Studies on Cnidophage, Specialized Cell for Kleptocnida, of Pteraeolidia semperi (Mollusca: Gastropoda: Nudibranchia)

概要

Generally, gastropods (Mollusca) have a calcareous shell that covers some to all of the upper surface of the animals. The molluscan’s shell plays a defensive role against predators, which must contribute to their ecological success, although some gastropods have lost it (Bursca 2016). Members of the Opisthobranchia, namely sea slugs and allied molluscans, are characterized by their gills located posteriorly to heart unlike most of the other gastropods. The majority of opisthobranchian species have lost their shell partially or completely, and their exposed soft body parts often exhibit vivid coloration and/or peculiar morphology, which are thought to be involved in defense against natural enemies (Edmunds 1966). For example, Chromodoris sea slugs are known for their striking aposematic coloration presumably against visual predators (Zaccone 2019), which accumulate poisonous or distasteful chemicals obtained by feeding on sponges or algae (Faulkner and Ghiselin 1983). Other sea slug species are known for their Batesian mimicry, in which the non-poisonous species bear vivid coloration reminiscent of poisonous species, thereby avoiding predation (Edmunds 1966). On the other hand, inconspicuous coloration of many sea slugs, which is often derived from their food staff, seems to function as cryptic coloration (Wickler 1959; Faulkner and Ghiselin 1983; Edmunds 1987). Not only colors but also morphological traits are involved in defense and escape. For example, many cladobranchian sea slugs bear numerous dorsal protrusions called cerata, which are autotomized when attacked by enemies (Edmunds 1966). In addition, some cladobranchian species have evolved a unique defensive mechanism implemented in their dorsal cerata, called “kleptocnida”.

Most cladobranchian sea slugs feed on cnidarians such as hydrozoans, sea anemones, jelly fish,etc. Nematocysts are cnidarian-specific cell organelles used for feeding and defense, whose capsular envelop contains a poisonous stinger thread that is discharged by mechanical or chemical stimuli (Ewer 1947). Strikingly, many sea slug species belonging to the Aeolidioidea have the ability to incorporate and store the food-derived nematocysts within the tip of the dorsal cerata, and the nematocyst-armored cerata are used for defense against enemies. This phenomenon is called “kleptocnida” or “nematocyst stealing” (Grosvenor 1903; Kepner 1943; Edmunds 1966). How the sea slugs keep the nematocysts undigested, undischarged and stored is an enigma. It was suggested that the mucus of the sea slugs may have an activity to inhibit the nematocyst discharge, thereby enabling them to feed on cnidarians without injury (Greenwood 2004). Kleptocnida is an extremely unique and interesting phenomenon in that cell organelles of an organism are incorporated into other organism and function there, possibly representing an early stage of symbiosis. In this context, studies on the mechanism of kleptocnida is important in evolutionary biology. Previous attempts have shed some light on the mechanisms underlying kleptocnida. Histological studies revealed that the nematocysts are transported to the cnidosacs located at the tip of the cerata via the digestive gland, where the nematocysts are incorporated into the specialized cells for kleptocnida called the cnidophages (Grosvenor 1903; Martin 2003). It was proposed that the sea slugs can obtain and retain undischarged nematocysts by incorporating immature nematocysts into the cnidosacs and making them mature therein (Obermann et al. 2012), although this idea seems to be a mere hypothesis without solid evidence. At this stage, it is totally unknown how Aeolidioidea sea slugs can selectively incorporate and maintain intact nematocysts without digestion. According to the previous studies, the cnidophage is expected to play a role in selective incorporation and retention of the nematocysts. Hence, the basic characteristics of the cnidophage, such as developmental origin, ultrastructure, gene expression, etc. should be investigated in detail.

Thus far, no studies have collected and histologically inspected a sufficient number of immature cerata without cnidosacs and/or cnidophages. The main reason for the lack of knowledge is that rearing and maintenance of cladobranchian sea slugs are generally difficult. Recently, a long-term culturing method for the cladobranchian Pteraeolidia semperi has been established in our laboratory. In this study, therefore, I investigated P. semperi, which is large in size (up to 5 cm), easy to collect around the Misaki Marine Biological Station, Japan, and maintainable in the laboratory. First, focusing on the large number of dorsal cerata in which kleptocnida occurs, I investigated the formation and increase patterns of cerata during the growth of P. semperi, and found that the number of cerata continuously increases even in mature individuals, and the body regions where new cerata are formed are predictable to some extent (Part I). Next, I attempted to get insight into the properties of cnidophage by examining the developmental process of cnidosacs and the detailed internal structure of cnidophages (Part II). Finally, I performed transcriptomic and proteomic analyses of the cnidosac samples using RNA-sequencing and mass spectrometry, thereby attempting to identify genes and proteins that are specifically up-regulated in cnidosac/cnidophage where kleptocnida occurs (Part III).

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