Ultrastructure of nuchal organs in some marine polychaetes

Abstract

Polychaetes normally possess one pair of nuchal organs at the posterior edge of the prostomium or peristomium. They have been regarded as chemosensory organs. The nuchal organs of four marine polychaete species with different habits were investigated by electron microscopy. Although the shapes of nuchal organs can vary greatly from simple ciliary bands (Scolelepis squamata, Spionidae) to retractile tongue-like, piston- or finger-shaped forms (Eteone longa, Anaitides mucosa, Phyllodocidae; Heteromastus filiformis, Capitellidae), the structural components, including the ciliated supporting cells, sensory cells, and nuchal epidermal cells, are essentially similar. The differences basically concern 1) the position of the sensory cells with relation to the ciliated supporting cells, 2) the location and structure of the nuchal nerve, and 3) the structure of the nuchal cuticle. The diverging nature of this modified cuticle is described and discussed in detail. Comparisons are made with the fine structure of nuchal organs of other polychaete species. Similarities of cellular components of nuchal organs are found not only in the four species studied here but also in all nuchal organs investigated so far. This is hypothesized to be due to the fact that the polychaete stem species already possessed nuchal organs with the respective cell types. Differences in the number and distribution of cellular components and in the overall shape of nuchal organs are thought to have evolved in correlation with the equipment of other cephalic appendages and with different habits and modes of nutrition.

Ultrastructural investigations on the cephalic and metameric nuchal organs of Spio cf. filicornis (Polychaeta, Spionidae)

“…In S. filicornis they are located peripheral to the ciliated supporting cells, thus oriented towards the nuchal nerve, along the whole length of the ciliated area. This is consistent with observations from other Spionidae (Jelsing 2003;Rhode 1990;Schlötzer-Schrehardt 1987, 1991.…”

“…The paving-stone-like cover of the ciliated supporting cells seems to be a special character of Spionidae (Rhode 1990;Schlötzer-Schrehardt 1986, 1987, 1991 and Protodrilida (Purschke 1986(Purschke , 1990. This structure, which overlays and protects the sensory elements, has so far not been reported from the nuchal organs of species belonging to any other group (Purschke 1997).…”

“…The ultrastructural components from both the cephalic and metameric ciliated bands correspond to the general structure of nuchal organs. Differences in the specific configuration of substructures such as the position, arrangement and number of cells were encountered, but such interspecific variations are well known from comparative investigations on other polychaetes (see, for example, Purschke 1997;Rhode 1990;Whittle and Zahid 1974). Structural dissimilarities of the cephalic and metameric nuchal organs are restricted to the clustering of dendritic processes, the size of the olfactory chamber and the contiguous cephalic nuchal nerves.…”

“…These organs are usually paired structures which consist of ciliated supporting cells and bipolar primary sensory cells. Sensory processes terminate in a subcuticular olfactory chamber overlaid by a specialised protective cuticular cover, and paired nerves innervate the organs directly from the brain (Hessling and Purschke 2000;Lewbart and Riser 1996;Purschke 1986Purschke , 1990Purschke , 1997Purschke , 2000Purschke and JouinToulmond 1994;Rhode 1989Rhode , 1990Schlötzer-Schrehardt 1986, 1987, 1991West 1978;Whittle and Zahid 1974).…”

Observations of serotonin and FMRFamide‐like immunoreactivity in palp sensory structures and the anterior nervous system of spionid polychaetes

“…Among the polychaetes, nuchal organs are typically considered to function in chemoreception based on histological, ultrastructural, and positional criteria (Storch and Schlötzer-Schrehardt, 1988;Rhode, 1990;Purschke, 1997Purschke, , 2005, but see Fewou and Dhainaut-Courtois, 1995 for a potential osmoregulatory function). Although physiological evidence for chemoreception is limited, several researchers have linked nuchal organs to chemoreceptive behaviors.…”