aquiferous system and eurypylous choanocyte chambers; ectosome thin, proportion of mesohyl to chambers ------------------------------------------------------------------------------------------------- Oscarella
With leuconoid aquiferous system and diplodal choanocyte chambers; ectosome thick, proportion of mesohyl
to chambers ------------------------------------------------------------------------------------------------- Pseudocorticium

Diagnosis References

Muricy, G. & Diaz, M. C. 2002. Order Homosclerophorida Dendy, 1905. Family Plakinidae Schulze, 1880. pp. 71-82 in Hooper, J.N.A. & van Soest, R.W.M. (eds). Systema Porifera: A guide to the classification of sponges. New York : Kluwer Academic/Plenum Publishers Vol. 1. [71]

General References

Gazave, E., Lapébie, P., Renard, E., Vacelet, J., Rocher, C., Ereskovsky, A.V., Lavrov, D.V. & Borchiellini, C. 2010. Molecular Phylogeny Restores the Supra-Generic Subdivision of Homoscleromorph Sponges (Porifera, Homoscleromorpha). PLoS ONE (Public Library of Science) 5(12): 1-15 [e14290] [1]

Muricy, G. & Diaz, M. C. 2002. Order Homosclerophorida Dendy, 1905. Family Plakinidae Schulze, 1880. pp. 71-82 in Hooper, J.N.A. & van Soest, R.W.M. (eds). Systema Porifera: A guide to the classification of sponges. New York : Kluwer Academic/Plenum Publishers Vol. 1.

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