Cocconeis mascarenica, Riaux-Gobin & Compere. SV, 2008

COCCONEIS MASCARENICA COMPLEX AND COCCONEIS SP.

A small-sized taxon with strong similarities to Cocconeis mascarenica Riaux-Gobin & Compère (2008 , figs 33–40, 48–51), here listed as C. cf. mascarenica ( Table 2), showed several morphs, i.e. with SV apical rows of areolae ( Fig. 12 View Fig ), one apical and marginal row of longer SV areolae ( Fig. 10 View Fig arrow), and dense (up to 6) and dash-like SV areolae with no particular arrangement ( Figs 7, 9, 11 View Fig ). The above mentioned morphs have their valvocopulae devoid of true fimbriae (RVVC, Fig. 8 View Fig arrowhead; SVVC, Fig. 11 View Fig arrowhead). Note the presence of an RV marginal hyaline rim ( Fig. 8 View Fig arrow). All these morphs have a largely elliptic valve shape and biometrics similar to what was originally observed ( Table 3).

From a pearl oyster farm in Gatavake Bay (owner Michel Teakarotu), Cocconeis sp. ( Table 3, Figs 13–15 View Fig ) was found as subtidal, epizoic on Pinctada margaritifera , also present on ropes lying at the same depth (i.e. 15 m, Rik15). This taxon had small dimensions, an oblong-elliptic to linear valve shape ( Figs 13–14 View Fig ), blunt apices, measurements close to that in the C. mascarenica type ( Table 3). Cocconeis sp. had short and dense dash-like SV areolae (up to 6 per stria, with no axial alignment), SV hymenes with marginal short slits ( Fig. 15 View Fig ), and elliptic SV sternum. Valves measured 8.0–9.4 µm in length and 3.5–4.3 µm in width, with 32–36 SV striae in 10 µm, regularly spaced (no areolae on SV apex), and 39–45 RV striae in 10 µm, regularly spaced, with a marginal hyaline area and low with presence-absence, biotopes and location, presented as ‘supplementary material’.

Taxon acronym

Achnanthales

Achnanthes cf. brevipes Agardh acbre

Achnanthidium glyphos Riaux-Gobin, Compère & Witkowski acgly

Achnanthidium glyphos morph acgly2

Achnanthidium pseudodelicatissimum Riaux-Gobin, Witkowski & Compère acpse

Achnanthidium sp. aff. Achnanthes fogedii Håkansson acfog

Amphicocconeis clypeus Riaux-Gobin & Witkowski amcly

Amphicocconeis cf. mascarenica Riaux-Gobin & Compère ammas

Amphicocconeis rodriguensis Riaux-Gobin & Al-Handal amrod

Amphicocconeis ruatara Riaux-Gobin amrua

Amphicocconeis sp. amsp

Astartiella sp. assp

Cocconeis angularipunctata Riaux-Gobin, Romero, Compère & Al-Handal coang

Cocconeis cf. borbonica Riaux-Gobin & Compère cobor

Cocconeis carinata Riaux-Gobin, Ector & Witkowski cocar

Cocconeis coralliensis Riaux-Gobin & Compère cocor

Cocconeis coronatoides Riaux-Gobin & Romero ‘type’ cocid

Cocconeis coronatoides discoid morph cocid2

Cocconeis cupulifera Riaux-Gobin, Romero & Al-Handal cocup

Cocconeis cf. delapunctata Hohn codel

Cocconeis cf. diaphana W.Smith codia

Cocconeis distans Gregory codis

Cocconeis guttata Hustedt & Aleem cogut

Cocconeis cf. geometrica Riaux-Gobin, Romero, Compère & Al-Handal cogeo

Cocconeis heteroidea Hantzsch cohet

Cocconeis margaritata Riaux-Gobin & Al-Handal comar

Taxon acronym

Cocconeis mascarenica Riaux-Gobin & Compère comas

Cocconeis mascarenica forma conew

Cocconeis cf. molesta Kützing comol

Cocconeis paucistriata Riaux-Gobin, Romero, Compère & Al-Handal copau

Cocconeis peltoides Hustedt copel

Cocconeis peltoides var. archaeana Riaux-Gobin & Compère coarc

Cocconeis placentula Ehrenberg complex copla

Cocconeis pseudodiruptoides Foged copsd

Cocconeis pseudograta Hustedt copsg

Cocconeis pseudomarginata Gregory copsm

Cocconeis santandrea Riaux-Gobin, Witkowski & Bemiasa cosan

Coccconeis scutellum Ehrenberg coscu

Cocconeis sigillata Riaux-Gobin & Al-Handal cosig

Cocconeis suzukii Riaux-Gobin, Compère, Coste, Straub & Taxböck cosuz

Cocconeis sp. 5 (= sp. 4 in Riaux-Gobin et al. 2015 c) cosp5

Madinithidium flexuistriatum (Riaux-Gobin, Compère & Witkowski) mafle

Madinithidium scalariforme (Riaux-Gobin, Compère & Witkowski) masca

? Majewskaea Van de Vijver, Robert, Witkowski & Bosak maj

Planothidium cf. delicatulum (Kützing) Round & Bukhtiyarova pldel

Planothidium mathurinense Riaux-Gobin & Al-Handal plmat

Planothidium rodriguense Riaux-Gobin & Compère plrod

Planothidium sp1 oblong without fascia plsp1

Planothidium sp2 elliptic sternum pls2

Schizostauron citronella (Mann) Górecka, Riaux-Gobin & Witkowski sccit

Olifantiella

Olifantiella cf. rodriguensis Riaux-Gobin olrod

Olifantiella pilosella Riaux-Gobin olpil

Olifantiella muscatinei (Reimer & Lee) Van de Vijver, Ector & Wetzel olmus helictoglossae. Figs 17–18 View Fig are possibly illustrating the RV of C. sp., with no RV central area, RVVC with undulated edge and no fimbria ( Fig. 17 View Fig insert, twin arrowheads), but with the SVVC not observed. Cocconeis sp. was rare and observations on several characteristics are still lacking to finalize its exact taxonomic position. This taxon may pertain to the C. mascarenica complex, but with a subtidal ethology. One SV found in Hao ( Fig. 16 View Fig , intertidal locality) had some similarities with the SV of C. sp., but the adjoining RV was not found. There are some similarities between C. sp. and C. neothumensis var. marina De Stefano, Marino & Mazella ( De Stefano et al. 2000, Sar et al. 2003), but the latter is less linear, with more acute apices, and RV striae ( 26 to 32 in 10 µm, ref. cit.) denser than the SV striae ( 20 to 26 in 10 µm, ref. cit.). Also there are some similarities with the SV of C. coralliensis Riaux-Gobin & Compère 2008 , with lower stria density ( 36–40 in C. coralliensis versus 32–36 in C. sp.), round SV endings (versus sharp in C. coralliensis ), a lower number of areolae per stria, no areolae on apices versus areolae perpendicular to the margin on apices in C. coralliensis , and a SV sternum slightly elliptical in C. sp., versus narrow and straight in C. coralliensis . Note that the RV of C. coralliensis has no marginal hyaline area (ref. cit.).

Remarks: Cocconeis mascarenica ( Riaux-Gobin & Compère 2008) was originally compared to C. neodiminuta Krammer & Lange-Bertalot 1991 and to C. neothumensis Krammer 1990 , but the RV of C. neothumensis ( Krammer 1990, fig. 34) was also attributed to C. neodiminuta ( Krammer & Lange-Bertalot 1991, pl. 55, fig. 3). Furthermore, the shape of the SV areolae in C. mascarenica differs from that of C. neodiminuta (ref. cit., pl. 55, fig. 1) in being much shorter, and the SV sternum in C. mascarenica being narrow-straight in place of largely elliptical ones. Note that a picture illustrating C. neothumensis in Krammer (1990, fig. 39) was also used to illustrate C. neodiminuta by Krammer & Lange-Bertalot (1991, pl. 55, fig. 1), causing taxonomic confusion between C. mascarenica and the two latter taxa. Furthermore, the freshwater taxa C. neodiminut a and C. neothumensis have dissimilar RV and SV stria densities, while C. mascarenica has the same stria density on both valves (see discussion in Riaux-Gobin et al. 2018). Also, there are some similarities with the later described C. neothumensis var. marina ( De Stefano et al. 2000, figs 53–65), but with differences (see above remarks). In the original description of C. mascarenica off Mascarenes, the taxon demonstrated an obvious phenotypic plasticity, with two morphs described from Rodrigues (morphs 1 and 2 in Riaux-Gobin et al. 2011c). The Cocconeis mascarenica complex probably includes several species that can only be separated via genetics.

In addition, Cocconeis mascarenica and C. cf. mascarenica and all the above-mentioned morphs present in Mangareva ( Tables 2–3, Figs 7–18 View Fig View Fig ) have some similarities with C. placentula sensu lato (as referred to by Potapova & Spaulding 2013), but with overall smaller valve dimensions, higher stria densities on both valves, and valvocopulae without conspicuous fimbriae [ C. placentula sensu lato (see ref. cit. above) has 20–24 striae in 10 µm on RV, and 15–25 on SV]. Note that the bibliography of the C. placentula sensu lato complex is complicated, with several new taxa that can only be distinguished through SEM, such as the recently described C. fetscheriana Stancheva (2022 , 128). The morphological plasticity in such taxa can also be pro parte imputed to the reproductive stages of the taxon (see Jahn et al. 2020), rendering cultures and genetics essential in definitively splitting taxa.