The structure of cnidosacs in nudibranch mollusc Aeolidia papillosa (Linnaeus, 1761) and presumable mechanism of nematocysts release

Abstract

The structure of cnidosacs in nudibranch mollusc Aeolidida papillosa (Linnaeus, 1761) before and after the discharging of kleptocnidae has been studied. In the apical zone of the cnidosac, the basal laminae of epidermis and gastrodermis are interrupted, and the muscle layers of the cnidosac and the epidermis are absent. We suggest the formation of a temporary channel during the discharging of the cnidosac. Through this channel, nematocysts move from the cnidosac to the cnidopore, which forms on the top of the ceras.

General and fine structure of Aeolidia papillosa cnidosacs (Gastropoda: Nudibranchia)

“…The process of nematocysts extrusion from cnidosac remains poorly understood. While it is widely accepted that cnidosac discharging relates tomuscular contraction controlled by the nervous system (Greenwood & Mariscal, 1984; Goodheart & Bely, 2017; Vorobyeva et al, 2017) and our data also support this viewpoint (Figures 3, 10–13), there is no strict consensus on how kleptocnides are released from cnidophages into the seawater. Several authors suggested that cnidophages located close to cnidopore area may detach from basal laminae and be expulsed through a temporary cnidosac exit (Kälker & Schmekel, 1976; Greenwood & Mariscal, 1984; Martin et al, 2009).…”

“… Aeolidia papillosa , schematic drawing of the structure of the cnidosac, longitudinal section (modified from Vorobyeva et al, 2017). *, cnidopore; bl, basal lamina; ci, cilia; cmc, circular muscular layer of cnidosac; cmb, circular muscular layer of body; cnph, cnidophages; dg, digestive gland; ep, epidermis; he, haemocoel; ic, interstitial cell; lmb, longitudinal muscular layer of body; lmc, longitudinal muscular layer of cnidosac; lu, lumen; mv, microvilli; nc, nematocyst; nu, nucleus; sph, sphincter; va, vacuoles with chitinous spindles…”

“…At the same time, knowledge on nematocysts obtaining and other defensive strategies in aeolid nudibranchs remains scarce. Aeolid nudibranchs and few other independent lineages are able to sequester cnidarian nematocysts, incorporate them into their tissues as organelles so‐called “kleptocnida” and use them for their defense (Glaser, 1910; Goodheart & Bely, 2017; Kälker & Schmekel, 1976; Martin et al, 2009, 2010; Vorobyeva et al, 2017). Sequestered kleptocnids are stored in specific structures called cnidosacs, occupying the distal part of each ceras.…”

“…Sequestered kleptocnids are stored in specific structures called cnidosacs, occupying the distal part of each ceras. Once a mollusk is threatened, kleptocnids are extruded from cnidosacs and defend it from predators (Frick, 2005; Goodheart & Bely, 2017; Greenwood, 2009; Greenwood & Mariscal, 1984; Vorobyeva et al, 2017).…”

“…In its proximal part a cnidosac connects with muscular lining of a ceras, forming a cnidopore zone. Through the cnidopore zone sequestered nematocysts are released to the outer surface after musculature contraction, and discharge (Goodheart et al, 2018; Goodheart & Bely, 2017; Greenwood, 2009; Greenwood & Mariscal, 1984; Vorobyeva et al, 2017). Nudibranchs also developed specific chemical and mechanical protection helping them against injury from stringing cells during feeding process and cnidosac discharging.…”