Amalthea mahilanii F.P.Cabrera & A.R.Sherwood, 2025

Amalthea mahilanii F.P.Cabrera & A.R.Sherwood , sp. nov. ( Fig. 4)

EXAMINED MATERIAL. — United States • Hawai‘i, Ho‘okuleana (Penguin Banks); 106 m depth; 17.XI.2006; collected by Terry Kerby; ARS 10059; holotype , BISH 791136 About BISH ; female gametophyte ( Fig. 4A, C) • same collection data as for the holotype; ARS 10059; isotype , BISH 791137 About BISH ; female gametophyte ( Fig. 4D) .

ADDITIONNAL EXAMINED MATERIAL. — United States • Hawai‘i, Ho‘okuleana (Penguin Banks); 107 m depth ; 17.XI.2006; collected by Terry Kerby; ARS 10058; BISH 791138 About BISH ; female gametophyte ( Fig. 4E) • Hawai‘i, Hōlanikū (Kure Atoll), Mokupāpapa , PMNM; 90 m depth ; VI.5.2016; collected by John Hansen & Brian Hauk; ARS 09490; BISH 791139 About BISH ; male gametophyte ( Fig. 4F) .

TYPE LOCALITY. — Hawai‘i, Ho‘okuleana (Penguin Banks); at 106 m.

ETYMOLOGY. — The epithet “mahilanii ” honors Uncle Jerry Leroy Mahilani Kaluhiwa, also known as Na Koa Limu, for his significant role in conserving and preserving Hawaiian limu (algae) and traditional Hawaiian values that emphasize the importance of cultural stewardship in conserving Hawaiian biodiversity.

HABITAT. — Amalthea mahilanii F.P.Cabrera & A.R.Sherwood , sp. nov. inhabits the mesophotic zone at 90-107 m depth, attaching to rhodoliths or pebbles on sandy sediments. In the PMNM, they occur at a mean temperature of 18-19°C at 90 m and currents of 10-15 cm−1 at 70-90 m depths. In the MHI, the mean temperature is 20-21°C at 100-120 m, with stagnant currents at 100-110 m depths.

DISTRIBUTION. — Mesophotic and Hawai‘i-endemic.

SEASONALITY. — Collected from June (Kau/ Summer) to November (Hooilo/Winter) in Hawai‘i.

DESCRIPTION

Thallus typically foliose, solitary, and expanding broadly, growing up to 50 cm in length and widths ranging from 3 to 25 cm, arising from a narrow-cuneate stipe (3-7 mm long,

3-4 mm wide) attached to a discoid holdfast. Female and male gametophytes are isomorphic and dioecious. Blade color transitions from chestnut red to dark mauve when living, eventually drying to a mauve to taffy pink hue. Blade surfaces incised with various-sized round perforations. Thallus margins typically eroded, torn and wavy. Texture firm, cartilaginous, and leathery, and in surface view, the thallus appears compact with 1-2 layers of isodiametric outer cortical cells. Thallus thickness remains uniform between the basal and distal portions. Elongated spermatangial parent cells occur in both sides of the thallus. Cystocarps are embedded in the medulla and cause protuberances on one side of the thallus surfaces as small hemispherical blisters measuring up to 500 µm. Carposporangia are spherical to ovoid, 6-10 µm in diameter, and are released through an ostiole. Tetrasporic blades absent.

Vegetative characteristics

Collected blades of Amalthea mahilanii F.P.Cabrera & A.R.Sherwood , sp. nov. were predominantly mature, characterized by a single thickened blade reaching lengths of up to 50 cm and widths ranging from 3 to 25 cm ( Fig. 4 A-F). The internal anatomy of younger blades was not observed. The thallus emerges from an inconspicuous narrow-cuneate tough stipe ( Fig. 4D, E, arrowheads) measuring 3-7 mm in length and 3-4 mm in width, firmly attached to a discoid holdfast ( Fig. 4F, arrowheads). When living, the thallus exhibits a striking chestnut red to dark mauve color. Blades collected in Holaniku retained their dark mauve hue until pressed, while those collected in Ho‘okuleana dried to a taffy pink shade. The blade has various-sized round perforations, eroded margins, and a firm to rough, leathery thallus with compact outer cortical cells ( Fig. 4G). The cortex consists of 9-10 layers of ovoid cortical cells (1-3 µm in diameter) and is accompanied by 4-8 fascicles of stellate subcortical cells ( Fig. 4H, J). Below the cortex, the subcortex forms a layered structure with stellate cells, creating a network parallel to the surface ( Fig. 4H). Within the medulla, anticlinal and intertwined filaments are densely arranged, accompanied by medullary stellate cells connecting inner cortical cells and medullary filaments. No tetrasporophytes were collected.

Reproductive features

Elongated spermatangial parent cells are found on both sides of the thallus surface ( Fig. 4H, J), which undergo division to form rosettes of 8-12 parent cells ( Fig. 4I). Upon maturity, each spermatangial parent cell yields a spermatangial initial, which subsequently elongates, occasionally bifurcating, to give rise to 1-2 ovoid spermatangia at the distal ends ( Fig. 4J, arrowheads). Carpogonial branches and early post-fertilization stages were not observed in mature female specimens. When cystocarps are present, they are nestled within the medulla, inducing protuberances on one side of the thallus surface, manifesting as small

Halymenia maculata NC _046751

Polypes affinis OR 635817 * * ARS 09490 PP777198 Amalthea mahilanii * ARS 10058 PP777199 F sp.P.. Cabrera nov. & A.R.Sherwood, * ARS 10059 PP777200 Pachyleniopsis lanceolata OR 635818 * Grateloupia asiatica OR 635816

* Grateloupia taiwanensis NC _021618 * * Grateloupia turuturu NC _066971 * Grateloupia filicina NC _037841

Rhodymenia pseudopalmata KX 284709

hemispherical blisters, measuring up to 500 µm ( Fig. 4K). Within these cystocarps, the carposporangia are spherical to ovoid, measuring 6-10 µm in diameter. Carposporangia are released through apparent ostioles ( Fig. 4G, arrowhead; 4L), or distinct openings in the thallus surface ( Fig. 4G, arrowhead).