Salp Species Research Articles

Reproductive biology, elemental composition and diel vertical migration of the cosmopolitan warm-temperate pelagic tunicate Soestia zonaria

Abstract Pelagic tunicates (appendicularians, doliolids, pyrosomes, salps) are cosmopolitan members of open-ocean food webs that serve as a link to the microbial loop and play a disproportional role in vertical carbon flux. Soestia zonaria is an oceanic salp species studied for more than a century, but little information exists on its ecology. Specimens of Soestia collected between 2008 and 2021 during four research expeditions (three to the Chatham Rise, New Zealand, and one to the Northeast Atlantic) using MOCNESS-1 net and large midwater trawl were analyzed for reproductive biology, stoichiometry and vertical distribution. Populations at the Chatham Rise sampled in early winter were more developed than in spring/early summer and dominated by small and medium-sized sexually reproducing blastozooids. Whole Soestia specimens had high organic content (mean ± SD = 32.8 ± 7.5%) and carbon-to-nitrogen values (6.8 ± 0.9) compared with other salp species, indicating a stronger dependency on carbon to meet its nutritional needs. Depth-stratified sampling showed that Soestia is primarily a low-amplitude diel vertical migrator occurring in the top 150 m of the water column, but also found at depths exceeding 500 m. Soestia is primarily an epipelagic salp species adapted to living in warm-temperate nutrient-depleted and more productive ocean regions.

Elemental composition and stoichiometry of krill and salps

AbstractKrill and salps play a pivotal role in marine food webs and ocean biogeochemistry, yet little is known about their stoichiometry, which is crucial for our understanding of the consequences of key grazer population shifts for ecosystem functioning. To assess differences in stoichiometric requirements, we compared the elemental composition of carbon (C), nitrogen (N) and phosphorus (P) in body tissue and gut content of Euphausia superba and Salpa thompsoni collected along the Northern Antarctic Peninsula and conducted a systematic literature review on the stoichiometry of 33 euphausiid and 19 salp species. Our data revealed significantly lower body C:P and N:P ratios but higher gut content N:P in S. thompsoni compared with E. superba, suggesting higher P uptake. In contrast, literature data showed higher C:N, C:P and N:P ratios in salps, potentially indicating lower nutrient demand compared with krill. However, literature data exhibited high intra- and interspecific variability potentially driven by organism size, seasonal or latitudinal variations, emphasizing that nutritional requirements are not only related to the target species, but also to its habitat. Our study provides valuable stoichiometric information for future experimental and modeling studies, fostering our ability to predict the future role of krill and salps for ocean biogeochemistry.

A New Molecular Phylogeny of Salps (Tunicata: Thalicea: Salpida) and the Evolutionary History of Their Colonial Architecture.

Salps are marine pelagic tunicates with a complex life cycle, including a solitary and colonial stage composed of asexually budded individuals. These colonies develop into species-specific architectures with distinct zooid orientations, including transversal, oblique, linear, helical, and bipinnate chains, as well as whorls and clusters. The evolutionary history of salp colony architecture has remained obscured due to the lack of an ontology to characterize architectures, as well as a lack of phylogenetic taxon sampling and resolution of critical nodes. We (1) collected and sequenced eight species of salps that had never been sequenced before, (2) inferred the phylogenetic relationships among salps, and (3) reconstructed the evolutionary history of salp colony architecture. We collected salp specimens via offshore SCUBA diving, dissected tissue samples, extracted their DNA, amplified their 18S gene, and sequenced them using Sanger technology. We inferred the phylogeny of Salpida based on 18S using both Maximum Likelihood and Bayesian approaches. Using this phylogeny, we reconstructed the ancestral states of colony architecture using a Bayesian ordered Markov model informed by the presence and absence of specific developmental mechanisms that lead to each architecture. We find that the ancestral salp architecture is either oblique or linear, with every other state being derived. Moreover, linear chains have evolved independently at least three times. While transversal chains are developmentally basal and hypothesized to be ancestral, our phylogenetic topology and reconstructions strongly indicate that they are evolutionarily derived through the loss of zooid torsion. These traits are likely critical to multijet locomotory performance and evolving under natural selection. Our work showcases the need to study the broader diversity of salp species to gain a comprehensive understanding of their organismal biology, evolutionary history, and ecological roles in pelagic ecosystems.

Zooplankton summer composition in the southern Gulf of Mexico with emphasis on salp and hyperiid amphipod assemblages

AbstractMesoscale features within the Gulf of Mexico (GOM) are known to influence zooplankton dynamics. Here we describe the composition of the zooplankton assemblage off shelf during summer in relation to environmental conditions, with emphasis on hyperiid amphipods and salps. Zooplankton samples were collected in summer of 2015 and 2016 in the central and southern GOM and in the Yucatan Channel in 2015. Two anticyclonic gyres were present in the north and less intense coupled cyclonic-anticyclonic gyres in the south. Zooplankton abundances differed temporally and spatially. Copepods were the dominant group (>55% of total abundance), while several less abundant taxa contributed to inter-annual and spatial differences. Amphipods and salps comprised <3% and their abundances were positively correlated. Fifty-six hyperiid and 10 salp species were identified. The dominant amphipod species were:Lestrigonus bengalensis(summer 2015),Anchylomera blossevilleiandPrimnospp. juveniles (summer 2016). Dominant salp species wereIhlea punctata,Iasis cylindricaandThaliaspp. Lower salp and amphipod species richness and abundance were associated with anticyclonic structures. Spatial and temporal differences were partly associated with symbiotic relationships between the groups. This study supports previous evidence of high spatial and temporal variability in zooplankton abundance in off-shelf waters of the GOM.

Salp metabolism: temperature and oxygen partial pressure effect on the physiology of Salpa fusiformis from the California Current

AbstractSalps are pelagic tunicates that play an important role in carbon cycling by filter feeding and packaging waste into dense fecal pellets that sink rapidly to the deep ocean. There has been limited research on salp physiology and no studies that examine how changes in environmental factors such as temperature and dissolved oxygen impact basic physiological processes. Here I examine temperature and oxygen partial pressure effect on metabolism in blastozooids of Salpa fusiformis. Routine metabolic rates of 1.66 and 3.95 μmol O2 g−1 h−1 wet weight at 10°C and 17°C, respectively, resulted in a Q10 = 3.45. The observed decrease in metabolism associated with decreased temperature, as well as hypoxia tolerance, is explored in the context of observed vertical migrations into hypoxic waters in the California Current, and potential impacts on carbon output. Metabolic rates for S. fusiformis are compared to metabolic rates published for other species of salps and gelatinous zooplankton. Expansion of this work across a broader set of species is critical to quantify the impact climate change may have on salps and their role in marine carbon cycling.

Spatial distribution of Salpa thompsoni in the high Antarctic area off Adélie Land, East Antarctica during the austral summer 2008

The salp Salpa thompsoni has the potential to alter the Southern Ocean ecosystem through competition with krill Euphausia superba. Information on the reproductive status of S. thompsoni in the high Southern Ocean is thus essential to understanding salp population growth and predicting changes in the Southern Ocean ecosystem. We carried out stratified and quantitative sampling from the surface to a depth of 2000 m during the austral summer of 2008 to determine the spatial distribution and population structure of S. thompsoni in the Southern Ocean off Adélie Land. We found two salp species, S. thompsoni and Ihlea racovitzai, with the former being dominant. S. thompsoni was distributed north of the continental slope area, while I. racovitzai was observed in the neritic zone. Mature aggregates and solitary specimens of S. thompsoni were found south of the Southern Boundary of the Antarctic Circumpolar Current, suggesting that S. thompsoni is able to complete its life cycle in high Antarctic waters during the austral summer. However, S. thompsoni was sparsely distributed in the continental slope area, and absent south of the Antarctic Slope Front, suggesting that it is less competitive with krill for food in the slope area off Adélie Land, where krill is densely distributed during the austral summer.

High abundance of salps in the coastal Gulf of Alaska during 2011: A first record of bloom occurrence for the northern Gulf

Atypical high abundances of two salp species occurred in the coastal Gulf of Alaska during 2011. Salpa aspera dominated numerically in aggregate form during spring, and became uncommon during summer, while Cyclosalpa bakeri increased from low during spring to high abundance during summer. Both species were absent, or nearly so, by fall. C. bakeri abundance was positively correlated to surface temperature in spring and summer, and both species abundances were negatively correlated to chlorophyll a. The proportion of aggregate forms of both species was higher than that of solitary forms during spring and summer. The length–frequency of S. aspera aggregate individuals ranged primarily from 10 to 50mm, and solitary forms reached 130mm, while C. bakeri aggregates were 10–25mm, with solitary forms up to 75mm. Estimated biomass of S. aspera was 0.35±0.64mg Cm−3 in southeastern Alaska during spring then decreased to 0.03±0.12mg Cm−3 during summer. Estimated biomass of C. bakeri was 0.03±0.06mg Cm−3 over the entire sampling domain during spring, then rose to 0.15±0.25mg Cm−3 during summer. The volume of water filtered daily by S. aspera was estimated to be up to 17% of the 200m water column at some stations during spring, but only up to ~3.5% during summer. Substantially higher grazing impact was possible if animals were largely confined to the surface mixed layer (typically 20–30m thick). The average volume filtrated was higher during spring for S. aspera, but for C. bakeri it was higher during summer. We propose that the combined effect of the northward transport of seed populations, their rapid biomass increase through asexual reproduction, and the high clearance rate of salps contributed to atypically low chlorophyll a in the Gulf of Alaska during spring and summer of 2011. This unusual event impacted ecosystem function during 2011, and might be expected to increase in frequency as the Gulf continues to respond to climate variations.

Molecular analyses of gut contents: elucidating the feeding of co-occurring salps in the Lazarev Sea from a different perspective

AbstractThe diet of Antarctic salps was elucidated by investigating their gut content using automated ribosomal intergenic spacer analysis (ARISA) and 454-pyrosequencing. Salp samples were collected during the Lazarev Sea Krill Study in the western Weddell Sea (summer 2005–06 and 2007–08, autumn 2004 and winter 2006). Two salp species,Salpa thompsoniandIhlea racovitzai, both occur in the Southern Ocean and can overlap geographically and seasonally. We provide evidence that, despite the non-selective feeding mechanism, the two co-occurring salp species might have different niches within a habitat. ARISA-patterns of 93 gut content samples revealed strong differences between the two salp species, even at the same sampling site. These differences were confirmed by 454-pyrosequencing of the V4-18S rDNA of ten salps. The pyrosequencing data indicate that flagellates, in particular dinophyceae, constitute a high proportion of the sequence reads identified in the gut content of both salp species. However, within the dinophyceae, differences in the read composition were detected between the two salp species. This supports the findings of a previous study where fatty acid signatures indicate a flagellate-based diet, even though microscopic analyses identified diatoms as the dominant component of salp gut contents.

Morphological and molecular characterization of salps (Thalia spp.) from the Tristan da Cunha archipelago

The identification of salp species by morphological features alone can be challenging. This study combined morphological and molecular approaches to characterize different salp reproductive forms (oozooids and blastozooids) collected from the remote Tristan da Cunha archipelago. Novel cox1 barcoding primers developed in this study proved successful for the identification of Thalia salp species and generated much needed barcoding data for this taxonomic group.

Influence of monsoon-driven hydrographic features on thaliacean distribution in waters around Taiwan, western North Pacific Ocean

Abstract Background The mesoscale distribution of thaliaceans associated with hydrographic conditions in the waters around Taiwan was investigated during two different monsoon seasons in 2004 in order to elucidate possible influences of hydrological conditions driven by seasonal monsoons on distributional patterns of thaliaceans. Results In total, 18 thaliacean species, belonging to 12 genera and 3 families, were identified in our samples. Thalia rhomboides, Doliolum denticulatum, Doliolum nationalis, Thalia orientalis, Pyrosoma verticillatum, and Thalia democratica were the six predominant species and contributed 93% to the total thaliacean collection by number. Thaliacean assemblages were similar in composition between seasons, but abundances and species numbers were higher in summer than in winter. Spatial distribution patterns of doliolids and salps clearly differed and were closely associated with hydrographic characteristics. Doliolids were mainly found in lower-salinity and nutrient-rich shelf and neritic waters; among them, D. denticulatum could be used as an indicator species of the China Coastal Current. Most salp species showed higher abundances in warm oceanic waters, such as the Kuroshio Current, Kuroshio Branch Current, and South China Sea Surface Current. Conclusions This study showed that the succession of water masses driven by monsoons affects seasonal and particularly spatial distributions of abundances of the thaliacean assemblage in the area studied.

Spatial distributions and population dynamics of two salp species, Ihlea racovitzai and Salpa thompsoni, in the waters north of Lützow-Holm Bay (East Antarctica) during austral summers of 2005 and 2006

Information on the sequences of generations and reproductive states of salps in the Southern Ocean is essential for an improved understanding of salp population growth, although changes in distribution patterns of two species of salps, Salpa thompsoni and Ihlea racovitzai, in the Southern Ocean have the potential to alter the Southern Ocean ecosystem. We used stratified, quantitative sampling from the surface to 2,000 m depth with an RMT 8 net in January of 2005 and 2006 to determine the distribution and population structure of salps in the north of Lutzow-Holm Bay, East Antarctica. Ihlea racovitzai occurred in both 2005 and 2006, but S. thompsoni was found only in 2005. Ihlea racovitzai occurred abundantly along the ice edge where Antarctic Winter Water was well developed, whereas S. thompsoni was more abundant at northern stations affected by warm Modified Circumpolar Deep Water. Small solitary stages of I. racovitzai dominated in 2005, but they had declined significantly by 2006. The S. thompsoni population was composed of small immature aggregates and mature solitary stages, suggesting that the solitary stages were reproducing. We did not find mature aggregate and immature solitary stages in the present study and thus suggested that S. thompsoni was unable to complete its life cycle in the north of Lutzow-Holm Bay because of failure of sexual reproduction in the aggregate stage. The S. thompsoni population was therefore probably transported to our study area by advection.

Checklist of the salps (Tunicata, Thaliacea) from the Western Caribbean Sea with a key for their identification and comments on other North Atlantic salps

In waters of the Northwestern Atlantic pelagic tunicates may contribute significantly to the plankton biomass; however, theregional information on the salp fauna is scarce and limited to restricted sectors. In the Caribbean Sea (CS) and the Gulf ofMexico (GOM) the composition of the salpid fauna is still poorly known and this group remains among the less studiedzooplankton taxa in the Northwestern Tropical Atlantic. A revised checklist of the salp species recorded in the North At-lantic (NA, 0–40° N) is provided herein, including new information from the Western Caribbean. Zooplankton sampleswere collected during two cruises (March 2006, January 2007) within a depth range of 0–941 m. A total of 14 species wererecorded in our samples, including new records for the CS and GOM area (Cyclosalpa bakeri Ritter 1905), for the CS (Cy-closalpa affinis (Chamisso, 1819)), and for the Western Caribbean (Salpa maxima Forskål, 1774). The number of speciesof salps known from the CS and GOM rose to 18. A key for the identification of the species recorded in the region is provided. Studies on the ecological role of salps in several sectors of the NA are scarce and deserve further attention.

Salps in the Lazarev Sea, Southern Ocean: II. Biochemical composition and potential prey value

Two species of salps, Salpa thompsoni and Ihlea racovitzai, were sampled during three cruises to the Lazarev Sea, Southern Ocean, in summer (December–January) 2005/2006, Autumn (April–May) 2004 and Winter (July–August) 2006. Dry weight, carbon, nitrogen, protein, lipid and carbohydrate contents were measured to characterize the potential value of salps as a food source for predators in the Antarctic ecosystem. Biochemical composition measurements showed that despite having a high percentage of water (~94% of wet weight), both species had relatively high carbon and protein contents in their remaining dry weight (DW). In particular I. racovitzai showed high carbon (up to 22% of DW) and protein (up to 32% of DW) values during all seasons sampled, compared to lower values for S. thompsoni (carbon content only about 15% of the DW, protein content about 10% of the DW). At the same time, carbohydrates (CH) and lipids (Lip) only accounted for a small portion of salp DW in both species (1.4% CH and 3.6% Lip for I. racovitzai; 2.1% CH and 2.9% Lip for S. thompsoni). There was little variability in the biochemical composition of either salp species between the seasons sampled. Both biochemical composition and life cycle characteristics suggest that Antarctic salps, especially I. racovitzai, may be important prey items for both cold and warm-blooded predators in an environment where food is often very scarce.

Biology and life cycles of pelagic tunicates in the Lazarev Sea, Southern Ocean

Four grid surveys were carried out in the top 200 m layer of the Lazarev Sea during fall 2004, summer 2005–06, winter 2006 and summer 2007–08 onboard the R.V. Polarstern as a part of the German SO-GLOBEC. The distribution, abundance and biology of two species of salps, Salpa thompsoni and Ihlea racovitzai, were investigated. With the exception of fall 2004, I. racovitzai dominated the salp community although being represented by low densities (<20 ind. 1000 m −3). S. thompsoni was scarce during the summers of 2005–06 and 2007–08 and almost absent from the region during winter 2006. Nevertheless, it was modestly numerous during fall 2004 reaching densities of up to 33 ind. 1000 m −3 in the south-western stations of the grid. The data on the seasonal population structure and life cycle of I. racovitzai showed that this species followed the generalized pattern typical of S. thompsoni, i.e. sexual/asexual reproduction and spawning during fall. I. racovitzai densities were the lowest during summer, increased during fall and peaked in during winter. Numerous offspring were produced by I. racovitzai during fall, just before the area became ice-covered. Conversely, S. thompsoni was not able to complete its life cycle in the Lazarev Sea, with a high occurrence of stage X (unfertilized) aggregates present. Highest S. thompsoni densities in summer and fall, and its disappearance in winter are indicative of a population of the expatriate origin that is sustained by advection.

Biology of Salpa thompsoni in waters adjacent to the Ross Sea, Southern Ocean, during austral summer 2008

Depth-stratified vertical sampling was carried out during the New Zealand International Polar Year cruise to the Ross Sea on board the RV Tangaroa in February–March 2008. The distribution (horizontal and vertical), density and population biology of Salpa thompsoni were investigated. Salps were found at two of the four major sampling locations, e.g. near the continental slope of the Ross Sea and in the vicinity of seamounts to the north of the Ross Sea. Both abundance and biomass of S. thompsoni were highest near the seamounts in the Antarctic Circumpolar Current reaching ~2,500 ind 1,000 m−3 and 8.2 g dry wt 1,000 m−3 in the water column sampled. The data showed that S. thompsoni populations were able to utilize horizontal and vertical discontinuities in water column structure, in particular the warm Circumpolar Deep Water (CDW), to persist in the high Antarctic. Although salps appeared to continue migrating to the surface colder layers to feed, both aggregate chain and young embryo release seem to be restricted to the CDW. This study for the first time has provided evidence that low Antarctic salp species may successfully reproduce in the hostile high Antarctic realm.

Comparative jet wake structure and swimming performance of salps

Salps are barrel-shaped marine invertebrates that swim by jet propulsion. Morphological variations among species and life-cycle stages are accompanied by differences in swimming mode. The goal of this investigation was to compare propulsive jet wakes and swimming performance variables among morphologically distinct salp species (Pegea confoederata, Weelia (Salpa) cylindrica, Cyclosalpa sp.) and relate swimming patterns to ecological function. Using a combination of in situ dye visualization and particle image velocimetry (PIV) measurements, we describe properties of the jet wake and swimming performance variables including thrust, drag and propulsive efficiency. Locomotion by all species investigated was achieved via vortex ring propulsion. The slow-swimming P. confoederata produced the highest weight-specific thrust (T=53 N kg(-1)) and swam with the highest whole-cycle propulsive efficiency (eta(wc)=55%). The fast-swimming W. cylindrica had the most streamlined body shape but produced an intermediate weight-specific thrust (T=30 N kg(-1)) and swam with an intermediate whole-cycle propulsive efficiency (eta(wc)=52%). Weak swimming performance variables in the slow-swimming C. affinis, including the lowest weight-specific thrust (T=25 N kg(-1)) and lowest whole-cycle propulsive efficiency (eta(wc)=47%), may be compensated by low energetic requirements. Swimming performance variables are considered in the context of ecological roles and evolutionary relationships.

Seasonal variation in hyperiid amphipod abundance and diversity and influence of mesoscale structures off Baja California

Hyperiid amphipods collected during 4 IMECOCAL (Spanish acronym for Mexican Investigations of the California Current) cruises in January, April, July, and October 2005 were ana- lyzed for seasonal variation in abundance and diversity and to determine the influence of surface cir- culation. The number of species was high in all seasons (between 79 and 92), but abundances showed remarkable differences. The total number of amphipods was lowest in January and highest in October (geometric means were 82 and 606 ind. 1000 m -3 respectively). Multivariate analysis based on similar- ity indices demonstrated a tight coupling between species composition and mesoscale structures. The largest cluster joined 67 stations, with 40% of similarity being explained by 4 species (Vibilia armata, Lestrigonus schizogeneios, Eupronoe minuta, and Primno brevidens). The geographic position of these stations coincided with the main California Current (CC) flow from spring to autumn. Subgroups were defined by minor seasonal differences. In contrast, the CC flow during winter was reduced to a meander off north Baja California, and was surrounded by oligotrophic water as indicated by the pres- ence of the hyperiids Anchylomera blossevillei and Phrosina semilunata plus 3 of the dominant species (V. armata, E. minuta, and P. brevidens). To the south of this front, other clusters with high oceanic influence were formed. In 2005, 3 eddies were detected in the area: one anticyclonic eddy in April and 2 cyclonic eddies in July. The anticyclonic eddy located in the southwest had its own faunal cluster that had high diversity but low population density. The main species in the anticyclonic eddy was Platyscelus ovoides. Only one of the cyclonic eddies had a unique faunal identity, being strongly dom- inated by Vibilia armata; this was located in the north between the CC flow and the coast. The other eddy was large and situated in the middle of the area, with a faunal array analogous to that in the CC flow. Correlation between hyperiid and salp species abundances suggested that the increase in salps (Salpa fusiformis, Thalia orientalis, Cyclosalpa bakeri) in spring could be relevant, as many hyperiid species showed a strong increase at this time. However, the most correlated salp was Cyclosalpa danae, which occurred only in October. Chains of C. danae aggregates were firmly attached in the samples. This would promote the aggregation and survival of amphipod hosts, particularly small spe- cies such as Lestrigonus schizogenios and L. bengalensis.

Respiration in marine zooplankton-the other side of the coin: CO2 production

We measured respiratory release rates of CO2 from various taxonomic groups of zooplankton during three cruises in winter, spring, and summer in the North Atlantic Ocean. Zooplankton species collected comprised different species of thaliacea (salps), mollusc thecosomes, amphipod hyperiids, copepods, decapods, and euphausiids. Hourly, individual rates ranged from a minimum of 0.02 µl h−1 for the smallest copepods (Oncaea sp. and Acartia) to a maximum of 90.6 µl CO2 for the largest euphausiids (Meganyctiphanes norvegica), corresponding to a range of weight-corrected rates of 0.1 µl CO2 for the thecosome Cymbulia peronii and 5.6 µl CO2 for the smallest copepods. Size was the major factor controlling the recorded rates. Allometric coefficients varied between 0.6 and 0.7 for weight and 1.5 and 2.2 for length, which is in agreement with the theoretical values and the values recorded for oxygen consumption. RQ values (CO2 released/O2 consumed) were computed for each group from simultaneous measurements of both respiratory processes. Model II regressions yielded mean RQs of 0.87 ± 0.40 for copepods, 0.94 ± 0.40 for thecosomes, and 1.35 ± 0.08 for large crustacean and salp species. We propose that the increase in RQ value from copepod to large crustacean species is related to the development of the muscular mass and activity rather than a simple shift in respiratory metabolic substrate. Realistic RQ values, taking species differences into account, should be used to derive population and community CO2 release rates from simpler oxygen consumption measurements.

The endostyle and the feeding filter in salps (Tunicata)

This note examines the way the pharyngeal feeding filter is constructed within the salp endostyle. Three salp species (Pegea confoederata, Thalia democratica and Salpa fusiformis) have been examined: results from each are essentially similar. The endostyle is a deep ventral groove in the pharynx, divided by a line of long basal cilia. These are flanked by gland cells which secrete the ‘mucus” which forms one of the sets of filaments of the filter. Beside the openings of this basal row of gland cells (marked by bumps of curious ‘mucus’ rodlets), there is a fence of two adjacent rows of cilia. The first of these two rows, (nearest the basal mid-line), whose tips lie next to the rodlet bumps, are regularly spaced 1.0 μm apart along the endostyle, whilst the cilia of the second row are at around half this spacing. Passing upwards in the endostylar groove, this first ciliary fence is followed by a ciliated zone, and then a second row of glandular cells is linked to a second ciliary fence with a single row of cilia only.Filaments derived from the first set of gland cells pass obliquely over the ciliated zone, and continue over the second ciliary fence. The cilia of the second fence are often surrounded by very striking borders or curtains of rodlets which appear to dissolve, run down the cilia and pass off the tips as filaments, when they are joined by the first set of filaments to form a net with an elongate rhomboidal mesh. As this passes upwards, the mesh changes to become more or less square by the time it comes to overlie a striking asymmetrical band of cilia arranged in columns, when it is now composed of two sets of filaments running vertically and horizontally, spaced some 1.3 μm apart.Various aspects of the formation of this filter net are considered.

Two functionally different muscle fibre types in some salps?

This paper describes the structure and operation of the fibres in the locomotor muscle bands of several salp species. In many species, for example Thalia democratica or Pegea confoederata , all the muscle fibres of the locomotor muscle bands are similar in width and structure. In others, for example Salpa fusiformis and S. maxima, although fibre structure is similar, the marginal fibres edging the bands may be some 3-4 times the width of those in the centre of the band. In Ihlea punctata , not only is there a more striking difference in width between the marginal and central fibres of the bands, but also the two differ in structure. The marginal fibres are up to 10 times the width of the central fibres and the two differ in myofibrillar and mitochondrial content. Intracellular recordings from the fibres show that the normally compound spike potentials do not overshoot resting potentials (up to -70 mV), and are decremental. The two types of fibre may be separately activated. It is suggested that in Ihlea punctata , the wide marginal fibres may be involved in slow swimming, the central narrow fibres in `escape´ swimming.