Exploratory floristics of epiphytic diatoms from Revillagigedo Islands (Mexico)

Florística exploratoria de diatomeas epifitas de Islas Revillagigedo (México)

Siqueiros Beltrones, David A.^1*, Yuriko Jocselin Martínez1 & Alejandro Aldana-Moreno^{2, 3}

¹ Departamento de Plancton y Ecología Marina, ² Departamento de Pesquerías y Biología Marina, Centro Interdisciplinario de Ciencias Marinas-Instituto Politécnico Nacional. Av. IPN S/N, Col. Playa Palo de Santa Rita, La Paz, BCS. 23096. ³ Pelagios Kakunjá, Conservación Marina A.C., Laboratorio de Biotelemetría y Ecología de la Megafauna Marina. Email: dsiquei@gmail.com)

^*Corresponding author: email: dsiquei@gmail.com

Siqueiros Beltrones, D.A., Martínez, Y.J. & A. Aldana-Moreno. 2019. Exploratory floristics of epiphytic diatoms from Revillagigedo Islands (Mexico). Cymbella 5(1): 98-123. http://cymbella.mx

Introduction.

The high floristic potential of benthic diatoms for Mexican littorals has been recently confirmed. Studies in hitherto unexplored areas and the surveying of new substrata have yielded numerous new records (López-Fuerte et al. 2015; Martínez & Siqueiros-Beltrones 2018; Siqueiros Beltrones et al. 2017; Siqueiros Beltrones & Martínez 2017) that have enriched the current overall species list for the region (López-Fuerte & Siqueiros Beltrones 2016). This may be related to the great variety of substrata potentially used by benthic diatoms, which includes both live and inert surfaces. Among the former, many animals harbor high diversities of diatoms, but the surfaces of macroalgae provide by far a higher species richness of epiphytic diatoms which surpasses several hundreds of species. In spite of the latter, many macroalgae taxa are still to be surveyed for epiphytic diatom floristics.

In Mexico, most research on the subject has been carried out for the NW region and, although these are scarce there are even fewer for the tropical zones. Consequently, for the southern Gulf of Mexico only one ex profeso study on epiphytic diatoms of rhodophytes has been published. This exploratory study yielded 115 epiphytic diatom taxa from six rhodophyte species, that included six new records for Mexican waters (Siqueiros Beltrones & Martínez 2017). Thus, the possibility of observing substrata from remote tropical localities such as Islas Revillagigedo promises an increase in the number of new records of benthic diatoms for the Mexican littorals and the likeliness of new species. Likewise, it opens expectations on the particularities of benthic diatoms taxocoenoses as well demystification of the alleged endemism for the islands as marked by the Comisión Nacional de Areas Naturales Protegidas and the Secretaría de Medio Ambiente y Recursos Naturales, of Mexico (CONANP-SEMARNAT 2015).

Four islands make up the Revillagigedo archipelago, San Benedicto, Socorro, Roca Partida and Clarion (UNESCO 2018). Under the jurisdiction of the state of Colima, Mexico they are located 386 km south of Cabo San Lucas, Baja California Sur. Although on July 2016 the UNESCO declared it World Heritage Site, only recently (CONANP 2017) was there a previous study issued for justifying its declaration as a national park. In spite of the comprehensive character of the above study, much research is yet to be done in the archipelago, inasmuch its remote location contributes to the conservation of multiples habitats and distinct substrata used by benthic fauna and flora that are yet to be described.

A previous report on diatoms from the archipelago (Hanna & Grant 1926) deals with fossil forms. In this study, we focused on epiphytic diatoms living on red macroalgae, inasmuch the surface of macroalgae and particularly rhodophytes have been observed to harbor many diatom taxa (Siqueiros Beltrones & Argumedo Hernández 2014; Siqueiros Beltrones & Hernández-Almeida 2006; Siqueiros Beltrones & Martínez 2017). The benthic macroalgal flora for Revillagigedo Islands comprises at least 190 taxa, including 123 rhodophytes (Serviere-Zaragoza et al. 2007), although as much as 205 taxa (134 rhodophytes) have been identified (León-Tejera et al. 1996).

Seasonal variations in the region seem to be determined by the alternating influence of the California Current and the North-equatorial Current, whilst the rest of the year a transition between both states may be observed (Lluch-Cota et al. 1994). These environmental characteristics lead us to propose a hypothesis on the biogeographical affinity of the epiphytic diatom taxocoenosis, which in this case promised to be strongly tropical although with important temperate components.

Because the remoteness of the archipelago largely precludes access to the islands habitats in order to carry out a comprehensive sampling that is representative of the target taxa, we hoped that a small sample consisting of two macroalgae specimens collected ex profeso would provide an approximate idea of the species richness potential of the rhodophyte epiphytic diatoms for the islands. Thus, our objective was to construct an exploratory floristic list of epiphytic diatoms of rhodophyte macroalgae for the Revillagigedo archipelago that comprised as many taxa as could be surveyed to support further formal taxonomic and hypothesis driven studies on diatom floristics, ecology, and biogeography for the Revillagigedo Archipelago.

MATERIAL AND METHODS.

Two red algae specimens were collected manually by Scuba during the December 27-January 4, 2018 period at the rocky shore of San Benedicto Island (Revillagigedo Archipelago) at 15-25 m depth (Map 1). The specimens were sun-dried, transported in a plastic ziploc bag and identified in the laboratory following Abbot & Hollenberg (1976). Diatoms were brushed off from each specimen of red algae while rinsing with purified water. The resulting sample was placed in a 150-ml test tube and left to settle. Then, the sample precipitate was collected and oxidized with a mixture of commercial alcohol and nitric acid at a ratio of 1(sample): 2 (alcohol): 5 (acid), (Siqueiros Beltrones 2002). The oxidized material was rinsed repeatedly with purified water until it reached a pH ≥ 6. For each sample two double permanent slides were mounted using Pleurax (RI=1.7). Identification was done at 1000× under an Olympus CH-2 compound microscope with phase contrast illumination. Diatoms were identified following Cleve-Euler (1952, 1953a, 1953b, 1955), Desikachary (1988, 1989), Desikachary & Prema (1987), Desikachary et al. (1987), Foged (1975, 1978, 1984), Hernández-Almeida et al. (2013), Hernández-Almeida and Siqueiros-Beltrones (2008, 2012), Hustedt (1955, 1959, 1961-66), Joon et al. (2018), Hein et al. (2008), Loir and Novarino (2013), López-Fuerte et al. (2010), Moreno et al. (1996), Peragallo and Peragallo (1908), Siqueiros-Beltrones (2002, 2006), Siqueiros-Beltrones and Hernández-Almeida (2006), Siqueiros-Beltrones et al. (2014), Schmidt et al. (1874 -1959), Stepanek et al. (2016), Stidolph et al. (2012), Van Heurck (1881), Witkowski et al. (2000). Taxonomic status was updated according to the AlgaeBase web site (Guiry & Guiry 2018), and Round et al. (1990). A catalog of the recorded epiphytic diatoms was assembled with micrographs of specimens taken with a CMOS Konus digital ocular lens microscope at 1000×.

Introduction.

RESULTS.

A total of 208 (specific and infra-specific) epiphytic diatom taxa were identified in the samples from both specimens of Laurencia collected at Revillagigedo Islands; sixteen of these taxa could not be identified to species level and may represent new taxa. Also, 52 taxa are new additions to the diatom flora of the Mexican coasts and three to the Mexican Pacific coast: Cocconeis caribensis Romero et Navarro, Mastogloia affirmata (Leudiger-Fortmore) Cleve, and Synedrosphenia cuneata (Grunow) Peragallo (Table 1, Figs. 1-366). The genus with most species and infra-specific representatives was Mastogloia with 27 taxa.

DISCUSSION.

Previous floristic studies of benthic diatoms in unexplored areas of the Mexican littorals and the survey of new substrata have yielded numerous new records that have recently enriched the species inventory for the region (Estrada-Gutiérrez et al. 2017; López-Fuerte et al. 2016; López-Fuerte & Siqueiros Beltrones 2016; Martínez & Siqueiros Beltrones 2018; Siqueiros Beltrones & Argumedo-Hernández 2014; Siqueiros Beltrones & Martínez 2017; Siqueiros Beltrones et al. 2017). In this way, the first new diatom taxon for the Gulf of California was recently proposed: Halamphora primus López-Fuerte et Siqueiros Beltrones was collected from the skin of stone scorpion fish (López-Fuerte & Siqueiros Beltrones 2018). Thus, the survey of substrates from remote localities such as Islas Revillagigedo (Mexico) was expected to increase the number of taxa and to demystify their endemic potential. Said expectations are here supported by the new records of known diatom species, as well as the sixteen likely new taxa observed.

Our exploration yielded one of the highest species richness (S=208) ever recorded for two samples of epiphytic diatoms. In comparison, a single specimen of Ploclamium cartilagineum from the western coast of the Baja California Peninsula, heavily colonized with diatoms, yielded 46 taxa (Siqueiros Beltrones & Argumedo Hernández 2014), while in over eight samples of six rhodophyte taxa from southeastern (tropical) Mexico 115 taxa were identified (Siqueiros Beltrones & Martínez 2017). Another survey still, recorded 143 diatom taxa from ten specimens of Laurencia pacifica in five different dates (Siqueiros Beltrones & Hernández-Almeida 2006). Moreover, a much similar study carried out in Isla Guadalupe off Baja California, where transitional oceanographic conditions are characteristic, yielded less (119) taxa (including 11 % new records for Mexican littorals), although in many more samples (López-Fuerte et al. 2015). There, Mastogloia species, considered mostly of tropical affinity, comprised the higher number of species (13), whilst in the Revillagigedo Islands twice the number of species (27) of this genus were present. Thus, although the Revillagigedo Islands are located also within a transitional zone influenced by the California Current (CONANP-SEMARNAT 2015), the tropical component of the archipelago region does seem to be a major factor enhancing the species diversity of epiphytic diatoms.

Only one taxon, Grammatophora merletta Hanna & Grant, was recognized as previously recorded from the Revillagigedo Archipelago by Hanna and Grant (1926) who made a survey on diatoms from the Miocene. On the other hand, most of the identified diatom taxa in Revillagigedo islands have been recorded elsewhere in the Mexican Pacific, particularly in the NW littorals that harbor many temperate forms under transitional oceanographic conditions (López-Fuerte & Siqueiros Beltrones 2016). However, representatives of tropical affinity were also observed, such as the Mastogloia specific and infra-specific taxa (27). Plus, other tropical elements that were recorded previously only from the Gulf of Mexico such as Cocconeis caribensis, Mastogloia affirmata, and Synedrosphenia cuneata, and now recorded for the first time in the Mexican Pacific. This agrees with our working hypothesis. However, over 25 % of the taxa are new records hitherto not documented for Mexican littorals (López-Fuerte & Siqueiros Beltrones 2016; Martínez & Siqueiros Beltrones 2018; Siqueiros Beltrones & Martínez 2017; Siqueiros Beltrones et al. 2017) and whose biogeographic affinities are uncertain.

Our results may serve both to demystify the expected endemism and high species diversity of benthic diatoms in the archipelago, inasmuch for the latter the high species richness observed backs said assumption, but most of the recorded taxa are actually distributed worldwide. By contrast, the fact that ten taxa could not be identified after an exhaustive search leads us to infer that these represent new taxa that need formal taxonomic descriptions.

These results, derived from the few samples examined, suggest a likely occurrence of more biogeographycally isolated diatom taxa in the islands. As in the case of Grammatophora monilifera Tempere & Brun which has been considered solely as a fossil species (Guiry & Gury 2108) from Japan (Kociolek et al. 2019). However, although Desikachary (1987) recorded it (single image) in sediments from the Indian Ocean region, it was observed commonly in our samples.

In spite of the low number of hosts surveyed, this first floristic list of epiphytic diatoms of rhodophytes may serve as a reference for upcoming formal and hypothesis driven studies on diatom floristics, taxonomy, ecology and conservation, and biogeography for the Revillagigedo Archipelago. Undertaking the wanting diatom floristics may lead to expect finding over 500 epiphytic taxa on the 190 macroalgae taxa documented (Serviere-Zaragoza et al. 2007) in the archipelago, including a significant number of new and, likely, endemic diatom taxa that should require formal taxonomic treatment.

ACKNOWLEDGEMENTS.

The crew of Pelagios Kakunjá, Conservación Marina, A. C. and SOLMAR V provided support for collecting the macroalgae samples. Alejandra Pinón Gimate confirmed the identification of our rhodophyte specimens. The first author (DASB) is fellow of Comisión de Fomento a Actividades Académicas (COFAA) and Estímulos al Desempeño a la Investigación (EDI), Instituto Politécnico Nacional. The second (YJM) and third (AAM) authors received a postgraduate scholarship from CONACYT. Reviews by two anonymous referees are greatly acknowledged.

REFERENCES

Revisores 2 revisores anónimos
Sometido: 21 de enero de 2019
Revisado: 6 de agosto de 2019
Corregido: 8 de agosto de 2019