Zygotes ofSphaeroplea tenuis(Chlorophyceae)

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Abstract
Zygotes from the holotype collection of Sphaeroplea tenuis Fritsch have been studied utilizing combined techniques of light and scanning electron microscopy. Evidence is provided to support Fritsch's contention that these zygotes, found free in his original preserved material, are directly attributable to the alga he described as S. tenuis. Mature zygotes are typically spherical, range from 13–24 μm in diameter (including spines) and are ornamented with long (up to 4 μm or more), flattened, and sharply tapered spines that are closely spaced with their bases usually interconnected and forming an irregular reticulum of raised ridges. The tips of the spines are rarely bifurcate. Additional ridges and/or short papillae may supplement the pattern of ornamentation between spines. Younger zygotes are initially surrounded by a thin, smooth primary membrane, a feature characteristic of all members of the Sphaeropleaceae. Comparisons have been made with Atractomorpha echinata Hoffman which also produces spiny zygotes and shares with S. tenuis the unusual ability to form zygotes outside the female gametangia. Other similarities with the genus Atractomorpha are discussed, calling attention to the need to obtain S. tenuis in culture for comparative purposes. Presently it may be inferred that S. tenuis has the most primitive attributes of any known species of Sphaeroplea.