Acetabularia monodisca (Børgesen) Deshaprema & Dissanayake 2025

Deshaprema, Supun & Dissanayake, Sachini, 2025, Acetabularia monodisca, comb. et stat. nov. (Polyphysaceae): A taxonomic reclassification of an umbrella alga from Jaffna, Sri Lanka, with a key to Acetabularia species in the Indian Subcontinent, Phytotaxa 693 (3), pp. 245-254 : 248

Acetabularia monodisca (Børgesen) Deshaprema & Dissanayake , comb. et stat. nov. ( Figures 3 & 4 )

Basionym:— Acetabularia crenulata var. monodisca, Børgesen (1936 ; 65)

Type:— SRI LANKA. Jaffna District, Kankasanthurai, April 1929, E. C. T. Holsinger C-A-100359 (holotype C!)

Description:— Thallus 23.5–52.4 mm tall, calcified, attached to a substratum, forming dense tufts. Peduncle straight, cylindrical, slightly nodulate, 20.6–49.4 mm long, two verticils of sterile hairs or two rings of hair scars visible on peduncles below the disc of the gametangial rays, each peduncle bearing a light-green terminal disc. Disc rigid, less caliciform to planar in shape when mature, 7.3–13.5 mm in diameter, 3.5–4.9 mm in depth, formed by the lateral fusion of 59–86 gametangial rays. Gametangial rays long, cuneiform, and 3.0– 6.5 mm long with an apiculate apex. The apex of upper corona segments emarginate to round with each segment bearing two hairs. The apex of lower corona segments emarginate. Each fertile gametangial ray bearing 100–180 globose to spherical cysts.

Distribution and ecology:—Restricted to the shallow-intertidal sea next to Mathagal Beach, Kankasanthurai, Jaffna District, Sri Lanka ( Figures 5 & 6 ).

Etymology:— ‘Single disc’ (Greek mono =single, disca =disc). The specific epithet is based on the previous variety epithet ( var. monodisca ).

Additional specimens examined:— SRI LANKA. Jaffna District , Kankasanthurai, April 1929, E. C. T. Holsinger, C-A-100360 (isotype C!) ; Jaffna District , Kankesanturai, 9 o 47’3.8”N, 79 o 56’22.6”E, 25 May 2023, Supun Deshaprema, WCSG / AL/001 ( PDA!) ; Jaffna District , Kankesanturai, 9°47'6.7"N, 79°56'29.7"E, 25 May 2023, Supun Deshaprema, WCSG / AL/002 ( PDA!) ; Jaffna District , Kankesanturai, 9°47'10.5"N, 79°56'34.0"E, 25 May 2023, Supun Deshaprema, WCSG / AL/005 ( PDA!) ; Jaffna District , Kankesanturai, 9°47'4.5"N, 79°56'24.4"E, 25 May 2023, Supun Deshaprema, WCSG / AL/007 ( PDA!)

Figures

FIGURE 3. Acetabularia monodisca (A) General characteristics of the tuft, scale bar 5 mm; (B) Thallus (side view), scale bar 5 mm; (C) Disc of gametangial rays (top view), scale bar 2 mm; (D) Cuneiform gametangial rays with apiculate apex, scale bar 2 mm.

FIGURE 4. Illustration of A. monodisca, by E. C.T.Holsinger, 1932 (Courtesy of the Natural History Museum, University of Copenhagen, Denmark)

FIGURE 5. Shallow sea next to Mathagal Beach, Kankesanturai, the typical habitat of A. monodisca.

FIGURE 6. Distribution of A. monodisca in Sri Lanka.