(Figs 4 a–f, 5 a–d)
Material
Holotype: 6230 20 1 (52 ° 28 ’ 8.5 ’’N, 173 ° 35 ’ 52.9 ’’W, 34.8 km north of Amlia Island, 190 m depth). The holotype is deposited at the National Museum of Natural History, Washington, D.C., U.S.A. under the registration number USNM 1082996.
Description
The sponge is spherical, lightyellow coloured, covered with whitish, translucent fistules (Fig. 4 a). It is completely white in ethanol, 6 cm in diameter. The surface is covered with numerous thinwalled, bulbous fistules, 4–8 mm high and up to 4 mm in width, with the largest width observed at the top.
Skeleton. The ectosome is a translucent white layer, easily detachable, 400–650 µm in thickness. Here smooth tylotes are partially arranged perpendicular to the surface, oriented in all directions to some degree. From the choanosome there are tracts of tylotes fanning out towards the ectosome. Tylotes are also found in the choanosome besides the acanthostyles. Choanosomal acanthostyles are singly distributed without recognizable orientation.
Spicules. Tylotes with smooth ends (Figs. 4 b, c) measure 640–680 x 10–12 µm. Choanosomal acanthostyles (Figs. 4 c, d) measure 660–670 x 20–30 µm. Microscleres are three categories of isochelae: large isochelae with fimbriae (Figs. 4 e,f), 65–93 µm, large isochelae of the same size but with dented outer margins (Figs. 5 a, b), small anchorate isochelae (Figs. 5 c, d), 23–25 µm.
Distribution
Known only from the typelocality.
Etymology
Named after the spherical growth form of the species and the lightyellow color, from latin globo–sphere and latin gilvus–light yellow.
Discussion
The new species fits into the narrowed concept of Myxillidae suggested by Van Soest (2002: 602) which includes “only those genera which combine the possession of anchorate chelae (or polydentate derivations) with diactinal ectosomal tornotes and choanosomal styles in a reticulate arrangement.” However, there is no genus in the Myxillidae to fully accommodate placement of this species. We decided to assign it to Melonanchora because there is a considerable number of shared characters: subglobular growth, surface covered with fistules, the surface is parchmentlike, ectosomal tylotes are smooth, the choanosomal skeleton is poorly defined, with vague tracts, occurrence of anchorate isochelae and the presence of fimbriae on two categories of isochelae. On the other hand, there are also some differences to known species of Melonanchora: The diagnosis of the genus (Van Soest, FIGURE 4. Melonanchora globogilva sp. nov. a, holotype. Small quadrats equal 1 cm 2. b, ectosomal tylote among microscleres. c, close up of smooth end of tylote and pointed end of acanthostyle. d, acanthostyles. e, large anisochela, shaft and inner margins of alae with fimbriae. f, close up of fimbriate inner margins of alae of isochelae.
2002: 609) demands smooth choanosomal styles, whereas our species has acanthostyles and microscleres should include spherancoras. The fimbriae or dents on two categories of isochelae in M. globoblanca also occur on spherancoras of other species of Melonanchora and the unusual shape of these isochelae leads us to regard these isochelae as reduced or possibly original melonanchoras. To accommodate this new species the diagnosis of Melonanchora has to be modified to include the possession of choanosomal acanthostyles. There are four known species of Melonanchora, M. elliptica Carter, 1874, M. emphysema (Schmidt, 1875), M. kobjakovae Koltun, 1958 and M. tetradentifera Koltun, 1970. As already elaborated on above, all differ from our new species in having smooth choanosomal styles. It should be mentioned that the NAtlantic M. elliptica and M. emphysema have fully developed spherancoras while the N Pacific M. kobjakovae and M. tetradentifera have, judging from Koltun´s figures, somewhat reduced spherancoras where the alae of the spherancoras are not fused. Koltun (1970) reported dermal tornotes with slightly spinulose ends for M. tetradentifera. Consideration of this species as a true Melonanchora would require expanding the genus diagnosis to also include the possession of tylotes with acanthose ends.