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Pictures used with the consent of copyright holder & tour companion Norman Behr
As the old observation was stuck with wrong IDs (Tricomycteridae) I have duplicated it to restart ID'ing...
ID Phreatobius given by Flavio Lima.

Species identified: Aulacodiscus kittonii Arnott ex Ralfs 1861: 844, pl. 8: fig. 24

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:

Aulacodiscus kittonii Arnott ex Ralfs 1861: 844, pl. 8: fig. 24

Description and Morphometric data: Cells are heavily silicified, solitary and drum-shaped in girdle view with conspicuous external marginal processes. Circular shape in valve view with submarginal processes (rimoportulae). Valve center has a flat central area 1/3 of radius with a central rosette without a hyaline area with gradually rising elevated submarginal processes. Between the processes are concavities. The reported diameter range is 42.6 to 180 µm (Hendey 1964, Holmes and Mahood 1980, Tiffany 2008, Rivera et al. 2016). External processes are four to eight, most common is four and rarely 0-3 (Holmes and Mahood 1980). The external processes consist of doomed hoods with three slits akin to a trident. Internally, the top of each rimoportulae has a raised double horse-shoe-shaped structure and each is subtended by a hyaline ray which continues to the central area. Coarse areolae, 5 in 10 µm in the centre to 6-7 in 10 µm at the margin (Holmes and Mahood 1980, Rivera 2016) arranged in radial rows from the center of the valve. There are five to six nonperforated open bands with a long ligula and the valvocopula is 5.0-6.4 μm wide. The green-brown chloroplasts are green-brown colour, numerous and discoid 5.5-9.0 μm in diameter. Widely distributed in tropical and temperate waters. (Pritchard 1861, Rattray 1888: 375, Cupp 1943, Hendey 1964, Holmes and Mahood 1980, Round et al. 1990, Tiffany 2008, Rivera et al. 2016, Guiry and Guiry 2023).

Location: Spanish Hills Wharf, Trincomali Channel, North Galiano Island, Southern Gulf Islands, British Columbia, Canada. Dec. 29, 2010.
Morphometric data for Galiano Island samples:
Circular shape in valve view with submarginal processes (rimoportulae). Valve center has a flat central area 1/3 of radius with a central rosette without a hyaline area with gradually rising elevated submarginal processes. Between the processes are concavities. Diameter is 111.4 µm. External processes are seven. The external processes consist of domed hoods with three slits akin to a trident. A hyaline ray extends from the external process to the central area. Coarse areolae, 6 in 10 µm in the centre to 7-8 in 10 µm at the margin arranged in radial rows from the center of the valve.
Images: SHW-zrax-H202-Aulacodiscus kittonii-0002-Dec 29-2010.jpg; SHW-zrax-H202-Aulacodiscus kittonii-0014-Dec 29-2010.jpg; SHW-zrax-H202-Aulacodiscus kittonii-0021CR2-Dec 29-2010.jpg

Previous reports of Aulacodiscus kittonii along the Pacific Ocean, west coast of North and Central America: Cupp 1943 (southern Calif.), Lewin 1973 (Oregon), Shim 1976 (Strait of Georgia, Salish Sea), Holmes and Mahood 1980 (Oregon, Calif.), Maher et al. 2023 (Canada West Coast), Tiffany 2008 (Calif.), Rivera et al. 2016 (northern Chile).

Previous reports: Aulacodiscus spp. along the Western Pacific of North, Central and South 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:
Found in a general plankton sample collected at Spanish Hills Wharf, Trincomali Channel, North Galiano Island, Southern Gulf Islands, British Columbia, Canada, Dec. 29, 2010. Cells cleaned in concentrated hydrogen peroxide (30%) for 2 hours at 100° C to remove organics, then rinsed multiple times in ddH20 to a neutral pH. Mounted in ZRAX (ZRAX was made and distributed by Prof. Bill Daily of the University of Pennsylvania) on coverslips then slides for light microscopy. Imaging with a Leitz Dialux and a Canon Rebel T1i SLR with a Martin Microscope (https://www.martinmicroscope.com/) camera adapter. Collection, preparation, taxonomy, identifications, and image adjustments by Mark Webber.

References:
Boyer, C.S. 1926-1927. Synopsis of North American Diatomaceae. Proceedings of the Academy of Natural Sciences of Philadelphia 78(1): 1-228 (1926), 79(2): 229-583 (1927).
Campbell, E. E. (1996). The global distribution of surf diatom accumulations. Revista Chilena de Historia Natural. 69:495-501.

Cupp, E.E. 1943. Marine plankton Diatoms of the west coast of North America. Bulletin Scripps Institution of Oceanography 5(1): 1-238.

Guiry, M.D. in Guiry, M.D. & Guiry, G.M. 2023. AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. http://www.algaebase.org; searched on December 16, 2023.

Hasle, G.R. & Syvertsen, E.E. (1996). Marine diatoms. In: Identifying Marine Phytoplankton. (Tomas, C.R. Eds), pp. 5-385. San Diego: Academic Press.

Hendey, N. I. (1964). An Introductory Account of the Smaller Algae of British Coastal waters. Part V: Bacillariophyceae (Diatoms). Her Majesty's Stationery Office, London. 317 pp.

Hernandez-Becerril, D. U. and Miranda , N. P. (1994). Morphology and Variation of the Diatom Aulacodiscus margaritaceaus (Bacillariophyta). Acta Botánica Mexicana., 26:11-19.

Hernández-Becerril, D. U., Barón-Campis, S. A. and Ortiz-Lira, H. (2012) "A morphological study of epipsammic diatoms (Bacillariophyta) from the tropical Mexican Pacific, including two species of Aulacodiscus, and a description of the new species Cerataulus simsae Hernández-Becerril et Barón-Campis" Botanica Marina, vol. 55, no. 1, , pp. 73-84. https://doi.org/10.1515/bot.2011.124

Holmes, R.W. and Mahood, A. 1980. Aulacodiscus kittonii Arnott– Distribution and morphology on the West Coast of the United States. British Phycological Journal 15: 377-389.

Lewin, J. 1973. Blooms of surf-zone diatoms along de Coast of the Olympic Peninsula, Washington. III. Changes in the Species Composition of the Blooms since 1925. Nova Hedwigia Beiheft 45: 251-257.

Lewin, J. and Schaefer, T. (1983). The role of phytoplankton in surf ecosystems. In: McLachlan, A and Erasmus, T. (ed). Sandy beaches as ecosystems.

Rattray, J. (1888) A revision of the genus Aulacodiscus Ehrb. Journal of the Royal Microscopical Society 1888: 337–382.

Rivera, P., Cruces, F. and Gallardoo, V. A. (2016). First report of a surf zone diatom accumulation in the Eastern South Pacific: Aulacodiscus kittonii Arnott ex Ralfs (Bacillariophyta) as the dominant and exclusive species. Gayana Bot. 73(1): 1-8.

Round, F.E., Crawford, R.M. and Mann, D.G. (1990), The Diatoms, Biology & Morphology of the Genera, pp. 138-139. Cambridge University Press, Cambridge, UK.

Sancetta, C. and Calvert, S. E. (1988). The annual cycle of sedimentation in Saanich inlet, British Columbia: implications for the interpretation of diatom fossil assemblages. Deep Sea Research Part A. Oceanographic Research Papers, 35(1), 71–90. doi:10.1016/0198-0149(88)90058-1

Sims, P.A. and Holmes, R.W. 1983. Studies on the "kittonii" group of Aulacodiscus species. Bacillaria 6: 267-292.

Sims, P.A. (ed.) (1996). An atlas of British diatoms arranged by B. Hartley based on illustrations by H.G. Barber and J.R. Carter. pp. 274-275, Bristol: Biopress Ltd.

Stidolph, S.R. (1998) A light and electron microscopical study of the diatom Aulacodiscus petersii Ehrenberg, from New Zealand coastal marine habitats and identification of the holotype. Botanica Marina 41: 399–409.

Tiffany, M.A. (2008). Valve development in Aulacodiscus. Diatom Research 23(1): 185-212.