Family SPONGODISCIDAE Haeckel, 1862
Spongodiscida Haeckel, 1862: 239, 452, 460 [as a tribe]; Haeckel 1882: 461 [as a subfamily]; Haeckel 1887: 409, 573-575 [as a family]. — Stöhr 1880: 117 [as a family]. — Dunikowski 1882: 190 [as a subfamily]. — Bütschli 1889: 1964 [as a family]. — nec Rüst 1892: 172 [as a family]. — Schröder 1909: 3 [as a family]. — Anderson 1983: 24 [as a family].
Spongodiscidae – Pantanelli 1880: 49. — Popofsky 1908: 226; 1912: 143. — Clark & Campbell 1942: 47; 1945: 25 — Campbell & Clark 1944a: 27; 1944b: 18. — Frizzell & Middour 1951: 25-26. — Campbell 1954: D93. — Orlev 1959: 449-450. — Chediya 1959: 146. — Riedel 1967b: 295 (sensu emend.); Riedel 1971: 654. — Riedel & Sanfilippo 1971: 1588. — Petrushevskaya & Kozlova 1972: 528. — Sanfilippo & Riedel 1973: 523- 524. — Nakaseko et al. 1975: 169. — Petrushevskaya 1975: 547. — Nakaseko & Sugano 1976: 125. — Tan & Tchang 1976: 255. — Riedel & Sanfilippo 1977: 866; 1977: 866. — Kozur & Mostler 1978: 156 (sensu emend.). — Dumitrica 1979: 25; 1984: 100; 1995: 26. — Petrushevskaya 1979: 111-112. — De Wever 1982a: 190. — Tan & Su 1982: 157. — Anderson 1983: 39. — Sanfilippo et al. 1985: 660. — Blueford 1988: 252. — Takahashi 1991: 83. — van de Paverd 1995: 148-149. — Chen & Tan 1996: 151. — Hollis 1997: 50. — Boltovskoy 1998: 31-32. — Cordey 1998: 92. — Kiessling 1999: 42. — Kozlova 1999: 93. — Tan & Chen 1999: 230. — Amon 2000: 33-34. — Anderson et al. 2002: 1002. — De Wever et al. 2001: 158, 160. — Afanasieva et al. 2005: S288. — Afanasieva & Amon 2006: 131. — Ogane et al. 2009a: 84. — Chen et al. 2017: 145.
Spongotrochida Haeckel, 1882: 461 [as a tribe]; Haeckel 1887: 575 [as a subfamily]. — Dunikowski 1882: 190 [as a tribe]. — Schröder 1909: 50 [as a subfamily].
Spongotrochinae – Clark & Campbell 1942: 48; Clark & Campbell 1945: 26. — Campbell & Clark 1944a: 27; 1944b: 18. — Campbell 1954: D94. — Chediya 1959: 147.
Spongodiscinae – Frizzell & Middour 1951: 26. — Campbell 1954: D93. — Tan & Tchang 1976: 255. — Kozur & Mostler 1978: 157 (sensu emend.). — Tan & Su 1982: 157. — Tan 1998: 236. — Tan & Chen 1999: 230.
Spongodiscudae [sic] – Tan 1998: 236 (= Spongodiscidae).
Spongolonchidae Afanasieva & Amon in Afanasieva, Amon, Agarkov & Boltovskoy, 2005: S281 [nomen dubium]. — Afanasieva & Amon 2006: 121.
TYPE GENUS. — Spongodiscus Ehrenberg, 1854b: 237 [type species by subsequent designation (Frizzell & Middour 1951: 26): Spongodiscus resurgens Ehrenberg, 1854b: 246].
INCLUDED GENERA. — Spongaster Ehrenberg, 1861b: 833 (= Spongastrella with the same type species; Histiastrella n. syn.). — Spongodiscus Ehrenberg, 1854b: 237 (= Spongodisculus with the same type species). — Spongotrochus Haeckel, 1861b: 844 (= Spongotrochiscus with the same type species).
NOMINA DUBIA. — Pseudocephalis, Spongolonche, Stylotrochiscus, Stylotrochus.
DIAGNOSIS. — Shell with a circular to rounded rectangle outline, complemented by a spinose microsphere and a dense, homogenous, concentric structure throughout the test. Large numbers of radial beams develop to join the adjacent concentric wall structure at very high to vertical angles. These radial beams barely penetrate through the concentric walls.
Endoplasm fills the entire spongy shell. Its color varies from green to red in Spongaster and dark brown to gray for Spongodiscus and Spongotrochus. A robust, long axoflagellum emerges out the nonwalled pylome on the peripheral edge for Spongaster. This has not been confirmed for Spongodiscus and Spongotrochus. Dinoflagellate symbionts are found in Spongaster tetras tetras as well as cyanobacteria in Spongaster tetras irregularis. No algal symbionts have been determined in Spongodiscus and Spongotrochus. Pseudopodia radiate throughout the shell, and brown granules of an unknown origin are present in some bundles of pseudopodia in Spongaster. Isolated skeleton fragments are scattered in bundles of pseudopodia in Spongotrochus.
STRATIGRAPHIC OCCURRENCE. — Early Eocene-Living.
REMARKS
Spongodiscus is often misinterpreted as Spongocyclia (Litheliidae). The former is distinguishable from the latter by its very distinctive and systematic concentric structure as well as its straight radial beams that penetrate through the concentric structure, the absence of a thick crust cover, a tunnel-like set of pores straightly disposed along the outermost concentric structure. Under a light microscope, the circular- outlined polycystines between the Spongodiscidae and Spongopylidae are commonly misidentified. The misidentification can be attributed to the overlooking of exact superficial shape and appendages. In the case of Spongotrochus (Spongodiscidae) and Spongospira (Spongopylidae), the former is flat with cover-like appendages (Yamauchi 1986: pl. 3, figs 15, 16) while the latter is “ringed” by a repeating pattern of thicker and thinner parts (Cheng & Yeh 1989: pl. 1, fig. 19). The importance of the disk’s lateral profile will be discussed in the remarks regarding Trematodiscoidea.
The genus Spongaster is easily recognizable, regularly encountered in plankton samples from tropical to subtropical oceans and is relatively easy to maintain in culture. Consequently, this genus was used for various environmental control tests: growth, seasonality and opal productivity (Anderson et al. 1989a); trophic activity and primary productivity of symbiont (Anderson et al. 1989c); temperature, salinity and light influence on growth and survival (Anderson et al. 1989b); recognition of silicalemma in warping the siliceous skeleton (Anderson 1994: fig. 24); growth experiment (Anderson 1994: figs 25, 26); as well as analysis of the internal skeletal structure (Sugiyama et al. 1992: pl. 7, fig. 5). A living image was illustrated for Spongaster (Casey 1993: fig. 13.6; Matsuoka 1994: fig. 2B; 2007: fig. 4.D; 2017: figs 10.1, 10.2 Sugiyama & Anderson 1997a: pl.1, figs 1, 2; pl. 2, figs 1, 3; Takahashi et al. 2004: fig. 3; Suzuki & Aita 2011: fig. 4M; Suzuki & Not 2015: fig. 8.8.1; Matsuoka et al. 2017: appendix A), Spongodiscus (Suzuki & Not 2015: fig. 8.10.2), and Spongotrochus (Suzuki et al. 2013: fig. 7.3). Protoplasm and algal symbionts were documented by epi-fluorescent observation with DAPI dyeing in Spongodiscus (Zhang et al. 2018: 14, fig. 26, p. 16, figs 2-5), Spongaster (Suzuki et al. 2009b: figs 3M, 3N; Ogane et al. 2010: figs 1.3, 2.3; Zhang et al. 2018: 16, fig. 9). Several genera remain undescribed (e.g., Nakaseko & Nishimura 1982: pl. 43, fig. 1; Takahashi 1991: pl. 17, figs 12-16; van de Paverd 1995: pl. 41, fig. 1; Onodera et al. 2011: pl. 6, figs 3, 4).
VALIDITY OF GENERA
Spongaster
Histiastrella is marked by a quadrangular shell, a patagium, four undivided distally blunt arms (Campbell 1954: D87-88).
The type species of Histiastrella, Histiastrum quadrigatum, is characterized by the presence of seven to nine dividers in each arm but this character is not used as a distinguishing feature at the genus-rank (Haeckel 1887: 544). Presence of patagium and arms distally blunt are considered as intraspecies variations.