Ultrastructure of gill epithelial cells of Diopatra neapolitana (Annelida, Polychaeta)

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.

The effects of salinity changes on the Polychaete Diopatra neapolitana: Impacts on regenerative capacity and biochemical markers

Polychaetes have been identified by several authors as a group of marine invertebrates that respond rapidly to anthropogenic stressors. However, studies investigating alterations in Polychaetes affected by climate changes are scarce. Thus, the present study aimed to assess the impact of salinity changes (14, 21, 28, 35, 42g/L) on the physiological and biochemical performance of the Polychaete Diopatra neapolitana, evaluating the species regenerative ability and biochemical alterations. The results obtained demonstrated that organisms exposed to extreme salinity conditions (14, 21 and 42g/L) presented higher mortality rates, needed more days to completely regenerate the missing body region and also regenerated less chaetigers, when compared to organisms exposed to salinities 28 and 35g/L. The present study further demonstrated that D. neapolitana presented significantly lower glycogen and protein content at salinities 21 and 42g/L, which can be explained by higher energy expenditure in the physiological and biochemical processes. A marked impairment of the glutathione redox status was also recorded at salinities 21 and 42g/L. Increased antioxidant enzyme activities were observed at salinity 21g/L while LPO levels were increased at salinity 42g/L. Overall the present study demonstrated that the regenerative capacity of D. neapolitana can be used as a tool to assess environmental changes, namely salinity shifts. Moreover, stress related biomarkers revealed to be useful to evaluate the alterations in Polychaetes due to salinity changes. D. neapolitana revealed to be a good bioindicator to salinity alterations.