Acanthus ebracteatus

Overview

Natural populations of Acanthus ebracteatus are documented in mangroves. Separate studies tested seed germination and greenhouse seedlings under artificial tidal exposure, measuring germination, survival and growth. [S1, S3, S7]

Names and classification

Acanthus ebracteatus Vahl. Feng et al. (2024) retained it as a distinct species using morphology, ploidy, and molecular evidence, distinguishing it from A. ilicifolius and the newly described A. tetraploideus. Older var. xiamenensis labels and experiments under the former broad A. ilicifolius concept are not automatically transferred here. [S1, S2, S7, S8]

The 2024 revision distinguishes A. ebracteatus, A. ilicifolius, and A. tetraploideus using morphology, ploidy, and molecular evidence. Older var. xiamenensis records are not automatically included. [S1, S2, S7, S8]

Morphology and identification

A mangrove shrub with thick, spiny-toothed leaves, terminal inflorescences and capsule fruit; Flora of China describes the corolla as white, with blue or purplish variation. The 2024 taxonomic study distinguishes it from A. ilicifolius and A. tetraploideus using morphology, ploidy and molecular evidence. An old variety label is not a substitute for voucher-based identification. [S1, S2, S7]

Distribution, habitat and phenology

Flora of China describes the species from mangrove swamps. Feng et al. (2024) list modern mangrove collections in China and Southeast Asia, including Sungei Buloh Wetland Reserve in Singapore. Ragavan et al. (2015) report wild populations at Sippighat and Shoal Bay Creek in South Andaman, with vouchers PBL30969 and PBL30970, in landward mangrove zones near the high-tide line and in the forest understory. [S1, S2, S7, S8]

The A. ebracteatus entry in Flora of China, volume 19, page 379, gives flowering in April–May and fruiting in August–September. The following A. ilicifolius entry lists A. ebracteatus var. xiamenensis as a synonym and gives flowering in February–March and fruiting in August–September; that record belongs to the adjacent species, not A. ebracteatus. CNHP_1321 separately summarizes flowering and fruiting as spring–summer; this is a catalogue transcription and does not establish month-level dates or a uniform calendar across regions. [S1, S5]

Salinity in natural habitats

Several independent surveys record A. ebracteatus in mangroves in Southeast Asia and the Andaman Islands, supporting its occurrence in intertidal mangrove habitat. These population records do not measure salinity in the soil around this species. [S1, S7]

Salinity studies

Tong et al. collected seed from wild parent plants on Bailong Peninsula, Guangxi, in July 2023 and raised seedlings in freshwater for three months before transfer to peat pots (initial height 12.98±0.75 cm). In artificial tidal boxes, seedlings received artificial seawater made with commercial sea salt at 0, 5, 10, 15, 20, 25, 30 or 35‰; salt composition and purity were not reported. Water fully covered seedlings for 4 h/day, and freshwater or sea salt was added daily to maintain water level and salinity. The trial ran from October to December 2023, with exact duration unreported. Survival was 100% with better growth and root mass at 0–15‰, 87.5% with reduced growth at 20‰, and mortality at ≥30‰; basal diameter did not differ significantly among salinity treatments. These are artificial-seawater values in ‰, not pure NaCl; substrate ECe was not measured, so the result applies to the greenhouse seedling tidal simulation. [S3]

Tong et al. also tested seeds from a wild Guangxi population in artificial seawater made with commercial sea salt at 0, 5, 10, 15, 25 and 35‰. Seeds were soaked for 24 h, then germinated on filter paper with same-salinity solution added daily for seven days. Germination exceeded 90% at ≤15‰ and was markedly inhibited at ≥25‰. This is a seed-germination result, not a test of long-term plant growth; the original ‰ unit is not converted to PSU. [S3]

The species-level eHALOPH record lists a maximum salinity of 35.5 PSU, but its linked material provides no checkable treatment medium or endpoint. This database value is reported separately from the artificial-sea-salt trials given in ‰. [S4]

Species-level China Halophyte Catalogue entry CNHP_1321 lists 500 mM in a field labelled seawater salinity and attributes the value to a database. It gives no salt identity, measurement metric, material, medium or exposure duration. The unit may have been misapplied; the value is not a measured tolerance concentration and cannot be interpreted as 500 mM NaCl or equated with 35.5 PSU. [S5]

The GRHC species entry for A. ebracteatus describes beach mangroves and reports soil salinity above 0.6%, but gives no salt identity, percentage basis, sample, measurement method or time scale. The same page separately describes A. ebracteatus var. xiamenensis; that account is not transferred to the species. The percentage cannot be converted to EC, PSU or NaCl concentration. [S6]

Reported mechanisms and ecological adaptations

The greenhouse seedling trial measured survival and growth under artificial tidal treatments; a separate filter-paper assay measured seed germination and timing, using a different life stage and setup. No single molecular mechanism was established, and the artificial seawater was not resolved as pure NaCl. [S1, S3, S7]

Conclusions and evidence gaps

Natural mangrove records support an intertidal habitat. Quantitative records comprise filter-paper seed germination, a 2023 greenhouse seedling trial with 4 h of tidal submergence per day, and database fields of 35.5 PSU, 500 mM “seawater salinity” and >0.6% soil salt. The artificial-sea-salt trial did not measure substrate ECe, and the salt composition was not reported; the database fields are not linked to specific experiments. [S1, S2, S3, S4, S5, S6]