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Introduction
Identification of diatom species and construction of floristic lists are still much needed activities for the Mexican coasts which are actually improving on the basis of comprehensive taxonomic works that incorporate the latest advances in diatom systematics. It is evident that the lack of floristic studies on benthic diatoms in much of the Mexican littoral precludes insight of their ecology and distribution (Siqueiros-Beltrones et al., 2017a), inasmuch floristics relates taxonomy with ecology. Besides carrying out continuous floristic surveys that will allow establishing ecological relations and biogeographic affinities of benthic diatom flora in the coasts of Mexico, more specific surveys dealing with particular groups, mainly the conspicuous ones may prove to be useful while addressing the above topics. This approach has been recently realized focusing on the genus Lyrella for the northwestern Mexican Pacific coasts, responding to controversial statements on the geographical distribution and indicative properties of Lyrella spp., and taking advantage of their observed diversity and abundance in a coastal lagoon in northwestern Mexico (Siqueiros-Beltrones et al., 2017b). In the present case a similar pattern was noted while describing the assemblage structure of epiphytic diatoms of Thalassia testudinum along the coast of Campeche, Mexico (southern Gulf of Mexico), where many Mastogloia species were observed in the mounted samples (López-Mejía, 2016).
Mastogloia species are classified in the order Mastogloiales (Bacillariophyceae). Their naviculoid isopolar frustules are readily recognized and discriminated by the presence of partecta, i.e., loculate marginal septa that bear 1 or many chambers in the valvocopulae.
Live cells exhibit 2 plastids, 1 towards each pole (Round et al., 1990). Most species of Mastogloia are ubiquitous marine forms although some are common in brackish water and freshwater habitats. Most species are benthic and are abundant in epipelic and epiphytic assemblages (Round et al., 1990), particularly in subtropical and tropical latitudes where they appear in great numbers and showing high species richness (Frankovich et al., 2006).
Regionally, Meave-del Castillo et al. (2003) compiled 25 Mastogloia taxa records in the tychoplankton of the Mexican Pacific, while Moreno et al. (1996) accounted for 38 Mastogloia taxa in phytoplankton samples of the Gulf of California. Also, Bahía Magdalena in the Mexican northwestern Pacific on the other side of the Baja California Peninsula, was proposed as a “hot spot” for diatom species diversity, in which the high number (S = 20) of Mastogloia taxa (and of Lyrella spp.) marked the difference with other ecosystems in the region (López-Fuerte et al., 2010), while further north, off the west coast of Baja California a taxocoenosis from Isla Guadalupe for which a biogeographical affinity could not be established yielded many tropical species including 13 Mastogloia spp. (López-Fuerte et al., 2015).
In contrast, for the Mexican Atlantic region and the highly diverse Caribbean Sea, Navarro and Hernández-Becerril (1997) compiled 89 Mastogloia taxa records in a survey of 47 studies. Also, Loir and Novarino (2013) recorded a similar number of species (> 80) for several islands in the Caribbean Sea, which they described and illustrated, while Hein et al. (2008) recorded 75 Mastogloia taxa from the Bahamas Archipelago. Moreover, in a previous study on epiphytic diatoms of T. testudinum off the coast of Quintana Roo (Caribbean Sea) López-Fuerte et al. (2013) recorded 30 Mastogloia species. These accounts set a reference as to the expected species variety in any given area for the genus Mastogloia, particularly within tropical regions. Our objective, thus, was to determine the species richness of Mastogloia thriving on T. testudinum blades in a tropical environment exhibiting also their hitherto unattended taxonomic potential.
Materials and methods
The study area is located in the northern coast of the state of Campeche (19º19’43.90” - 20º20’23.30” N, 90º47’48.90” - 90º29’43.30” W). It includes 3 localities: Champotón, City of Campeche, and the Biosphere Reserve Los Petenes (Fig. 1), which show extensive T. testudinum beds. Campeche is divided by the Champotón River into the southern part where rivers are a characteristic trait, and the northern part where surficial rivers are scarce due to the permeability of the lime rock (Villalobos-Zapata & Mendoza-Vega, 2010).
Figure 1 Study area comprising the 3 sampled localities: Champotón, City of Campeche, and the Biosphere Reserve Los Petenes.
The dry season in the coastal zone of Campeche extends from November to April, while the warm, rainy season (September-April) is seasonally influenced by cold northwesterly winds which cause mixing of the water column (Zavala-Hidalgo et al., 2014). Temperature varies from 29.7 ºC in Summer to 24 ºC in Winter (Callejas-Jiménez et al., 2012). Mean salinity values are higher than 36 for these periods (Limoges et al., 2013).
Sampling was carried out in 2013 during the dry season in April (Petenes), April-May (Champotón), and in the rainy season during August (Costa, Campeche). Sites were selected on the basis of the distribution of Thalassia testudinum (Fig. 2). In each site, 3 bundles of seagrass were collected randomly and stored in 1 L plastic jars. Overall, 23 sites were sampled: 6 at Champotón, 7 at Costa, and 10 at Petenes (López-Mejía, 2016). Overall, more than 300 blades of T. testudinum were processed; these ranged in size from 36 to 70 cm2; no statistical differences between blade size from the 3 sampled sites were found (López-Mejía, 2016). Thus, considering that between 3,000 and 4,000 diatoms cells may be found on 1 mm2 of a host surface (Siqueiros-Beltrones et al., 2002), and that the random sampling of the blades compensated for the patchy distribution of diatom assemblages (Siqueiros-Beltrones, 2002), confidence exists on the representativeness for the upcoming estimation of proportionality or relative abundances of the distinct taxa in the diatom assemblages occupying this substrate, specifically of the identified Mastogloia infrataxa.
Figure 2 Mastogloia angulata (a, b), M. belaensis (c, d), M. Corsicana (e, f), Mastolgoia cf. Ovulum (i, j), M. ovata (g, h). Scale bars = 10 µm; i, j have a different scale.
In the laboratory, the epiphytic diatoms were scraped off the T. testudinum blades with a spatula, cleaned by oxidizing the organic matter with nitric acid, and rinsed with distilled water until reaching a pH > 6 (Siqueiros-Beltrones, 2002). Afterwards, the diatom frustules were mounted on permanent slides using synthetic resin Pleurax (RI = 1.7), i.e., 3 slides per site (69 slides).
Mastogloia species and infra-specific taxa were identified on the basis of frustule morphology following the classic works by Peragallo and Peragallo (1908), Hustedt (1966), Cleve-Euler (1953), Schmidt et al. (1959), and recent literature: Foged (1975, 1978, 1984), Moreno et al. (1996), Witkowski et al. (2000), Siqueiros-Beltrones (2002), Metzeltin and García-Rodríguez (2003), Siqueiros-Beltrones et al. (2004), Hein et al. (2008), Novelo et al. (2007), López-Fuerte et al. (2010, 2013), Stidolph et al. (2012), Loir and Novarino (2013), Lobban et al. (2012). All species names were checked in the Guiry and Guiry (2016) online Algaebase.
Considering the entire epiphytic diatom taxocoenosis, i.e., > 300 diatom species (López-Mejía, 2016), the different Mastogloia taxa were classified according to their estimated proportions or relative abundances among all the observed diatom taxa. This was computed on the basis of a sample size of n = 500 valve count in a slide (Siqueiros-Beltrones, 2002; Siqueiros-Beltrones et al., 1985), i.e., 1,500 valves per site, and > 34,500 valves for the 3 localities; taxa were classified as: rare (1< x ≤ 15), frequent (15 < x ≤ 100, abundant (100 < x ≤ 300), and very abundant (> 300).
Results
The genus Mastogloia exhibited the highest number of taxa with 75 out of 301 identified diatom taxa found as epiphytes on Thalassia testudinum blades from the sampling area (López-Mejía, 2016), which were distributed among 70 diatom genera. In fact, Mastogloia taxa represented > 25 % of the total taxa found within the epiphytic taxocoenosis. Only 51 Mastogloia species or infra-specific taxa could be formally identified. In contrast 25 were left as cf. or sp. status (Table 1, Appendix). Petenes and Champotón, sampled during the dry season, yielded 38 and 52 Mastogloia taxa, respectively, while in Costa, which was sampled during the rainy season, 55 taxa were recorded. Mastogloia crucicula was the most abundant taxon (very abundant) in the 3 locations. Seven Mastogloia taxa had not been hitherto observed for the southern Gulf of Mexico region comprising the study area: Mastogloia angusta, M. belaensis, M. depressa, M. ovalis, M. pseudolacrimata, M. regula, and M. urveae. While 20 of the 76 identified constitute new records for de Gulf of Mexico, and 15 are new records overall for the Mexican coasts. Several of these taxa stand out, such as M. urveae (very abundant) and M. ovalis (abundant).
Table 1 List of species of Mastogloia recorded for the north coast of Campeche State.
| Class: Bacillariophyceae | |||
|---|---|---|---|
| Order: Mastogloiales | |||
| Family: Mastogloiaceae | |||
| Genus Mastogloia: Thwaites ex W. Smith, 1856 | Petenes | Champotón | Costa |
| M. angulata Lewis (Fig. 2a, b) | r | f | r |
| M. asperula Grunow ex Cleve (Fig. 10a) | r | f | |
| M. asperuloides Hustedt * (Figs. 9 c-e, 10 d, e) | f | r | f |
| M. bahamensis Cleve (Fig. 9a, b) | r | f | r |
| M. barbadensis (Geville) Cleve (Fig. 10j) | r | r | - |
| M. belaensis Voigt ** (Figs. 2c, d; 10b, c) | r | f | r |
| M. binotata (Grunow) Cleve (Fig. 9f, g) | f | r | r |
| M. biocellata (Grunow) G.Novarino & A.R.Muftah (Figs. 8c; 12c-f) | f | f | f |
| M. borneensis Hustedt (Fig. 8m) | r | - | - |
| M. cf. angusta Hustedt (NI) | f | f | f |
| M. cf. asperula Voigt (NI) | - | f | f |
| M. cf. asperuloides Hustedt (NI) | - | - | - |
| M. cf. braunii Grunow (Fig. 10f-i) | - | - | r |
| M. cf. borneensis Hustedt (Fig. 9h) | - | - | - |
| M. cf. corsicana (Grunow) Peragallo & Peragallo (NI) | - | r | - |
| M. cf. crucicula (Grunow) Cleve (Fig. 4p) | - | f | r |
| M. cf. delicatissima Hustedt (Figs. 10m, q; 13f, g) | - | - | - |
| M. cf. erythraea Grunow (Fig. 11f) | - | - | - |
| M. cf. foliolum Brun (NI) | r | - | - |
| M. cf. mauritiana Brun (NI) | r | - | - |
| M. cf. ovulum Hustedt (Fig. 2i, j) | - | - | - |
| M. cf. ovum-paschale (A.Schmidt) A.Mann (Fig. 3a, b, i, j) | - | - | r |
| M. cf. pulchella Cleve (NI) | - | - | r |
| M. cf. rimosa Cleve (Fig. 11a-c) | - | r | - |
| M. cf. staurophora Hustedt (Figs. 6a-c; 7h, i) | f | - | - |
| M. cf. tenera Hustedt (Fig. 8a, b) | f | f | r |
| M. cocconeiformis Grunow (Fig. 4a, b) | f | f | r |
| M. corsicana (Grunow) Peragallo & Peragallo (Figs. 2e, f; 4c, e) | a | a | a |
| M. corsicana (Grunow) Peragallo & Peragallo var. 1 (Fig. 11j, k) | - | r | r |
| Class: Bacillariophyceae | |||
| M. corsicana (Grunow) Peragallo & Peragallo var. 2 (Fig. 11l, m) | - | - | r |
| M. corsicana (Grunow) Peragallo & Peragallo var. 3 (Fig. 11o-s) | - | - | r |
| M. cribrosa Grunow (Figs. 5g, h; 12m) | f | f | f |
| M. crucicula (Grunow) Cleve (Fig. 4j-o) | va | va | va |
| M. cuneata (Meister) R.Simonsen (Fig. 5r-t) | - | r | r |
| M. cyclops Voigt ** (Fig. 4f, g) | - | f | r |
| M. decipiens Hustedt * (Fig. 11g) | f | f | f |
| M. decussata Grunow ** (Fig. 11n) | - | r | - |
| M. depressa Hustedt ** (Figs. 4 h-i; 13 c-e) | r | r | r |
| M. emarginata Hustedt (Fig. 5e, f) | - | r | r |
| M. erythraea Grunow (Figs. 5i-k; 12a, b, k, l) | - | - | r |
| M. erythraea Grunow for. 1 (Fig. 12g, h) | f | f | f |
| M. erythraea Grunow for. 2 (Fig. 12i, j) | - | - | - |
| M. erythraea var. grunowii Foged ** (Fig. 5a-d) | - | r | f |
| M. foliolum (Brun) A.Schmidt ** (Figs. 10r-t; 12n-q) | - | - | r |
| M. frickei Hustedt ** (Fig. 13a, b) | r | r | r |
| M. frimbriata (Brightwell) Grunow (Fig. 7a, b, d) | f | f | f |
| M. horvathiana Grunow (Fig. 6k, l) | f | r | r |
| M. ignorata Hustedt (Fig. 3c, d; 9k) | f | a | r |
| M. lanceolata Thwaites ex W.Smith (Figs. 3g, h; 8o, p) | r | f | f |
| M. mauritiana Brun ** (Fig. 13 j-l) | - | r | - |
| M. ovalis A.Schmidt ** (Figs. 5l-q; 13h, i) | a | f | f |
| M. ovata Grunow (Fig. 2g, h) | f | f | r |
| M. ovulum Hustedt (Fig. 10k, l, n-p) | r | r | r |
| M. ovum-paschale (A.Schmidt) A.Mann * (Fig. 8r, s) | f | f | f |
| M. parva Hustedt ** (Fig. 8n) | f | f | r |
| M. pseudolacrimata Yohn & Gibson** (Fig. 3e, f) | - | - | r |
| M. pulchella Cleve (NI) | - | r | - |
| M. punctifera Burn (Fig. 9l-o) | f | f | f |
| M. pusilla var. linearis Ostrup * (Fig. 11h, i) | - | - | f |
| M. pusilla (Grunow) Cleve var. pusilla Moreno *(Fig. 7q-t) | - | f | f |
| M. pusilla var. subcapitata Hustedt (Fig. 7m-p) | a | f | r |
| M. regula Hustedt ** (Fig. 6d, e) | - | - | - |
| M. rimosa Cleve** (Fig. 11d, e) | - | f | f |
| M. robusta Hustedt (Fig. 13m-p) | r | f | - |
| M. similis Hustedt ** (Fig. 7l) | - | r | - |
| M. sp.1 (Fig. 8q) | f | r | f |
| M. sp.2 (NI) | - | - | - |
| M. sp.3 (NI) | - | - | r |
| M. sp.4 (NI) | - | - | r |
| M. sp.5 (NI) | - | r | - |
| M. sp.6 (NI) | - | - | f |
| M. sp.7 (NI) | - | - | - |
| M. sp.8 (NI) | - | f | f |
| M. subaffirmata Hustedt (Fig. 8k, l) | f | f | f |
| M. urveae Witkowski ** (Fig. 6o-r) | a | a | f |
| M. varians Hustedt (Fig. 6f-j, m, n) | f | f | f |
Figure number and their occurrences are included. f: Frequent; r: rare; a: abundant; va: very abundant; -: absent; *: new record for the Gulf of Mexico; **: new record for the Mexican coasts; NI: no image.
Figure 3 Mastogloia cf. ovum paschale (a, b, i, j), M. ignorata (c, d), Mastogloia pseudolacrimata (e, f), M. lanceolata (g, h). Scale bars = 10 µm; e, f, have a different scale.
Figure 4 Mastogloia cocconeiformis (a, b), M. corsicana (c-e), M. cyclops (f, g), M. depressa (h, i), M. crucicula (j-o), Mastogloia cf. crucicula (p). Scale bar = 10 µm for all specimens.
Figure 5 Mastogloia erythraea var. grunowii (a-d), M. emarginata (e, f), M. cribrosa (g, h), M. erythraea (i-k), M. ovalis (l-q), M. cuneata (r-t). Scale bar = 10 µm for all specimens.
Figure 6 Mastogloia cf. staurophora (a-c), M. regula (d, e), M. varians (f-j, m, n), M. horvathiana (k, l), M. urveae (o-r). Scale bar = 10 µm for all specimens.
Figure 7 Mastogloia frimbriata (a, b, d), Mastogloia sp. 5 (c), Mastogloia sp. 3 (e-g), Mastogloia cf. staurophora (h, i), Mastogloia sp. 4 (j, k), M. similis (l), M. pusilla var. subcapitata (m-p), Mastogloia pusilla var. pusilla (q-t). Scale bar = 10 µm for all specimens.
Figure 8 Mastolgoia cf. tenera (a, b), M. biocellata (c), Mastogloia sp. 6 (d-j), M. subaffirmata (k, l), M. borneensis (m), M. parva (n), M. lanceolata (o, p), Mastogloia sp. 1 (q), M. ovum-paschale (r, s), Mastogloia sp. 2 (t). Scale bar = 10 µm for all specimens.
Figure 9 Mastogloia bahamensis (a, b), M. asperuloides (c-e), M. binotata (f, g), Mastogloia cf. borneensis (h), Mastogloia angusta (i, j), M. ignorata (k), M. punctifera (l-o). Scale bar = 10 µm for all specimens.
Figure 10 Mastogloia asperula (a), M. asperuloides (d, e), M. belaensis (b, c), Mastogloia cf. braunii (f-i), M. barbadensis (j), Mastogloia ovulum (k, l, n-p), Mastogloia cf. delicatissima (m, q), M. foliolum (r, s, t). Scale bar = 10 µm for all specimens.
Figure 11 Mastogloia cf. rimosa (a-c), M. rimosa (d, e), Mastogloia cf. erythraea; (f) M. decipiens (g), M. pusilla var. linearis (h, i), M. corsicana var. 1 (j, k), M. corsicana var. 2 (l, m), M. decussata (n), M. corsicana var. 3 (o-s). Scale bar = 10 µm for all specimens.
Figure 12 Mastogloia erythraea (a, b, k, l), M. biocellata (c-f), M. erythraea f. 1 (g, h), M. erythraea f. 2 (i, j), M. cribrosa (m), M. foliolum (n-q). Scale bar = 10 µm for all specimens.
Figure 13 Mastogloia frickei (a, b), M. depressa (c-e), Mastogloia cf. delicatissima (f, g), M. ovalis (h-i), M. mauritiana (j-l), M. robusta (m-p). Scale bar = 10 µm for all specimens.
New records for the Gulf of Mexico* and Mexican coasts**
The following descriptions and distributions of Mastogloia taxa were complemented following Witkowski et al. (2000), Hein et al. (2008), Loir and Novarino (2013), whose descriptions coincide with our observations in each taxon. Also, Guiry and Guiry (2016) AlgaeBase was consulted for distribution data.
*Mastogloia asperuloides Hustedt (Figs. 9c-e, 10d, e): valves lanceolate with cuneate to cuneate sub-rostrate apices, 14.1-31 μm long, 8.5-16 μm broad. Striae, 18/10 μm; partecta, 4/10 μm. Raphe slightly wavy, external central endings slightly bent in one side. Axial area very narrow, central area small, circular or transapically elongated. Distribution: reported in the Caribbean Sea, Canary Islands, Biscayne Bay, Puerto Rico, Samoa, and Baja California.
**Mastogloia belaensis Voigt (Figs. 2c, d, 10b, c): valves lanceolate with obtusely rounded apices, 55-92 μm long, 14-19 μm broad. Raphe undulating, axial area moderately broadly linear, at apices narrowed, central area rectangular connected to hyaline lateral areas. Transapical striae coarsely punctate radiate, at apices parallel, 13-14 in 10 μm, puncta 12-13 in 10 μm. Partectal ring displaced towards the raphe sternum, variable in size, 1.5 to 3 μm broad; 5 to 10 in 10 μm. Distribution: reported for brackish-water near Karachi in Pakistan and Moroccan coasts.
**Mastogloia cyclops Voigt (Fig. 4f-g): valves elliptic-lanceolate with slightly protracted apices, 25 -46 μm long, 11-13 μm broad. Raphe strongly wavy, axial area very narrow, central area small, asymmetrically semi-circular. Transapical striae parallel throughout, 20-22 in 10 μm, crossed by longitudinal, slightly wavy ribs. Partectal ring 1.5-2.0 μm broad, terminating just below apices, partecta transapically rectangular with inner margin slightly convex, 10 in 10 μm. Distribution: described from Japan
*Mastogloia decipiens Hustedt (Fig. 11g): valves elliptical with rostrate to rostrate-capitate apices, 20-33 μm long, 9.5-12 μm broad. Raphe straight to slightly undulating, external central endings approximate, axial area narrow, central area small, narrow, transapically elongated. Transapical striae parallel to slightly radiate towards the apices, 26-28 in 10 μm crossed by some equidistant longitudinal wavy lines, 14-17 in 10 μm. Partecta inseparable, equal, square to transapically rectangular, 7.5-9 in 10 μm, 1.3-1.8 μm broad, extending to a short distance from the apices. Inner margins flat to slightly convex. Distribution: France, China, Caribbean Sea, Mediterranean, Tanzania, Guadalupe, Seychelles Islands, Tahiti, and Baja California.
**Mastogloia decussata Grunow (Fig. 11n): valves lanceolate with slightly protracted broadly rounded apices, 70-130 μm long, 22-27 μm broad. Raphe slightly undulating, axial area very narrow, central area small ovoid or slightly transapically, widened. Transapical striae parallel, 26-30 in 10 μm, composed of 3 areolae row crossing each other. The number of decussate rows equals that of the transapical one. Partecta ring ca. 4 μm broad, terminates at some distance below apices; partecta uniform in shape, transapically elongated, rectangular with inner margin straight, 9-10 in 10 μm. Distribution: commonly found on the tropical coasts of the Indian and Pacific Oceans, Caribbean, Kuwait, Tanzania, Seychelles Islands, and Tahiti.
**Mastogloia depressa Hustedt (Figs. 4h-i, 13c-e): valves elliptical with subrostrate apices, 14-25 μm long and 6.1-10.3 μm broad. Raphe branches are very gently sinuous. Trasapical striae monoseriate, 20-28 in 10 μm, slightly radiate at center and strongly radiate at poles. Partecta are apically rectangular, gently convex on the free margin, more or less similar in shape and size (0.8-1.0 μm in width). Distribution: Mediterranean coasts, Philippines, and South China Sea.
**Mastogloia erythraea var. grunowii Foged (Fig. 5a-d): valves lanceolate to rhombic-lanceolate with cuneate to protracted apices, 23-89 μm long, 6-18 μm broad. Raphe strongly undulating, axial area very narrow, central area small asymmetric. Transapical striae in the middle parallel, but slightly radiate towards the apices, slightly convergent at the apex, 21-25 in 10 μm, crossed by slightly wavy longitudinal ribs, 16-20 in 10 μm. Partecta large, one at each margin of the valve, at opposite sides of the central area. Distribution: the littoral of Cuba (Although Witkowski mentions it as common in the Gulf of Mexico, we did not find any records).
**Mastogloia foliolum Brun in A. Schmidt (Figs. 10r-t, 12n-q): valves elliptical with more or less rostrate apices, 26.36 μm long, 12-14 μm broad. Raphe straight, axial area narrow. Central area transversely expanded into a widened fascia to about one half of the valve width. At both sides of the raphe, semi-lanceolate lateral areas cover about half of the valve surface Transapical striae within marginal areas, 25-27 in 10 μm. Partecta inseparable, sub-equal, square to rectangular, extending shortly below the apices. Groups of 3 to 5 central larger partecta, 4-5 in 10 μm. Distribution: reported for Biscayne Bay.
**Mastogloia frickei Hustedt (Fig. 13a-b): valves lanceolate with cuneate slightly protracted apices; 52-65 μm long, 12-16 μm in width. Raphe wavy, central area asymmetric; middle slightly large at only one side. Transapical striae 16-21 in 10 μm, radiate in the middle and convergent toward the apices, crossed by longitudinal wavy ribs. Partectal ring 1.5 μm broad continuing up to the apices; partecta rectangular to square, 8 in 10 μm. Distribution: common in the Caribbean and allegedly recorded from Campeche bay, although, we did not find any records.
**Mastogloia mauritiana Brun (Fig. 13j-l): valves lanceolate with rounded apices, 20-75 μm long, 11.5-22 μm broad. Raphe very undulated, external central endings slightly expanded. Axial area narrow, central area small. Transapical striae radiate, 21-24 in 10 μm, crossed by longitudinal lines, 17-20 in 10 μm. Partecta inseparable, sub-equal, extending up to a short distance from the apices. Distribution: Adriatic Sea, Australia, Colombia, Caribbean, France, Guadalupe, Mediterranean, Philippines, China, Tanzania.
**Mastogloia ovalis A. Schmidt (Figs. 5l-q, 13h-i): valves elliptical 9.5-22.5 μm long, 6.5-11 μm broad. Raphe straight or slightly sinuous, external central endings expanded, distant. Axial area narrow, central area narrow, apically elongated. Transapical striae radiate to strongly radiate at apices, 20-22 in 10 μm, composed of rounded areolae, 21-23 in 10 μm. A central group of 2 to 7 partecta inseparable, equal, apically elongated, 4.3-55.5 in 10 μm. Distribution: Albania, France, China, Guam, Caribbean Sea, Biscayne Bay, and Philippines.
*Mastogloia ovum-paschale (A. Schmidt) A. Mann (Fig. 8r-s): valves elliptical, 32-52 μm long, 21-30 μm broad. Raphe sinuous, external central endings expanded, moderately distant. Axial area very narrow, lined by 2 series of transapically elongated areolae. Central area transapically widened. Transapical striae radiate, 12-16 in 10 μm, composed of transapically elongated areolae, 8-10 in 10 μm. Partecta inseparable, equal, rectangular, transapically slightly elongated, 5-6 in 10 μm continuing up to the apices. Distribution: reported from Bahia de la Ventana Baja California, Caribbean Sea, Adriatic Sea.
**Mastogloia parva Hustedt (Fig. 8n): valves linear-elliptic with protracted apices, 20-23 μm long, 9-10 μm broad. Raphe straight, axial area very narrow, central area very small. Transapical striae in the middle parallel, towards apices becoming slightly radiate, 21-23 in 10 μm, crossed bay slightly sinuous longitudinal ribs. Partectal rings 0.75-1.0 μm broad, terminating close to the apices, 4-6 in 10 μm. Distribution: Australia.
**Mastogloia pseudolacrimata Yohn & Gibson (Fig. 3e-f): valves elliptical with acute apices, 53 μm long, 15 μm wide. Transapical striae coarsely punctate, middle parallel in the, becoming slightly radiate towards apices 15/10 μm, partecta 9/10 μm, with partectal ring ending close to the apices. Distribution: Bahamas.
*Mastogloia pusilla var. linearis Östrup (Fig. 11h-i): valves linear with parallel margins and rostrate ends, 40 μm long, 5-6 μm broad. Transapical striae 20-24 in 10 μm. Partectal ring terminates close to the apices, partecta variable in size, the largest partecta located in the middle, 4 in 10 μm, the other partecta smaller 6-8 in 10 μm. Distribution: wide, Australia, Gulf of California, Colombia, Danish coast, Tasmania.
*Mastogloia pusilla (Grunow) Cleve var. pusilla Moreno (Fig. 7q-t): valves linear-elliptic to lanceolate with broadly rounded apices, 15-35 μm long, 5-7 μm broad. Raphe straight, axial area very narrow, central area very small, lanceolate. Transapical striae in the middle parallel towards apices becoming radiate, 18-20 in 10 μm, crossed by longitudinal ribs, 24 in 10 μm. Partectal ring terminates close to the apices, partecta variable in size, the largest partecta located in the middle (usually 1-2), 4 in 10 μm, the other partecta smaller 6-8 in 10 μm. Gulf of California.
**Mastogloia regula Hustedt (Fig. 6d-e): valves linear with almost parallel margins and broadly rounded, 18-23 μm long, 4-7 μm broad. Raphe straight to slightly wavy, axial area very narrow, central area very small. Transapical striae parallel in the middle becoming slightly radiate at the apices, 20-22 in 10 μm. Partecta ring 0.5-1 μm wide terminating close to the apices, 8 in 10 μm. Distribution: Bosnia-Hertzegovina, Mediterranean coasts.
**Mastogloia rimosa Cleve (Fig. 11d, e): lanceolate valves, very acute, 37 μm long and 13 μm broad. Transapical striae parallel 11 in 10 μm. Axial area very narrow. Raphe straight. Partecta ring 2 μm broad, partecta small, rectangular transapically, 10 in 10 μm. Distribution: Bahamas, China, and Guam.
**Mastogloia similis Hustedt (Fig. 7l): valves linear-lanceolate with rostrate to rostrate-capitate apices, 29-39 μm long, 9-10.5 μm broad. Raphe straight to barely sinuous, external central endings. Axial area narrow, central area small. Transapical striae 30-32 in 10 μm. Partecta inseparable, rectangular apically elongated. Large central partecta 4.5-6.3 in 10 μm. Smaller partecta towards the apices, 8-9.5 in 10 μm, extending to some distance below the apices. Inner margins flat, external margins slightly convex. Distribution: Caribbean Sea, Borneo, Philippines, Kuwait, Seychelles, France, and China.
**Mastogloia urveae Witkowski (Fig. 6o-r): valves linear-lanceolate with rounded apices, 9.2-17.8 μm long, 3.6-4.8 μm broad. Raphe straight to barely sinuous, external central endings distant. Axial area and central area very narrow. Transapical striae parallel to barely radiate towards the apices, 25-29 in 10 μm. Partecta inseparable, a single large median partecta, 4-6 in 10 μm. Distribution: Gulf of Mexico (USA).
Discussion
Although differences occurred in the number of Mastogloia taxa from the 3 localities from the coast of Campeche, in each one they represented less than 30% of the recorded diatom taxa (López-Mejía, 2016), which may be serendipitous, rather than related to differences in the environment. This and the common substrate (T. testudinum) allows for considering the 3 localities as a homogeneous study area.
Overall, the Mastogloia species in this report comprised > 25% of the total taxa found within the epiphytic taxocoenosis living on Thalassia testudinum, collected in 3 dates solely in the Campeche area, and represents the highest species richness for this genus hitherto recorded in Mexican waters on the basis of >30 studies by the first author (López-Fuerte & Siqueiros-Beltrones, 2016; López-Fuerte et al., 2017; Siqueiros-Beltrones, 2002; Siqueiros-Beltrones et al., 2017a), and a previous checklist by Navarro and Hernández-Becerril (1997). This percentage is similar to those recorded elsewhere by Frankovich et al. (2006), and López-Fuerte et al. (2013), i.e., 24.6% and 27%, respectively. By comparison, Navarro and Hernández-Becerril (1997) compiled 89 Mastogloia taxa records from the highly diverse Caribbean Sea in a survey of 47 studies. However, only 30 species from said survey correspond to the Gulf of Mexico (Loir & Novarino, 2013). Another study carried out by the latter authors at the French Lesser Antilles yielded a similar number of species (> 80) but comprised a 7-year sampling effort. While, Hein et al. (2008) recorded 75 Mastogloia taxa from the Bahamas archipelago. Finally, the closest survey to the present one, carried out in the coast of Quintana Roo, around the continental division between the Gulf of Mexico and the Caribbean Sea, Mexico, yielded 30 Mastogloia species epiphytic on T. testudinum (López-Fuerte et al., 2013), out of which, 19 were not observed in our Campeche samples; whereas 23 taxa from our study were not observed in the Quintana Roo samples. These add to around 106 Mastogloia species and varieties between both localities, sharing approximately 40% of the taxa. However, neither sampling efforts may be considered exhaustive so as to suggest a definitive distribution.
The recent survey by López-Fuerte and Siqueiros-Beltrones (2016) showed that, out of the > 1,100 benthic diatom taxa ever recorded for Mexican waters, 73 Mastogloia species are included. Thus, considering that 30 of these taxa correspond to the Mexican Caribbean (López-Fuerte et al., 2013), only a little over 40 Mastogloia taxa from Mexican non tropical waters are left, mainly from the Mexican northwestern coasts. In summary, according to the present account, plus the latest observations (Hein et al., 2008; Loir & Novarino, 2013; López-Fuerte et al., 2013), it is more likely that Mastogloia species are more diverse and abundantly distributed in tropical and subtropical marine regions as proposed by Frankovich et al. (2006) than on a global perspective as suggested by Round et al. (1990).
Finally, the many (25) unidentified Mastogloia forms in this survey suggests the need for a formal taxonomic treatment of this taxon, inasmuch the available literature proved to be an insufficient reference for identifying all the specific and infra-specific taxa, including the 3 undetermined (informal) varieties of Mastogloia corsicana (Grunow) Peragallo and Peragallo and the 2 (informal) forms of Mastogloia erythraea Grunow recorded (Table 1, Appendix). In comparison, Loir and Novarino (2013) experienced the same difficulty in their more comprehensive survey, inasmuch they left 21 forms as unidentified species of Mastogloia, out of over 80 taxa. Likewise, Hein et al. (2008) left 22 forms unidentified out of 75 taxa.
This study corroborates that Mastogloia species are mostly distributed in tropical and subtropical habitats, and shows that much floristic and formal taxonomic studies are still required on this genus and overall for benthic diatoms from all types of substrata in general for the Mexican littorals.
