Circulation of hemocoelic fluid during slow and fast swimming in the pteropod mollusc Clione limacina

In the pteropod mollusc Clione limacina Phipps 1774, individuals possess an open circulatory system that fills their body cavities and functions as a hydrostatic skeleton. Individuals of C. limacina demonstrate two distinct swimming behaviors, slow and fast swimming, and their wings are supported by their hydrostatic skeleton. We investigated the circulation of fluid within the body cavities of individuals of C. limacina by injecting dye into the hemocoelic compartments to visualize flow during both slow swimming and serotonin‐induced fast swimming. Hemocoelic fluid was observed to have a defined pattern of flow: rostrally from the heart into the wings and head, then following a dorsal pathway caudally into the body and tail before being taken up by the heart again. During patterned attack behavior, the neck constricted in width as the head's buccal cones were hydraulically inflated with hemocoelic fluid.     

Impact of feeding and starvation on the lipid metabolism of the Arctic pteropod Clione limacina

Feeding and starvation experiments were carried out with Clione limacina sampled in Kongsfjorden (Svalbard, Arctic) during summer 2002. Dry mass and lipid mass, lipid class and fatty acid compositions were analysed. Specimens of C. limacina used for the feeding study had a mean length of 25 mm, a dry mass (DM) of 13.7 mg, and a moderate lipid content of 12.1%DM. Animals were allowed to ingest only one individual of its exclusive prey, Limacina helicina which had 8.0 mm in diameter, 21.4 mg DM and 8.7% lipid of ash-free DM. Five days after feeding, the dry mass of C. limacina had increased from 13.7 to 25.3 mg which corresponds to an uptake of about 80% of the ash-free DM (14.3 mg) of L. helicina. Lipid mass increased from 1.5 to 3.9 mg which is almost two times more the ingested lipid from L. helicina (1.2 mg lipid). Thus, the major portion of lipids was synthesised de novo by C. limacina from non-lipid compounds. These lipids were triacylglycerols (TAG) and 1-O-alkyldiacylglycerol ethers (DAGE), increasing from low proportions of 6.1% and 5.7% to 42.3% and 25.8%, respectively. Considerable de novo synthesis was observed for the monounsaturated fatty acids 16:1(n − 7), 17:1(n − 8), 18:1(n − 9), and 18:1(n − 7) and the alkyl moiety 16:0. The increase in the polyunsaturated fatty acids 22:6(n − 3), 20:5(n − 3), and 18:4(n − 3) corresponded with the amount available by ingestion of L. helicina, supporting that C. limacina is not able to synthesise polyunsaturates. After 15 days of digestion, dry mass and lipids dropped almost back to the initial values.During the 100-day starvation experiment, two groups of animals were separately considered as storage lipid-rich and lipid-poor animals because of their large differences in the amount and proportion of TAG and DAGE. Storage lipid-rich C. limacina were only found until day 50, whereas lipid-poor animals were present throughout the experiment. In the lipid-rich specimens, the levels of TAG were about twice that of DAGE. The proportions of TAG decreased considerably during the 50 days of starvation (from 48.3% to 25.1% of total lipid). DAGE, varying between 16.5% and 20.5%, showed only a small decrease. The lipid-poor animals survived 100 days of starvation, exhibiting low initial amounts and proportions of storage lipids which were nearly exhausted at the end. In all C. limacina specimens, the total lipid content remained almost constant showing that lipid and non-lipid components were simultaneously utilised. This implies that body shrinkage may be an important adaptation to long-term starvation. Based on these results, it is possible to estimate the potential survival period of lipid-rich C. limacina under food limitation. A model, which considers maturity and reproduction (egg production), reveals that lipid-rich specimens might be able to survive up to 260 days without food.     

Exceptional long-term starvation ability and sites of lipid storage of the Arctic pteropod <Emphasis Type="Italic">Clione limacina</Emphasis>

The Arctic pteropod Clione limacina was collected in Kongsfjorden, Svalbard, in mid June 2004, to study the lipid metabolism within the sites of lipid storage structures during long-term starvation. Animals survived in an aquarium without any food for nearly 1 year (356 days). Size, number of lipid droplets, dry and lipid mass, lipid class and fatty acid compositions of C. limacina were determined and separately analysed for the digestive gland and the remaining integument. During the starvation period, animals shrunk from 22.4 to 12 mm in length on average, and the number of lipid droplets decreased from 1,600 to 1,000 per animal. Dry mass (DM) and total lipid mass both dropped by about 80% from day 200 to the end. The lipid content as percentage DM of the total organism did not decrease significantly ranging from 43.8 to 32.3%DM. The lipid content of the trunk was moderate with about 20%DM. The digestive gland was very rich in lipids with more than 70%DM throughout the experiment and is the major site of lipid metabolism and storage. Triacylglycerols (TAG) decreased, in the total organism, from high initial levels of 62.6 to 43% of total lipid at the end. In contrast, the proportions of 1-O-alkyldiacylglycerols [diacylglycerol ethers (DAGE)] remained almost constant, varying between 20.4 and 28.4%. In the digestive gland, TAG ranged from 60.3 to 64.8% and DAGE from 23.6 to 32.2% from day 200 to the end of the experiment. TAG and DAGE of the trunk were most likely located in the lipid droplets and were almost depleted at the end of starvation. Besides their function as lipid deposit DAGE may also act as protecting substance against bacterial and fungal infections. During the first 200 days of starvation, the fatty acid compositions showed only small variations. Thereafter, fatty acids typical for storage lipids decreased in all body compartments. In adaptation to long periods of food scarcity, C. limacina has evolved various strategies as body shrinkage, utilisation of body constituents not essential for survival, a very low metabolism and slow lipid consumption.     

Trophodynamics of Southern Ocean pteropods on the southern Kerguelen Plateau

Pteropods are a group of small marine gastropods that are highly sensitive to multiple stressors associated with climate change. Their trophic ecology is not well studied, with most research having focused primarily on the effects of ocean acidification on their fragile, aragonite shells. Stable isotopes analysis coupled with isotope‐based Bayesian niche metrics is useful for characterizing the trophic structure of biological assemblages. These approaches have not been implemented for pteropod assemblages. We used isotope‐based Bayesian niche metrics to investigate the trophic relationships of three co‐occurring pteropod species, with distinct feeding behaviors, sampled from the Southern Kerguelen Plateau area in the Indian Sector of the Southern Ocean—a biologically and economically important but poorly studied region. Two of these species were gymnosomes (shell‐less pteropods), which are traditionally regarded as specialist predators on other pteropods, and the third species was a thecosome (shelled pteropod), which are typically generalist omnivores. For each species, we aimed to understand (a) variability and overlap among isotopic niches; and (b) whether there was a relationship between body size and trophic position. Observed isotopic niche areas were broadest for gymnosomes, especially Clione limacina antarctica, whose observed isotopic niche area was wider than expected on both δ¹³C and δ¹⁵N value axes. We also found that trophic position significantly increased with increasing body length for Spongiobranchaea australis. We found no indication of a dietary shift toward increased trophic position with increasing body size for Clio pyramidata f. sulcata. Trophic positions ranged from 2.8 to 3.5, revealing an assemblage composed of both primary and secondary consumer behaviors. This study provides a comprehensive comparative analysis on trophodynamics in Southern Ocean pteropod species, and supports previous studies using gut content, fatty acid and stable isotope analyses. Combined, our results illustrate differences in intraspecific trophic behavior that may be attributed to differential feeding strategies at species level.     

Smooth muscle fiber types and a novel pattern of thick filaments in the wing of the pteropod mollusc Clione limacina

Summary Wing (parapodial) retraction in the pteropod mollusc Clione limacina is a reflex triggered by tactile stimulation. Light and transmission electron microscopy revealed three groups of smooth muscles in the wing hemocoel that participate in retraction movements: transverse, longitudinal, and dorsoventral. Among these, two subtypes of muscle cells were identified. The first (type A) appears in all three groups and forms a well-organized lattice-like structure. The second (type B) is the major component of transverse muscles and runs in one direction only. Quantitative ultrastructural comparisons of dimensions, abundance, and organization of dense bodies, thick and thin filaments, membrane invaginations, sarcoplasmic reticulum, and mitochondria suggest that type A cells are able to contract and relax more quickly with less endurance whereas type B cells are capable of generating stronger contractions with more endurance and slower relaxation speed. Furthermore, type A cells have a unique pattern of thick filament organization, here referred to as pseudosarcomeres. The roles played by the different cell types in wing retraction are discussed.     

Regeneration of central and peripheral synaptic connections in the locomotor system of the pteropod molluscClione limacina

The neural network underlying rhythmic wing movements in the molluscClione limacina is well-studied. Two different groups of motoneurons innervate two distinct groups of wing muscles. The locomotor rhythm generated in the left and right pedal ganglia is synchronized by interneurons. When the axons of the locomotor motoneurons are crushed, numerous fine neurites sprout towards the denervated muscles and reach them in 8–15 days. At this stage motoneurons project to and synapse on not only correct but equally incorrect muscle targets. After 2 weeks of regeneration the number of incorrect neurites and synaptic connections begins to decrease and following 1.5–2 months all incorrect connections are eliminated, incorrect axons are withdrawn and the behavioral deficit is compensated. In this study the regeneration of interneurons and the growth profiles of inter- and motoneurons were also studiedin vitro. Two individually isolated pedal ganglia were co-cultured in three different configurations: a) the wing nerve stump from one ganglion was fixed against the commissural stump from another ganglion; b) the wing nerve stumps were fixed against each other; c) the commissural stumps were fixed against each other. Under the above experimental conditions we found that the interneurons were able to cross only the contact between two commissural stumps, and in this case found their original targets, restored correct connections and synchronized the rhythm in two pedal ganglia. In contrast, motoneurons were able to cross all types of contacts.