Ilanga Herbert, 1987

Vilvens, Claude & Williams, Suzanne T., 2020, New species of Ilanga (Gastropoda: Trochoidea: Solariellidae) from the Indo-West Pacific, Zootaxa 4732 (2), pp. 201-257 : 207-208

Ilanga Herbert, 1987

Type species. Trochus laevissimus Martens, 1881 (by original designation)— Recent , South Africa .

Diagnosis. Following Herbert (1987) and using the additional Indo-Pacific species described in this work, the main conchological features of this genus are:

• shell width 5 mm to about 20 mm;

• shape of shell conical, sometimes slightly cyrtoconoidal or coeloconoidal, with a moderately elevated to strongly depressed spire; periphery from rounded to subangulate;

• protoconch typically solariellid of about 1–1.25 whorl, diameter 200–500 µm, often with several spiral lines;

• teleoconch of up to 5 more or less convex whorls; sculpture of a few spiral cords on first whorl, usually quickly vanishing on next whorls, making an almost smooth surface on last whorl;

• umbilicus wide, deep, open to apex, with a rounded or angulate margin, spiral cords and occasionally thin axial threads inside; axial pleats around umbilicus present or absent, sometimes visible on some parts of umbilicus but absent from others;

• aperture with an incomplete peristome; outer and inner lip thin; inner lip usually with weak basal thickening against umbilical rim; interior nacreous;

• colour variable, often with flames, chevron-like patterns and spiral bands.

Remarks. The genus Ilanga was initially defined and used for species living in the south-western Indian Ocean ( Herbert 1987). The present work, based on molecular and morphological study, shows that species assigned to Ilanga are in fact distributed throughout the tropical Indo-West Pacific. The genus does not occur in the Atlantic or in temperate regions. Its depth range is given from 16–1280 m by Herbert (1987), with 40–1280 m for living specimens; in this study we found living specimens from 50–616 m, which is shallower than the depth range of some other solariellid genera (e.g. Bathymophila 326–1570 m, Archiminolia 91–1385 m, Zetela 139–2421 m; Marshall 1999). There are no apparent biogeographic radiations in the Ilanga taxa included in molecular analyses ( Williams et al. 2013, Sumner-Rooney et al. 2016, Fig. 2 ). For instance, Indian Ocean species are found scattered throughout the phylogenetic tree of Ilanga and do not form a clade. Most species have been sampled from only one locality (which suggests low sampling intensity) and only I. oxeia n. sp., I. corrineae and I. fulgens are more widespread. There are two instances of species pairs, where one species found in the North West Pacific is sister to a species found in the South West Pacific ( I. harrytaylori & I. eurystoma ; I. comes n. sp. & I. navakaensis ). Similar patterns have also been noted in other solariellid taxa ( Williams et al. 2013). However, sampling to date is limited and needs to be expanded before meaningful biogeographic studies can be undertaken,

Herbert (1987) figured living specimens of Ilanga and provided details about their feeding habits. He observed animals using a ring of digitate palps around the mouth to sort and process sediment. He also observed some specimens swimming short distances to avoid predators. All species examined to date appear to have functional eyes with open apertures and pigment ( Williams et al. 2013, Sumner-Rooney et al. 2016, this study). Shells are typically flattened and wider than high. Most species have shells with some colour (usually brown or pinkish-brown, occasionally yellow) and pattern, although a few species lack any colour.

Etymology. Ilanga (Zulu) , the sun, feminine.

Figures

FIGURE 2. Relationships among 16 Ilanga species. Bayesian tree based on COI sequences, with outgroups (Elaphriella leia, MNHN IM-2007-18539; Elaphriella sp. 7, YK1384; and E. khantaros, MNHN IM-2009-15192) removed for clarity. Taxon labels are GenBank accession numbers and support values are posterior probabilities (PP). New GenBank sequences are in red type. Branches with PP <50% were collapsed, PP not shown for intraspecific relationships. Note that Ilanga herberti n. sp., I. helicoides n. sp. and Ilanga 18 are missing from the tree as although molecular data exist, there are no COI sequences for these species. Ilanga 19 was erroneously published as Microgaza fulgens in Williams et al. (2008) but is not discussed in this study as there is no voucher material. Other species are missing because there are no live-collected specimens.