Aglaothamnion halliae (Collins) Aponte, D.L. Ballantine & J.N. Norris (A)

This species forms dark red tufts, 1-4 cm high, and lacks a distinct percurrent main axis (Image A). In the upper thallus branching is alternate and distichous (Image B), which imparts a feathery appearance to the species (Image A). Cells here are typically 19-25 µm wide by 26-52 µm long. Some tips were similar to other local Callithamniaceae (Image C) while others produced more angular cells through strongly oblique divisions (Image D). Mid thallus the cells vary from 52-55 µm wide by 162-170 µm long, and the branching remains alternate and distichous (Image E). Lower mid axes cells are up to 112 µm wide by 410 µm long (Image F), while cells on lower main axes are 100-112 µm wide by 130-326 µm long (tending to shorten toward the base) and branching remains largely alternate and distichous (Image G). Axes were ecorticate throughout the thallus although anchoring rhizoids were produced from the lowest axial cells (Image H). Tetrasporangia, 31-34 µm wide by 46-52 µm long, had a tetrahedral division pattern and were sessile on the lateral branches (Image I).

We have a single record with genetic data from RI. It was growing in the subtidal (8 m) on a coarse red alga. More study is needed to confirm this identification and fully document the features and distribution of this species in our flora.

Image A. Pressed specimen from the subtidal (8 m) on red algae, Fort Wetherill, RI (GWS005753).

Image B. Cells near the top of a main axis, branching distichous and alternate (GWS005753; rehydrated from press).

Image C. Divisions at tip transverse or only slightly oblique (GWS005753; rehydrated from press).

Image D. Oblique divisions at tip produce angular axial cells (GWS005753; rehydrated from press).

Image E. Cells mid thallus on a main axis, branching distichous and alternate (GWS005753; rehydrated from press).

Image F. Cells lower mid thallus (GWS005753; rehydrated from press).

Image G. Cells low on a main axis, branching distichous and alternate (GWS005753; rehydrated from press).

Image H. Descending rhizoids produced from lowermost main axis cells (GWS005753; rehydrated from press).

Image I. Tetrahedral tetrasporangium borne sessile on a branch (GWS005753; rehydrated from press).