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A general description is provided for male and female gametes, their gametangia, and thalli of five species of the siphonaceous green macroalga  Codium  from Atlantic and Pacific shores. Both types of gametes were biflagellate, the flagella arising from a cap at the tip of the colorless extension of the hyaline apex, which also contained the nucleus. Male gametes were small (mean diameter for five species, 2.5 μm; length ranged from 5 to 10 μm) and had few chloroplasts; female gametes were considerably larger (mean diameter 11 μm; length ranged from 11 to 21 μm) with many chloroplasts. Monthly collections of  C. isthmocladum  indicated that female seasonality was broader than male seasonality and that hermaphroditic thalli were frequent. Comparison of these reproductive traits with reports for the invasive  Codium, C. fragile  ssp.  tomentosoides,  collected from the northwestern Atlantic coast suggested that 1) in localized areas, two subspecies or varieties may coexist, and 2) no functional male gametes were produced by  C. fragile  populations from this coastline. We speculate that Tokyo and Sagami Bays on the Pacific shores of Honshu, Japan, might contain the source populations for the invasive ssp.  tomentosoides . Furthermore, we suggest that the extended presence of mature female thalli coupled with the apparent absence of mature male thalli might cause the prevalence of parthenogenesis in this genus.

Reproduction in the green macroalga Codium (Chlorophyta): characterization of gametes