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Nemacystus is a genus in the family Chordariaceae (Order Ectocarpales, Class Phaeophyceae) and contains 8 species (Guiry & Guiry 2020). The type species of the genus is N. ramulosus Derbès & Solier. There are relatively few studies on Nemacystus, the most complete one being that of de Jong & Prud'homme (1997), who did a review of the genus based on morphological characters and presumed phylogenetic relationships. A molecular phylogeny, however, has not yet been carried out for this genus. Species differ from each other on such subtle morphological traits as the dimensions and shape of the assimilatory filaments, plurilocular sporangia, color, ramification, and general habit (Jong & Prud'homme 1997).
In North America only two species have been reported, Nemacystus brandegeei (Setchell & Gardner) Kylin on the Pacific coast (Pedroche et al. 2008, Norris 2010) and Nemacystus howei (W.R. Taylor) Kylin on the Atlantic coast. On the Atlantic coast N. howei has been reported and described from Florida, North Carolina, and Bermuda (Taylor 1928, 1960, Schneider & Searles 1975, 1991, Schneider 2003). In Brazil, this species was recorded by Cassano & Pedrini (1992), which was the first report for the Atlantic coast of South America. Menezes & Campos de Paula (2010) later reported it from the southeastern coast of Brazil. It was reported and described from Florida by Taylor (1928, 1960) and Dawes & Mathieson (2008); for North Carolina it was reported and described by Schneider & Searles (1991), and for the Caribbean islands of Aruba and Bonaire by Vroman & Stegenga (1988). For the Gulf of Mexico, Fredericq et al. (2009) reported N. howei as a synonym of “Eudesme howei” Taylor nom. inval., citing the report of Phillips & Springer (1960) for the Gulf coast of Florida. Finally, for the Atlantic coast of Mexico, Mateo-Cid et al. (2013) reported N. howei as a new record from the subtidal benthic macroalgae from the Campeche Banks.
Almost all the previous works with descriptions showed only illustrations of the species, especially of the assimilatory filaments and the plurilocular sporangia, except that of Cassano & Pedrini (1992), which also included photos of transverse sections of the thallus, depicting the central axis to be solid in younger parts and hollow in older parts. For the Atlantic coast of Mexico, however, despite being the first report, a description of the morphological characteristics or figures of this species has not yet been given for Mexico. So, this report is the first morphological descriptive account of this species in Mexico, and we provide the distribution of Nemacystus howei for the northern coast of the Yucatan Peninsula.
Materials and methods
Sampling was carried out on the northern coast of Yucatan during the dry season (with a 26.6 °C temperature average, 38 UPS and 8.8 pH) and the rainy season (with a 29.4 °C temperature average, 36 UPS and 7.9 pH ) at five localities: Ría Lagartos, 21° 38' 31.5'' N; 88° 10' 19.8'' W, San Felipe 5m (21° 37.080' N; 88°13.818' W), San Felipe 10 m (21° 39.344' N; 88° 16.344' W), Coloradas 10 m (21° 43' 17.2'' N; 87° 58' 187'' W), Chicxulub 10 m (21° 32.284' N; 89° 30.973' W), “Bajos 10” 10m (21° 21.210 N; 90° 09.125 W) and “Bajos 10” 5m (21° 20.870 N; 90° 08.940 W.) at Sisal reefs (Figure 1) from 2017 to 2019 at two depths (5 and 10 m) in the sublittoral, using SCUBA equipment. Specimens were separated, labeled, and preserved in a 4 % formaldehyde solution in seawater, silica gel and two dry vouchers were made from each sample, which are housed at the Herbarium Alfredo Barrera Marin of the University Autonomous of Yucatan (UADY). The determination was made using the keys and descriptions of Taylor (1960), Schneider & Searles (1991), Cassano & Pedrini (1992), and Dawes & Mathieson (2008).
Results
Six samples were collected at the five localities and each one contains a small mass of filamentous specimens of the species Nemacystus howei (W.R. Taylor) Kylin 1940, that belongs to: Kingdom Chromista, Phylum Ochrophyta, Class Phaeophyceae, Order Ectocarpales, Family Chordariaceae, Genus Nemacystus Derbès & Solier, 1850.
Basionym. Castagnea howei W.R. Taylor 1928: p. 111, Reported by Taylor 1960: p. 249, Earle 1969: p. 184, Dawes 1974: p. 107, Schneider & Searles 1991: p.138, Dawes & Mathieson 2008: p.145. Type locality: Dry Tortugas, Florida. Distribution in the Western Atlantic: The Carolinas, Florida, northern Gulf of Mexico, Gulf of Mexico, Brazil, Bermuda. Distribution on the northern coast of Yucatán Ría Lagartos, San Felipe, Coloradas, Chicxulub, and “Bajos 10” at Sisal reefs and Telchac (Figure 1). We observed N. howei to occur only in the dry season between March-June. It can be epiphytic or saxicolous, from 5 to 10 m depths (Table 1).
Table 1 Material examined.
| Date | Localities | Genus | species | I.D Number | Depth (m) |
|---|---|---|---|---|---|
| 06/05/2017 | Coloradas | Nemacystus | howei | UADY 1594 | 10 |
| 06/05/2017 | Ría Lagartos | Nemacystus | howei | UADY 1595 | 5 |
| 06/05/2017 | San Felipe | Nemacystus | howei | UADY 1596 | 5 |
| 06/05/2017 | San Felipe | Nemacystus | howei | UADY 1597 | 10 |
| 07/06/2019 | Chicxulub | Nemacystus | howei | UADY 1598 | 10 |
| 15/06/2019 | Bajos de 10 | Nemacystus | howei | UADY 1599 | 5 |
| 28/04/2017 | Bajos de 10 | Cladosiphon | occidentalis | UADY 1593 | 10 |
| 17/10/2010 | Telchac | Nemacystus | howei (reported as brandegeei) | UADY 1136 | - (Epiphyte of Sargassum) |
Examined Material. We collected six samples of specimens of Nemacystus howei (Table 1): UADY 1594 at Coloradas, UADY 1595 at Ría Lagartos, UADY 1596 at San Felipe, UADY 1597 at San Felipe, UADY 1598 at Chicxulub, UADY 1599 at “Bajos de 10” (Sisal Reefs).
Description. Thallus with a filamentous form, soft and mucilaginous entangled masses, up to 45 cm long, color light brown and somewhat greenish (Figure 2A-B). The axis is cylindrical, and the uniaxial structure is visible proximal to the apical cells. Branching is abundant, up to three orders, alternate to irregular. The internal structure is differentiated between a medulla (with 2-4 rows of colorless cells) and cortical cells. The main and older axes become hollow with a diameter between 0.3 - 0.6 mm (Figures 2C, 3A); the younger branches have a solid central axis, thin, cylindrical and often entangled, 0.1 - 0.3 mm in diameter (Figure 3C). The medullary cells are from 55-120 by 30-60 μm in diameter. The cortical cells are 12-25 by 12-45 μm in diameter.
Figure 2 Nemacystus howei. A) Habit, B) A pressed specimen, C) Transverse section with cortical cells (CC) ramified assimilatory filaments (AF) and showing sporangia and a hollow interior.
Figure 3 Nemacystus howei. A) Rows of medullary cells (MC) and cortical cells (CC), B) Transverse section showing a solid axis, C) Phaeophycean hairs (PH) and assimilatory filaments (AF), D) Plurilocular sporangia (PS).
The assimilatory filaments are not branched, 6-8(-13) cells long, with rounded moniliform or reniform cells, 10-15 μm near the apices and to 4-6 μm below. There are several discoid plastids per cell. These cells produce colorless hairs, 8-10 μm in diameter (Figure 3C). Unilocular sporangia were not observed. Plurilocular sporangia are uniseriate, arising from the base of the assimilatory filaments or directly from the cortical cells; 40-80 μm long with 8-23 loculi, which are 4-6 μm in diameter (Figure 3D).
Discussion
The review of the morphological descriptions of the specimens of Nemacystus howei by previous authors (Taylor 1960, Schneider & Searles 1991, Cassano & Pedrini 1992, de Jong & Prud’homme 1997, Dawes & Mathieson 2008), with respect to what was found in Yucatan, are very similar in size.
The reports of N. howei from the northern Gulf of Mexico, Florida, and Bermuda, showed a thallus length of approximately 45 cm, which is like our specimens, and with a diameter similar to those values for North Carolina specimens, as reported by Schneider & Searles (1991), who found specimens to be slender with main axes not exceeding a diameter of 0.5 mm. Also, our specimens were found at lower depths than other reports (Table 2 in Appendix 1).
Nemacystus howei (Figure 2A, 3C) has often been misidentified and easily confused with Cladosiphon occidentalis Kylin (Kylin 1940, Schneider & Searles 1975, de Jong & Prud’homme 1997) because of their very similar external morphology, ramification, and size (Figure 4A-B).
Figure 4 Cladosiphon occidentalis. A) Habit, B) Branch, C) Transverse section of C. occidentalis with cortical cells (CC) ramified assimilatory filaments (AF) and plurilocular sporangia (PS).
We found C. occidentalis (UADY 1593) at one locality (“Bajos de 10” at Sisal Reef) (Table 1), and it was necessary to make transverse sections to verify the identity. The main differences are the solid medulla of the younger branches and more medullary cells in the older branches of N. howei, whereas the cortex in C. occidentalis is formed of only one row of cells. The assimilatory filaments in C. occidentalis have a pedicel and then bifurcate two or three times; also, the sporangia are formed at the end of the filaments (Figure 4C), whereas in N. howei the filaments are uniseriate and the sporangia arise at the base of the filaments or directly on the cortical cells (Figure 3A).
At the UADY herbarium we found a single collection that had been identified as N. brandegeei (Setchell & Gardner) Kylinin Telchac, Yucatan (Figure 1), but it was misidentified and turned out to be N. howei (Table 1). There were no vouchers of Cladosiphon besides the ones that we collected.
Even though the external morphologies of these two species are very similar, it is relatively easy to distinguish them when their internal characteristics are checked. Their external resemblances have likely resulted in past confusion in their accurate determinations. Also, the paucity of specific works on the Phaeophyceae for the Yucatan Peninsula is another probable explanation why this is the first report of N. howei for the Yucatan Peninsula.
