Microxina myxa, Goodwin & Brickle, 2012

Goodwin, Claire & Brickle, Paul, 2012, Sponge biodiversity of South Georgia island with descriptions of fifteen new species, Zootaxa 3542, pp. 1-48 : 36-37

Microxina myxa sp. nov.

( Figure 20)

Type material: Holotype: Sample in 95% ethanol, tissue section and spicule preparation on slides. BELUM Mc 7691. Green Island , Stromness, Site 2, South Georgia (54°09.381’S, 36° 39.852’W); depth 17.4m; collected by C. Goodwin, J. Brown, and S. Brown, 28 th November 2010.

Comparative material examined: MNHN DT691 , DT692 , DT693 Microxina charcoti Topsent, 1916 Holotype. Slides of tissue section and spicule preparations.

Etymology: From the Greek myx meaning slime or mucus. Named for the copious amounts of mucus produced when sampled.

External morphology: In situ appearance: White vase shaped sponge with central aquiferous cavity up length terminating in a large oscule. Length of cylinder approximately 20cm. Surface covered in spiky projections formed from the ends of the skeletal fibres. Produced large amount of slime when collected and removed from water ( Fig. 20a).

Preserved appearance: Very soft with the appearance of matted fur. Produced copious slime even after some time in ethanol.

Skeleton: The choanosomal skeleton is an irregular mesh formed of ascending, anastomising, columns of oxeas (4–10 spicules thick), joined by paucipicular secondary columns (1–2 spicules thick) and single oxea ( Fig. 20b). The ectosomal skeleton is formed from the ends through the surface of the sponge ( Fig. 20c).

Spicules: Oxea: 225 (248) 271 by 9.0 (10.9) 13.7µm. Majority centrally curved ( Fig. 20d). No microscleres.

Remarks: We have assigned this specimen to the family Niphatidae on the basis of its skeletal form; this family can be distinguished from the closely related Callyspongiidae as it has a rather disordered choanosomal skeleton rather than a neat rectangular mesh ( van Soest & Hooper 2002). The family Niphatidae contains Haplosclerida with a three dimensional ectosomal skeleton of multispicular fibres. Nine genera are currently considered valid and separated on the presence of microscleres and the form of the ectosomal skeleton (Desqueyroux-Faúndez and Valentine 2002a), however, there remains some confusion between the genera and species assigned to the family require thorough revision (Desqueyroux-Faúndez and Valentine 2002a). Of those genera recorded in the South Atlantic and Antarctica, Hemigellius and Haliclonissa have finely hispid and Amphimedon a smooth surface whereas Dasychalina , Pachychalina and Microxina have connulose to spiny surfaces. Microxina is characterised by the presence of microxea and Dasychalina and Pachychalina may be separated as Pachychalina has a choanosomal skeleton with thick tracts connected by much thinner secondary tracts whereas in Dasychalina the fibres have no clear orientation and there is no distinction between primary and secondary tracts (Desqueyroux-Faúndez and Valentine 2002a).

This sponge has the strongly spined surface, tubular body with large cavity characteristic of the type specimen of the genus Microxina Topsent 1916 . However, microxea spicules are not present. Topsent (1916) originally erected the genus for M. charcoti , which has strong fibres of oxeas with smaller thinner ‘microxeas’ in between them and defined the genus as Gelliinae with microxea microscleres. There are currently seven species assigned to this genus ( Table 10). These have a variety of microscleres including sigma, toxa and microxea. Wiedenmayer (1989) synonomised Hemigellius Burton, 1932 with Microxina determining that separation based on microsclere type was not valid as some species have both microxea and sigmata (e.g. M. bendeni Topsent, 1901 ), this was followed by Calcinai and Pansini 2000 in their designation of two new species. Desqueyroux-Faúndez and Valentine (2002a) retained Hemigellius as a valid genus, characterised by a fine surface hispidation rather than strong spines and we also do not support the synonymy of Hemigellius and Microxina but we do support the view that the genus could contain species with a variety of microscleres. We would therefore propose that species without microscleres could also be included, with the defining characteristics of the genus being a loose skeleton of multispicular fibres joined by paucispicular tracts, the ends of which project through the surface to form pronounced spines. Therefore we tentatively place M. myxa in Microxea where it seems to share the characteristics of M. charcoti and M. benedeni . However it should be noted that this genus, like other Niphatidae , requires a complete revision, some species currently placed here such as M. sarai might be better transferred to Hemigellius .

Several other species in the family Niphatidae are found in the Antarctic and sub-Antarctic: Dasychalina magellanica ( Thiele, 1905) , Dasychalina validissima ( Thiele, 1905) , Pachychalina glacialis ( Burton, 1934) , Pachchalina erinacea Goodwin et al., 2011a , Haliclonissa verrucosa Burton, 1932 , Haliclonissa sacciformis Burton, 1932 , and several species in the genera Hemigellius and Amphimedon . Because of the confusion of the taxonomy of this family we compared this species with these. Dasychalina validissima has oxeas of a similar size (250µm), but these are much thicker and its skeleton is formed of very thick fibres (20–30 spicules thick). Dasychalina magellanica ( Thiele, 1905) has oxeas of a similar length (190µm), however, the ascending fibres terminate abruptly at the surface and it consequently lacks the spiny projections characteristic of this species. Pachychalina erinacea Goodwin et al., 2011 has smaller oxea (114–206µm), the terminal ends of the fibres form shorter spines and it is lobed rather than cup shaped. Pachychalina glacialis ( Burton, 1934) is a branching species with a ‘hirsute’ rather than connulose surface. Haliclonissa verrucosa has a spiny surface but much larger oxeas (369–422µm). Haliclonissa sacciformis has similar sized oxea (320 by 7µm) but has a ‘minutely pilose surface having the appearance of fine velvet’.