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

Abstract

Nudibranch mollusks (Gastropoda: Heterobranchia) are widely known for their ability to incorporate some active biochemical compounds of their prey, or even organelles and symbionts of the prey, which assured biological success of this group. At the same time, the process of nematocysts obtaining and incorporation into specific structures called cnidosacs by cladobranch mollusks remain poorly studied. This highlights a necessity of additional ultrastructural studies of cnidosac and adjacent organs in various aeolid mollusks using modern microscopic methods as they may provide new insight into the cnidosac diversity and fine‐scale dynamics of nematocysts sequestration process. The present study is focused on the general and fine structure of the cnidosac area in cladobranch Aeolidia papillosa (Aeolidiidae). Specific goals of our study were to provide a detailed description of histological and ultrafine structure of epidermis, upper parts of the digestive glands and the cnidosac, its innervation and proliferation using standard histological techniques, confocal laser scanning microscopy (CLSM) and transmission electron microscopy. Our results clearly demonstrated that A. papillosa cnidosac is a much more complex structure, than it was thought, especially compared with simple cnidosacs found in flabellinids and facelinids. Using CLSM for functional morphological analysis provides a better resolution in visualization of structural elements within a cnidosac compared with traditional histological techniques. We revealed the presence of two cell types in the cnidophage zone: cnidophages and interstitial cells, which differ in ultrastructure and function. Our results also document the presence of a specific cnidopore zone, lined with differentiated cuboid epithelium bearing long microvilli, which likely provides a unidirectional flow of nematocysts during kleptocnides extrusion. For the first time, occurrence of vacuoles containing protective chitinous spindles in the cnidosac epithelium was shown.

Movement and storage of nematocysts across development in the nudibranch Berghia stephanieae (Valdés, 2005)

“…The morphology of the adult cnidosac in Berghia stephanieae (Fig. 1 A, B) is broadly consistent with previous work in this species, and in the family Aeolidiidae [ 25 , 44 ]. This morphology includes a short, simple entrance into the cnidosac from the digestive gland, a thick (multi-layered) musculature surrounding the cnidosac, a putative proliferation zone of cnidophage cells at the proximal end of the cnidosac [ 25 ], multiple nematocysts housed within cnidophage cells along the lining of the cnidosac.…”

“…If the whole cell were sequestered, we would expect to see two double membranes (the nudibranch membrane and the cnidarian membrane). An exit was identified, but, B. stephanieae does not have a clear epithelial lining inside the cnidosac toward the tip ( i.e., a cnidopore), which is a structure commonly found in members of Aeolidiidae, including Aeolidia papillosa [ 44 ], Anteaeolidiella chromosoma , and Cerberilla amboinensis [ 25 ]. The cnidopore has been hypothesized to be a special adaptation for releasing the exceptionally long and narrow nematocysts sequestered from anemones [ 25 ].…”

A comparison of food sources of nudibranch mollusks at different depths off the Kuril Islands using fatty acid trophic markers

“…The nudibranch A. papillosa is known to prey on sea anemones, grabbing their soft tissues by highly denticulated uniserial radula ( Fig. 2A ), and consume their nematocyst (stinging capsular organelles) to protect against other predators ( Hall & Todd, 1986 ; Vorobyeva, Malakhov & Ekimova, 2021 ). The considerable levels of 22:5n-6 and C 20-22 MUFAs are characteristic for the FA composition of shallow- and deep-water sea anemones ( Kiyashko et al, 2014 ; Revel et al, 2016 ).…”

Nematocyst sequestration within the family Fionidae (Gastropoda: Nudibranchia) considering ecological properties and evolution

“…Different types of NCs differ in shape and size, and in the morphology of their shaft and tubule [ 21 ]. In aeolids that sequester nematocysts, the obtained organelles are transported to terminal muscular sacs called cnidosacs [ 5 , 12 , 22 – 24 ]. The cnidosac is commonly subdivided into three zones of different function: the proliferation zone, the cnidophage zone, and the cnidopore zone [ 20 , 24 , 25 ].…”

“…In aeolids that sequester nematocysts, the obtained organelles are transported to terminal muscular sacs called cnidosacs [ 5 , 12 , 22 – 24 ]. The cnidosac is commonly subdivided into three zones of different function: the proliferation zone, the cnidophage zone, and the cnidopore zone [ 20 , 24 , 25 ]. Each zone is present in the cnidosacs of most aeolid nudibranchs [ 20 ] but differ in size, proportions, and cell assemblage.…”