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Species identified: Aulacodiscus petersii Ehrenberg 1845

Genus: Aulacodiscus Ehrenberg, 1844, nom. cons.
Taxonomic classification:

Phylum: Heterokontophyta
Subphylum: Coscinodiscophytina
Class: Coscinodiscophyceae
Order: Coscinodiscales
Family: Coscinodiscaceae
Genus: Coscinodiscus (Guiry and Guiry 2023)

Type species: Aulacodiscus crux Ehrenberg 1844: 76 (as 'Crux')

Genus Summary:

Aulacodiscus is a marine and fossil diatom associated with coastal sediments, sands and surf zones. It is reported to form massive blooms in the surf zone, such as Oregon, California, Australia, northern Chile during certain years Lewin, 1974, Holmes and Mahood, 1980, Lewin and Schaefer 1983, Campbell 1996, Rivera et al. 2016). A mainly fossil genus (Cretaceous) exhibiting a large range of valve and rimoportula shapes. Cells solitary. In valve view the valves are generally circular. They have pronounced marginal processes, and some species are highly ornamented. In girdle view, the cells are often oblong and truncated with the marginal processes (external rimoportulae) prominent, possibly the most complex rimoportulae structures of any diatoms. Valves have a distinct central hyaline area, maybe coarse, with rows of radiating loculate areolae. Generally, the valve mantle is not differentiated from the rest of the valve (an exception is A. petersii that has a distinct rim). Areolae are closed by cribra externally and open internally by large foramina. The genus has a unique type of marginal rimoportula, being complex, exiting into a tube or into a cap with often fork-like slits; maybe either ‘E’ or ‘C’ internal shaped slits. Girdle bands (copulae) numerous, split, ligulate and antiligulate, fimbriate (at least on the valvocopula), with vertical rows of small areolae. Chloroplasts are described as discoid, numerous and large (A. kitonii). (Rattray 1888, Round et al. 1990, Hernández-Becerril et al. 2012, Rivera et al. 2016, ,Guiry and Giry 2023). There are 106 accepted species names; 1 accepted subspecies; 24 accepted varieties (Guiry and Giry 2023).

Species:

Guatemala specimens, El Paredon Beach surf bloom.

Aulacodiscus petersii Ehrenberg 1845: 361 (syn. Eupodiscus petersii (Ehrenberg) Kützing 1849: 135, Tripodiscus petersii (Ehrenberg) Kuntze 1898: 434.

Description and Morphometric data: Cells are heavily silicified, solitary and drum-shaped in girdle view with conspicuous external marginal processes. Circular shaped in valve view but with prominent sculptured undulations and in girdle view will appear in outline to be octagonal. Valve center is flat. Entire valve is covered in spherical granules and very small spines. The reported diameter range is 61-293 μm. External processes (rimoportulae) are four to five tubular and distinctly raised, close to the valve margins at the distal ends of the undulations. The highly silicified processes, 8-16 µm in height terminate at the apex of the valve elevations protruding out at an approximately 45º angle distally as a smooth semi-spherical cap with very small bumps. Inside the central opening of the processes there is a U-shaped, rolled tongue-like structure and some small, internal bulging or multiple ridges depending on the stage of tube morphogenesis. Internally the tubular processes have large labiate structures with a curved heart -shaped slit, nearly flush with the valve and each is subtended by a hyaline ray which continues to the central space. The spherical granules are denser and slightly bigger around the large external processes. The marginal rim is relatively free of granules and ornamentation. Coarse areolae, 5-7 in 10 µm arranged in radial rows from the center of the valve. The areolae are loculate, composed of a floral-like cribra formed by 13 – 14 small, elongated pores arranged in a circle. The areolae open internally through large foramina. Numerous girdle (copulae) bands are ornamented with rows of pores parallel to the pervalvar axis. The valvocopula has fimbriae whereas other bands have ligula and antiligula (Rattray 1888: 366-367, Round et al. 1990, Stidolph 1998, Tiffany 2008, Hernández-Becerril et al. 2012, Guiry and Guiry 2023).

Images: Aulacodiscus petersii-Guatemala EP5-Feb 10-2022_RR-4800_(Mar 7, 2022)-m015_3b.tif; Aulacodiscus petersii-Guatemala EP5-Feb 10-2022_RR-4800_(Mar 7, 2022)-m013_2.tif; Aulacodiscus petersii-Guatemala EP5-Feb 10-2022_RR-4800_(Mar 7, 2022)-m011_4b.tif; Aulacodiscus petersii-Guatemala EP5-Feb 10-2022_RR-4800_(Mar 7, 2022)-m003_3b.tif; Aulacodiscus petersii-Guatemala EP5-Feb 10-2022_RR-4800_(Mar 7, 2022)-m016_3b.tif; Aulacodiscus peterii-El Paredon-Guat-EP_3-Feb 10-2022-(Mar 10 2022-MQ)_m001_4.tif; Aulacodiscus petersii-El Paredon-Guat-EP_3-Feb 10-2022-(Mar 10 2022-MQ)_m002_2.tif; Aulacodiscus petersii-El Paredon Beach surf-Guat-EP_6-Feb 10-2022 TM4000-MW-Mar 1-2020_001(x500)_4.tif ;Aulacodiscus petersii-El Paredon Beach surf-Guat-EP_6-Feb 10-2022 TM4000-MW-Mar 1-2020_002(x2.0k)_3.tif; Aulacodiscus petersii-El Paredon Beach surf-Guat-EP_6-Feb 10-2022 TM4000-MW-Mar 1-2020_007(x800)_4.tif.
Morphometric data for Guatemalan samples: Cells are heavily silicified, solitary and drum-shaped in girdle view with conspicuous external marginal processes. Circular shaped in valve view with prominent sculptured undulations, and in girdle view it will appear in outline to be octagonal. The diameter is 91.5-145 μm in these specimens. The valve center is flat with a hyaline center absent of areolae, 3.5-4.5% of the valve diameter. Valve center and elevations are covered in spherical granules, that are almost absent from the lower spaces between undulations. In the lower spaces between elevations there occur very small spines approximately 0.5-0.8 µm high. External processes (rimoportulae) are five conspicuous, tubular and distinctly raised close to the valve margins at the distal ends of the undulations. The highly silicified processes, 13-16 µm in height protrude out at an approximately 45º angle. They terminate as distally smooth semi-spherical caps, approximately 10 µm across, with very small bumps and a row of bumps around the outer edge. Inside the central opening of the external processes there is a U-shaped, rolled tongue-like structure and seven-eight ridges (some are rolled tongue like) located in the inner surface of the tube proximate to the closed part of the tongue-like structure. Internally, the tubular processes have large labiate structures with a heart shaped slit, 8.7 µm long, 5.8 µm wide, nearly level with the valve and each has a hyaline ray which continues to the central area. The spherical granules are denser and slightly bigger around the large external processes at 3-4 in 10 µm. Granules are 1.9-3 µm diameter. The marginal rim is relatively free of ornamentation with a 4 µm width; one girdle view image shows it be a double rimed structure. Coarse areolae, 8-9 in 10 µm arranged in radial rows from the center of the valve. The areolae are loculate, composed of a floral-like cribra formed by 14–16 small, elongated pores arranged in a circle. The areolae open internally through large foramina. Four girdle bands are ornamented with rows of pores parallel to the pervalvar axis. The valvocopula has fimbriae whereas other bands have ligula and antiligula.

Previous reports of Aulacodiscus petersii along the Pacific Ocean, west coast of North and Central America: Hernández-Becerril et al. 2012 (Mexican Pacific Ocean). This is a first report of A. petersii in Guatemala Pacific Ocean coastal waters.

Previous reports: some Aulacodiscus spp. Along Western Pacific, North and Central America:

A. argus: Tiffany 2008 (Florida)
A. brownie: Tiffany 2008 (California)
A. kittonii: Cupp 1943 (southern Calif.), Lewin 1973 (Oregon), Shim 1976 (Strait of Georgia), Holmes and Mahood 1980 (Oregon, Calif.), Maher et al. 2023 (Canada West Coast), Tiffany 2008 (Calif.), Rivera et al. 2016 (northern Chile)
A. margaritaceaus: Hernandez-Becerril and Miranda (1994),
A. oregonus: Gran and Angst 1931 (Puget Sound), Tiffany 2008 (California)
A. petersii:Rattray 1888 (Japan, East Africa, South Africa, New Zealand), Stidolph 1998 (New Zealand, Australia), Tiffany 2008 (Australia), Hernández-Becerril et al. 2012 (Mexican Pacific Ocean)
A. scaber: Tiffany 2008 (California), Hernández-Becerril et al. 2012 (Mexican Pacific Ocean)

Methods:
Samples of a yellow-green bloom were collected by bottle off the surface in the surf zone at El Paredon Beach, Pacific Coast, Guatemala at 8-9 am, February 10, 2022. A concentrated sample was fixed with 3-4% formalin. Cells were cleaned with concentrated hydrogen peroxide (30%) or nitric acid (70%) at 100° C for 3-5 hours to remove organics, then rinsed multiple times in ddH20 to a neutral pH. A subsample was treated with concentrated hydrogen peroxide for 48 hrs at room temp to keep frustules intact. Mounted on SEM stubs or in Naphrax (Brunel, UK) on slides for light microscopy. Light microscopy imaging with a Nikon E800 with DIC objectives and a MU2003 20 MP camera. SEM imaging with either the Hitachi s4800 or Hitachi desktop TM4000 at Advanced Microscope Facility (AMF), at the University of Victoria (UVIC), B.C. Canada. My thanks go to Elaine Humphrey of the AMF, UVIC. Imaging with the Hitachi s4800 by Melanie Quenneville and Ron Read, imaging with the TM4000 by Mark Webber. Collection, preparation, taxonomy, identifications, and image adjustments by Mark Webber.

References:
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