Ultrastructural observations on the gills of polychaetes

Ultrastructure and functional morphology of the appendages in the reef-building sedentary polychaete Sabellaria alveolata (Annelida, Sedentaria, Sabellida)

Background

The sedentary polychaete Sabellaria alveolata, the sandcastle or honeycomb worm, possesses four different kinds of appendages besides the parapodia: opercular papillae, tentacular filaments, palps, and branchiae. It exhibits a highly specialized anterior end, the operculum, formed by the prostomium, peristomium, and two anterior segments. The operculum comprises opercular papillae, tentacular filaments, and palps. Paired branchiae are present from the second thoracic chaetiger onwards on the posteriorly following segments except for the last ones. Ultrastructural data on these appendages are either scanty, incomplete, or even lacking in Sabellariidae. In order to analyze their functional morphology, to bridge the data gap, and providing data for future phylogenetic and evolutionary analyses, we investigated the appendages of S. alveolata by applying light microscopy, confocal laser scanning microscopy, scanning, and transmission electron microscopy.

Results

In S. alveolata the entire body is covered by a thin cuticle characterized by the absence of layers of parallel collagen fibers with no differentiation between the various body regions including the branchiae. The opercular papillae bear numerous tufts of receptor cells and lack motile cilia. The tentacular filaments show a distinctive pattern of motile cilia. Their most conspicuous morphological feature is a cell-free cartilaginous endoskeletal structure enclosed by ECM. Besides musculature the filaments include a single coelomic cavity but blood vessels are absent. The palps are ciliated and possess two coelomic cavities and a single blind-ending internal blood vessel. Besides external ciliation and receptor cells, the coelomate branchiae are highly vascularized and equipped with numerous blood spaces extending deep between the epidermal cells resulting in low diffusion distances.

Conclusions

All appendages, including the branchiae, bear receptor cells and, as such, are sensory. The opercular papillae resemble typical parapodial cirri. In contrast, the tentacular filaments have a triple function: sensing, collecting and transporting particles. A similarity to branchiae can be excluded. The palps are typical grooved palps. A revised classification of polychaete branchiae is suggested; thereby, the branchiae of S. alveolata belong to the most common type comprising coelom, musculature, and blood vessels. The results indicate that diffusion distances between blood and environment have been underestimated in many cases.

Ammonia excretion in the marine polychaete Eurythoe complanata (Annelida)

Ammonia is a toxic waste product from protein metabolism and needs to be either converted into less toxic molecules or, in the case of fish and aquatic invertebrates, excreted directly as is. In contrast to fish, very little is known regarding the ammonia excretion mechanism and the participating excretory organs in marine invertebrates. In the current study, ammonia excretion in the marine burrowing polychaete Eurythoe complanata was investigated. As a potential site for excretion, the 100-200 µm long, 30-50 µm wide and up to 25 µm thick dentrically branched, well ventilated and vascularized branchiae (gills) were identified. In comparison to the main body, the branchiae showed considerably higher mRNA expression levels of Na⁺/K⁺-ATPase, V-type H⁺-ATPase, cytoplasmic carbonic anhydrase (CA-2), a Rhesus-like protein, and three different ammonia transporters (AMTs). Experiments on the intact organism revealed that ammonia excretion did not occur via apical ammonia trapping, but was regulated by a basolateral localized V-type H⁺-ATPase, carbonic anhydrase and intracellular cAMP levels. Interestingly, the V-type H⁺-ATPase seems to play a role in ammonia retention. A 1 week exposure to 1 mmol l^-1 NH₄Cl (HEA) did not cause a change in ammonia excretion rates, while the three branchial expressed AMTs showed a tendency to be down-regulated. This indicates a shift of function in the branchial ammonia excretion processes under these conditions.

Articulated halkieriids from the Lower Cambrian of North Greenland and their role in early protostome evolution

Articulated halkieriids ofHalkieria evangelistasp. nov. are described from the Sirius Passet fauna in the Lower Cambrian Buen Formation of Peary Land, North Greenland. Three zones of sclerites are recognizable: obliquely inclined rows of dorsal palmates, quincuncially inserted lateral cultrates and imbricated bundles of ventro-lateral siculates. In addition there is a prominent shell at both ends, each with radial ornamentation. Both sclerites and shells were probably calcareous, but increase in body size led to insertion of additional sclerites but marginal accretion of the shells. The ventral sole was soft and, in life, presumably muscular. Recognizable features of internal anatomy include a gut trace and possible musculature, inferred from imprints on the interior of the anterior shell. Halkieriids are closely related to the Middle CambrianWixaxia, best known from the Burgess Shale: this clade appears to have played an important role in early protostome evolution. From an animal fairly closely related toWixaxiaarose the polychaete annelids; the bundles of siculate sclerites prefigure the neurochaetae whereas the dorsal notochaetae derive from the palmates.Wixaxiaappears to have a relic shell and a similar structure in the sternaspid polychaetes may be an evolutionary remnant. The primitive state in extant polychaetes is best expressed in groups such as chrysopetalids, aphroditaceans and amphinomids. The homology between polychaete chaetae and the mantle setae of brachiopods is one line of evidence to suggest that the latter phylum arose from a juvenile halkieriid in which the posterior shell was first in juxtaposition to the anterior and rotated beneath it to provide the bivalved condition of an ancestral brachiopod.H. evangelistasp. nov. has shells which resemble those of a brachiopod; in particular the posterior one. From predecessors of the halkieriids known as siphogonuchitids it is possible that both chitons (polyplacophorans) and conchiferan molluscs arose. The hypothesis of halkieriids and their relatives having a key role in annelid—brachiopod—mollusc evolution is in accord with some earlier proposals and recent evidence from molecular biology. It casts doubt, however, on a number of favoured concepts including the primitive annelid being oligochaetoid and a burrower, the brachiopods being deuterostomes and the coelom being an archaic feature of metazoans. Rather, the annelid coelom arose as a functional consequence of the transition from a creeping halkieriid to a polychaete with stepping parapodial locomotion.

The fine morphology of the osphradial sense organs of the Mollusca. 1. Gastropoda, Prosobranchia

The comparative ultrastructure of osphradia is investigated in 51 species of prosobranch gastropods, representative of nearly all superfamilies as well as of various habits and environments. The essential results show that the sensory epithelium of the osphradium as a whole may reflect environmental conditions, whereas the ultrastructure of osphradial cell types reflects actual taxonomic relations. Accordingly, the following taxa can be differentiated on the basis of osphradial fine structure: (i) the validity of the recently established taxon Vetigastropoda (Pleurotomarioidea, Fissurelloidea, and Trochoidea) is confirmed; (ii) the Docoglossa appear as a very isolated group; (iii) the osphradia of the Neritopsina are similar to those of the Caenogastropoda with respect to the organization of the sensory epithelium, but differ in cell types; (iv) in Valvatoidea the osphradium is indifferently elaborated, and no close relationship to other groups can be stated; (v) the Viviparoidea possess a special type of osphradium distinct from that of other taenioglossan groups; (vi) Neotaenioglossa (= Mesogastropoda partly), Heteroglossa (Cerithiopsoidea, Triphoroidea, Epitonioidea, Eulimoidea), and Stenoglossa (= Neogastropoda) represent an (unnamed) monophyletic stock which is characterized by three special cell types with a constant mutual positional relationship within the osphradial epithelium. On the basis of its structure the function of the osphradium is suggested to be chemoreceptive (also in Archaeogastropoda).

Modified axonemes and ciliary membranes in three polychaete species

In living Ophryotrocha puerilis, Polyophthalmus pictus and Dinophilus gyrociliatus no modified cilia are present. Treatment with hyper- and hypotonic magnesium chloride solutions leads to the formation of either cilia with dilated tips or discocilia (paddle cilia). Discocilia show axoneme loops within distal swellings of the ciliary membranes. Both types of modified cilia regain their normal appearance if they are allowed to recover in seawater. The total number of discocilia and the diameter of the loops are inversely related to the osmolarity of the magnesium chloride solution used. Even isotonic solutions of magnesium chloride, which are usually used to anaesthetize marine worms, readily induce modified cilia. This indicates that the effect is not merely due to osmotic conditions. Glutaraldehyde and osmium tetroxide may act in the same way to induce modified cilia, a fact which may account for the numerous TEM and SEM documentations of modified cilia in various marine invertebrates. Which cilia in a particular species are modified varies from one specimen to another.