Morphology and function of the skin epithelium covering the giant keyhole limpet Megathura crenulata

Abstract

Mucus‐secreting epithelia protect marine gastropods from abrasive particles and microbes in seawater. We studied the morphology and function of the epithelium in the giant keyhole limpet Megathura crenulata. All exposed surfaces of the limpets were covered by a mucus‐secreting, simple columnar epithelium, in which most cells on the sole of the foot bore cilia, while the majority of cells on the side of the foot and three mantle regions bore microvilli. None of the coatings had antibacterial properties. The epithelium of the foot was distinct from that of other regions, in that the mucus secreted was used for locomotion and was left behind as the limpet moved. The glycocalyx bound to the microvilli of the mantle cells appeared to be clean in visual and SEM examinations, and attempts to enhance the binding of inert particles and bacteria were unsuccessful. Because studies have shown that standard tissue processing may cause artifactual shrinkage of mucous layers, cryostat sections and additives to standard fixatives were tested, but we found no change in the thickness of the mucous layers of the limpets compared to routinely processed tissues. Sloughing of the glycocalyx in vertebrate systems removes bound microbes, and alterations of the glycocalyx layers are associated with disease conditions. Sloughing of the glycocalyx on limpets was rarely observed. In one case, the outer mantle of the limpet was covered with silt and the glycocalyx appeared to be detaching. This process could be experimentally induced by dousing the outer mantle with talc particles. The types of secretory cells producing the mucus in each region of the skin were characterized using standard histological stains and lectin staining. Because analysis was hampered by their small size, classification of the types of secretory cells was based on TEM descriptions of granule morphology, which allowed for comparison to the secretory cells described from skin of the abalone. The possible roles of the numerous secretions, and the mechanisms of mucus production and loss, are compared to what is known in vertebrate systems and to our relative lack of knowledge regarding invertebrate systems.

Hemocyte activation and nodule formation in the giant keyhole limpet, Megathura crenulata

“…Fourteen limpets ranging 100-500 g in mass were used for an estimation of their blood volume. Data were collected from limpets used in this study and in two previous studies on the morphology of the gut and skin (Martin et al, 2010;Martin et al, 2018). Each limpet was sliced in half to separate the foot from the dorsal half containing the shell, mantle, and viscera.…”

“…Molluscs lack an adaptive immune system and rely on innate immunity to deal with foreign particles and pathogens that have penetrated surface barriers such as shells and mucous coatings (Gli nski & Jarosz, 1997;Loker, 2010;Martin et al, 2018). Infection by bacteria and other pathogens induces a rapid immune response comprising both cellular and humoral factors, but a body must first be recognized as foreign in order to initiate a response.…”

“…The giant keyhole limpet, M. crenulata, lives in rocky coastal environments along the Southern California coast (Morris et al, 1980), where it faces the challenge of exposure to microbial pathogens and environmental pollutants known to affect molluscan immune responses (Giron-Perez, 2010;Gopalakrishnan et al, 2009). The limpet is unusual in that most of its external surface, including the shell, is covered by a glycocalyx-coated epithelium (Martin et al, 2018). M. crenulata is a model for investigating the molluscan immune response.…”