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Introduction
Diatoms make up one of the most ecologically important and productive groups of microalgae in aquatic ecosystems (MacIntyre et al., 1996; Martínez et al., 2013; Popovich & Guinder, 2013). Nevertheless, although being considered one of the better-known primary producers (Jeffrey et al., 1997), current research has shown that much is yet unknown considering taxonomy alone. In this way, floristic studies of benthic diatoms along the littorals of the Gulf of California have become more comprehensive, even though most include only localities from the Baja California Peninsula (López-Fuerte et al., 2010; Siqueiros-Beltrones et al., 2011; Siqueiros-Beltrones & Argumedo-Hernández, 2015; López-Fuerte & Siqueiros-Beltrones, 2016; Siqueiros-Beltrones et al., 2017; Martínez & Siqueiros-Beltrones, 2018), while other regions such as the continental coasts have been hitherto neglected.
In accordance with the above, in the protected area known as Reserva de la Biosfera Marismas Nacionales, Nayarit (RBMNN) the study of diatoms is still incipient (Estrada-Gutiérrez et al., 2017; Hernández-Almeida et al., 2019). This area exhibits a great variety of habitats, including alluvial plains, coastal lagoons, estuaries, and mangrove forests (Blanco-Correa et al., 2011), which are used as a refuge, feeding and reproduction area of birds, mammals, reptiles, amphibians, giving to the area a great biological relevance (CONANP, 2013).
Thus, hypothesized that a high taxonomic diversity of benthic diatoms is to be found in the area, as it has been recorded in other parts of the Mexican region (Siqueiros-Beltrones, 2002; Siqueiros-Beltrones et al., 2017), where a large part of the rich diatom flora has yet to be discovered, including possible new records of diatom taxa.
Recently, in the mangrove systems of RBMNN, specifically in areas of production and fishing of the pleasure oyster Crassostrea corteziensis (Hertlein, 1951) and mullet Mugil curema (Valenciennes, 1836), the survey for benthic diatoms yielded a significant number of specimens of Cymatodiscus planetophorus (Meister) Hendey 1958, hitherto unrecorded for the Mexican region. Therefore, this study aimed to provide the first record of this taxon for Mexican coasts, particularly in the NW littorals. Meristic and morphometric traits were used to successfully discriminate Cymatodiscus planetophorus from Cymatotheca weissflogii (Grunow) Hendey 1958 using optic microscopy. Complementary ecological remarks on C. planetophorus are provided in order to discuss its ecological potential.
Material and Methods
Study area
Samples were collected at two estuarine flow wetlands fringed by mangroves (Figure 1) located within the RBMNN: 1) Pozo Chino (21°42'10.87" N, 105°28'0.95" W), a narrow and shallow (< 6 m) estuary, used in oyster culture; 2) El Rey (21°33'51.30" N, 105°19'30.30" W), which is part of a larger estuary used for fishing, that varies from 1-9 m in depth (Blanco-Correa et al., 2011; SAGARPA-CONAPESCA, 2010).
Samplings
At Pozo Chino estuary, samples were obtained monthly from different substrates from August 2017 to July 2018. Randomly, 120 oysters of Crassostrea corteziensis from suspended strings used in the commercial culture, were collected. Shells were removed from 10 oysters and their soft tissue was placed together in 10 % formaldehyde (one composed-sample per month). Likewise, 12 sediment samples were collected (one per month) by scuba using a PVC nucleator (2 cm height × 5 cm diameter), placed in plastic bags, and transported in ice to the laboratory. Simultaneously, 12 surface water samples (one per month) were collected using 1000 mL PET bottles and fixed with 4 % formaldehyde. Only the shells of the oysters collected in July were transported to the laboratory. Surface temperature and conductivity were measured using a HOBO ® U24 sensor only from November 2017 to July 2018. At El Rey estuary 4 specimens of mullet Mugil curema were gathered from the commercial trade (October 2016) and transported in ice to the lab. No environmental observations were made.
Sample processing
Contents from the digestive tracts of oysters and fish were retrieved by dissection. Water samples had scarce diatoms so monthly samples were combined for phytoplankton analysis. From the inverted PVC nucleator sediments of the first 0.5 cm were separated with a spatula. The convex surface of each oyster shell was brushed (toothbrush) while rinsing with distilled water. Frustules of diatoms present in samples of the digestive tract of oysters and fish (diatoms in the digestive tract), phytoplankton (planktonic diatoms), sediment (epipelic diatoms) and shell surface (epizoic diatoms), were cleaned using a modification of Siqueiros-Beltrones & Voltolina (2000) technique, in which organic matter was oxidized with nitric acid, sulfuric acid, and commercial alcohol at a ratio of 3:2:1. The oyster shells brushed-off material and sediment samples were first treated with 10 % hydrochloric acid to eliminate calcium carbonates. Samples were then rinsed with distilled water until reaching a minimum pH of 6.
Once cleaned, frustules of diatoms were mounted into permanent slides in Pleurax ® (I.R. = 1.7). These preparations were used for qualitative and quantitative analysis of diatoms under a Carl Zeiss GmBh microscope ® at 100×, 400× and 1000× equipped with phase contrast illumination and integrated camera. In each permanent slide a route was made that began in one of the margins to the opposite end and microphotographs of 50 specimens of C. planetophorus were taken. Each specimen was measured in valval view with program Photoshop CS6 ® using a calibrated-programed scale, calculated from micrographs captured in a calibration slide (Motic®) with the 0.07 mm magnitude at 1000×. Thereby, data were collected from the main meristic (areolas and fultortulas in 10 μm) and morphometric (length, width, shape of the valve) characters of the species. Finally, to estimate the relative abundance of C. planetophorus in the diatom assemblages from the different type of substrate, 500 valves were counted overall, except in the phytoplankton samples.
La identificación de C. planetophorus se tuvo como base la descripción original de Hendey (1958: Lám. V, Figura 8) y otras referencias: Desikachary et al. (1987: Lám. 265, Figura 16, 371), Ricard (1987: 162, Figura 154), Foged (1975: Lám. VII, Figura 10-11), Paulmier, (1993: Lám. 35, Figura 7-8), Tremarin et al. (2008: 1107, Figura 1-2), y Lehmkuhl et al. (2010: 320, Figura 26). También se consultó la base de datos en www.algaebase.org (Guiry & Guiry, 2021) para actualizar la nomenclatura taxonómica.
Results
Cymatodiscus planetophorus (Meister) Hendey 1958 (Figures 26)
Meristics and morphometrics characteristics
Valve elliptic, slightly depressed in the central area. Size range was 16.4-22.9 µm length, 12.1-16.6 µm wide, with 15-20 areolae in 10 µm and 1-3 fultoportulae in 10 µm (Table 1). The smallest specimens of C. planetophorus (Figure 2) were found on the shells of oysters, and the larger (Figure 6) in the sediments and plankton. Plankton specimens had the lowest number of areolae (15 areolae in 10 µm). And only one specimen found on an oyster shell showed 3 fultoportulae in 10 µm (Figure 3). Other distinctive traits of this taxon, that also discriminate between C. planetophorus from C. weissf logii, were a hyaline zone in the center of the valve (Figure 4), linear areolae becoming smaller towards the valval margins (Figure 5), and marginal ring of fultoportulae (Figure 6). These morphological characteristics differentiate C. planetophorus from C. weissflogii (Figures 7, 8).
2) Internal valve view, showing the smaller size in this study. 3) Internal valve view, showing three fultoportulae (arrows). 4) External valve view, showing the hyaline zone in the center of the valve (arrow). 5) Detail of the linear areolae becoming smaller towards the valval margins (arrows). 6) External valve view, showing the marginal ring of fultoportulae (arrows) and the bigger size to this study. Scale bar = 10 µm.
Figures 2-6 Various sizes of Cymatodiscus planetophorus.
8) Internal valve view of Cymatodiscus planetophorus showing the morphological characteristics that make it different from C. weissflogii (arrows); 1) hyaline central zone with 1 to 3-4 areolae; 2) radial areolae that become smaller towards the valve margins; 3) marginal ring of fultoportulae, usually two at each pole and two on the margin equidistant from the poles Scale bar = 10 µm.
Figures 7-8 Internal valve view of Cymatotheca weissflogii.
Ecological remarks
The specimens of C. planetophorus were collected in the shallow coastal zone (3-9 m) of both wetlands of Nayarit, as epipelic, epizoic, and planktonic, as well as in the digestive tract of C. corteziensis and M. curema (Table 1). At Pozo Chino estuary the taxon was present during all sampling periods at temperatures between 23.3-31.8 ºC and a salinity range of 21.1-33. The highest abundances were reached in October 2017 and March 2018 in the sediment at temperature intervals between 26.5-31.8 ºC and salinity of 25.8-33. At El Rey estuary, C. planetophorus ocurred in October 2016 in the digestive tract of M. curema (Figure 9).
Discussion
The first-ever description of C. planetophorus comprised specimens from the coast of Sierra Leona, Africa, but ever since, this taxon has been recorded in other tropical regions. For the Mexican region, a single potential record of the genus CymatodiscusHendey 1958 (Siqueiros-Beltrones & Hernández-Almeida, 2006) shows a single image of a specimen found as epiphyte of Laurencia johnstonii Setchell & N.L. Gardner 1924. However, said image does not show the details clearly enough, so there is no certainty on its identification. On the other hand, two previous studies on diatoms carried out in the area (RBMNN) specifically at Boca of Camichín estuary by Estrada-Gutiérrez et al. (2017) and Hernández-Almeida et al. (2019), did not record the presence of C. planetophorus. However, it should be noted that, in these studies, only samples of intestinal contents of C. corteziensis were inspected, unlike in this study that other types of samples (phytoplankton, oyster shells, sediment) were considered. Likewise, our results show that the highest abundances of C. planetophorus were recorded in sediments, which had no correspondence with the abundances recorded in the digestive tract of oysters (Figure 9). Therefore, the fact that C. planetophorus was not found at Boca de Camichin could be related to the type of sample that was used and even to the food preferences of oyster. Kasim & Mukai (2009) determined that this bivalve have in situ food preference for some species of microalgae.
In relation to the taxonomic difficulties facing this taxon, C. planetophorus was initially determined as Coscinodiscus planetophorus Meister (Hendey, 1958), which in turn was frequently mistaken for Coscinodiscus asymmetricus Meister. Detailed examination of both species resulted in the determination of distinct genera for these taxa: Cymatodiscus planetophorus (Meister) Hendey 1958 and Cymatotheca weissflogii (Grunow) Hendey 1958. Both have an elliptic valve and are similar in size (Table 1); however, characteristics that separate C. planetophorus from C. weissflogii are hyaline central zone with 1 to 3-4 areolae, radial areolae that become smaller towards the valve margins and a marginal ring of fultoportulae, usually two at each pole and two on the margin equidistant from the poles (Figures 7, 8). These meristic and morphometrics differences are clearly distinguishable under light microscopy and allow to discriminate between the two taxa with certainty.
Table 1 Comparison between meristic and morphometric traits of Cymatodiscus planetophorus collected in the wetlands of Nayarit, Mexico and other studies
| Measurements (min-max) | |||||
|---|---|---|---|---|---|
| Reference | Type of sample | Length (µm) | Width (µm) | Areolae (10 µm) | Fultoportulae (10 µm) |
| Hendey (1958) | Sediment | 24 | 16 | 6-12 | 1-2 |
| Bainbridge (1963) | Plankton | 17-24 | n.d. | n.d. | n.d. |
| DT/Echinoderm | 17-24 | n.d. | n.d. | n.d. | |
| Paulmier (1993) | Plankton | 25 | 17 | 11-12 | n.d |
| Tremarin et al. (2008) | Plankton | 17.4-26.1 | 12.6-17.4 | 11-16 | 2 |
| Epiphyte | 17.4-26.1 | 12.6-17.4 | 11-16 | 2 | |
| Lehmkuhl et al. (2010) | Plankton | 12-26 | 10-22 | 12-20 | 1-2 |
| This study | Sediment | 17.3-22.9 | 12.1-16.6 | 17-20 | 2 |
| DT/Oyster | 16.5-22.8 | 12.1-15.5 | 16-19 | 1-2 | |
| DT/Fish | 16.9-21.9 | 12.3-14.2 | 17-20 | 2 | |
| Plankton | 17.3-22.8 | 12.6-15.1 | 15-20 | 2 | |
| Epizoic | 16.4-21.1 | 12.2-14.8 | 17-20 | 2-3 | |
| C. weissflogii | 18-25 | 16-19 | 11-12 | 1-2 | |
n.d. = undetermined, DT = digestive tract content
The ranges in morphometric and meristic values of the examined specimens in this study agree with those reported elsewhere (Table 1), although no specimens as small (12 µm in length) as those reported by Lehmkuhl et al. (2010: 320, Figure 2-6) were found, nor as big (23-30 µm in length) as in Hendey (1958: Plate V, Figure 8), Paulmier (1993: Plate 35: Figure 7-8), or Tremarin et al. (2008: 1107, Figure 1-2).
Most reports on the habits of C. planetophorus refer it as a planktonic form, e.g.,Bainbridge (1963), Desikachary et al. (1987), Ricard (1987), Paulmier (1993), Lin & Yan (2007), Tremarin et al. (2008), Lehmkuhl et al. (2010), Costa-Böddeker et al. (2016) and Parizzi et al. (2016). But it has also been reported as epipelic (Hendey,1958; Sylvestre et al., 2004; Costa-Böddeker et al., 2016), and as epiphyte (Foged, 1975; Sylvestre et al., 2004; Tremarin et al., 2008)., Sylvestre et al. (2004) considered C. planetophorus a benthic form, as well as Parizzi et al. (2016), even though they collected it in plankton samples; whilst Costa-Böddeker et al. (2016) deemed it thycoplankton. Our observations confirm the benthic/thycoplanktic habits of C. planetophorus, and as a component in the diet of benthic feeding macrofauna, e.g., the sea cucumber Holuthurian sp. (Foged, 1975) and a fish, Ethmalosa fimbriata Browdich (1825) (Bainbridge, 1963). In the present study M. curema a mullet and C. corteziensis an oyster were added to the list of species that feed on C. planetophorus. Thus, assessment of its nutritional importance to commercial species should be considered, along with the ecological issues concerning this diatom species.
According to the recorded temperature range (23.3-31.8 ºC), which like that reported by Bainbridge (1963), Costa-Böddeker et al. (2016) and Parizzi et al. (2016), C. planetophorus is considered of tropical affinity (Valente-Moreira & Moreira-Filho, 1981; Ricard, 1987). Likewise, on the basis of salinity measurements, Valente-Moreira & Moreira-Filho (1981) and Ricard (1987) proposed that C. planetophorus was a brackish-water form. However, it has also been recorded at salinities of 14.9 (Hendey, 1958), 0-14 (Tremarin et al., 2008) and 0.2-15 (Lehmkuhl et al., 2010). Thus, although C. planetophorus seems to tolerate a wide range of salinities, our observations (21.1-33) as in Costa-Böddeker et al. (2016) and Parizzi et al. (2016) fall within the range of brackish-marine conditions.
Conclusions
This is the first record of C. planetophorus from Mexican coasts and North America overall. It widens the distribution range of this taxon, inasmuch until now it was recorded only for South America (Paulmier, 1993; Sylvestre et al., 2004; Tremarin et al., 2008; Lehmkuhl et al., 2010), Asia (Desikachary et al., 1987; Lin & Yan, 2007; Khustina et al., 2014; Costa-Böddeker et al., 2016), and Africa (Hendey, 1958; Bainbridge, 1963; Foged, 1975; Parizzi et al., 2016). This could be more related to the lack of comprehensive floristic studies worldwide than to other important taxonomic or biogeographical issues.
The confirmed benthic/thycoplanktic habits and brackish/marine water distribution of C. planetophorus in tropical environments provides a basis for further studies focusing on the ecological importance of this taxon for fisheries management policies in these environments. Hence, it is shown that floristic studies of benthic diatoms in Mexican coasts in general are mandatory, inasmuch they unveil taxonomic problems such as the one reported here that underlines relevant ecological implications.
