A Modified View on Octocorals: Heteroxenia fuscescens Nematocysts Are Diverse, Featuring Both an Ancestral and a Novel Type

Cnidarians are characterized by the presence of stinging cells containing nematocysts, a sophisticated injection system targeted mainly at prey-capture and defense. In the anthozoan subclass Octocorallia nematocytes have been considered to exist only in low numbers, to be small, and all of the ancestral atrichous-isorhiza type. This study, in contrast, revealed numerous nematocytes in the octocoral Heteroxenia fuscescens. The study demonstrates the applicability of cresyl-violet dye for differential staining and stimulating discharge of the nematocysts. In addition to the atrichous isorhiza-type of nematocysts, a novel type of macrobasic-mastigophore nematocysts was found, featuring a shaft, uniquely comprised of three loops and densely packed arrow-like spines. In contrast to the view that octocorals possess a single type of nematocyst, Heteroxenia fuscescens features two distinct types, indicating for the first time the diversification and complexity of nematocysts for Octocorallia

Light microscopy of cresyl-violet stained micrographs of <i>Heteroxenia fuscescens</i>.

<p><b>a</b>. Cross-section of polyp at pharyngeal level. <b>b</b>. Cross-section of tentacular pinnules. Nematocytes stained red (white arrows), pharynx (ph); epidermis (ep); mesoglea (me), gastodermis (ga), mesenterial filament (mf); gastrovascular cavity (gvc), zooxanthellae (white arrow heads).</p

Scanning-electron micrographs of nematocytes within the gastrovascular cavity of <i>Heteroxenia fuscescens</i>.

<p><b>a</b>. Nematocytes on the gastrodermis (white arrows); insert: cross-section of exposed gastovascular polyp cavity with mesenteries (mf), enlarged area of <b>a</b> indicated by rectangle. <b>b</b>. Enlarged view of a nematocyte, cnidocil (cn).</p

Size features of <i>Heteroxenia fuscescens</i> nematocysts.

<p>Size features (mean ±SD) of <i>Heteroxenia fuscescens</i> nematocysts: atrichous isorhiza (n = 17) and macrobasic-mastigophore (n = 15). Capsule and tubule length of atrichous isorhiza were significantly longer than those of macrobasic-mastigophore (t-test, <i>P</i><0.01), while the capsule width was similar for the two types. Shaft/capsule length ratio is given for macrobasic-mastigophore nematocysts.</p

Scanning-electron micrographs of novel macrobasic-mastigophore nematocyst type of <i>Heteroxenia fuscescens</i>.

<p><b>a</b>. Whole nematocyst with capsule (cp), spiral shaft and elongated thread. <b>b</b>. Enlarged view of spiral shaft covered with arrowhead-shaped spines (white arrow). <b>c</b>. View of the plain proximal thread tubule. Scale at <b>b</b> also applies to <b>c</b>.</p

Scanning-electron micrographs of discharged atrichous isorhiza nematocyst of <i>Heteroxenia fuscescens</i>.

<p><b>a</b>. Whole nematocyst with capsule (cp) and elongated thread tubule. <b>b</b>. Enlarged view of the tip (arrow) of the thread tubule with spines (s). <b>c</b>. Spines along the thread tubule (s).</p

Light microscopy of wet mounts of discharged nematocysts of <i>Heteroxenia fuscescens</i> stained by cresyl violet.

<p><b>a</b>. Atrichous isorhiza nematocysts isolated from polyp cavity, zooxanthellae indicated by arrow. <b>b</b>. Novel type of nematocysts adjacent to surface of tentacular pinnules, zooxanthellae indicated by arrow. <b>c</b>. Enlarged view of a discharged atrichous isorhiza nematocyst-type from the gastrovascolar cavity. <b>d</b>. Novel macrobasic-mastigophore nematocyst-type from pinnules.</p