Brine Channels Research Articles

Effects of Incubation Temperature on Growth and Production of Exopolysaccharides by an Antarctic Sea Ice Bacterium Grown in Batch Culture

The sea ice microbial community plays a key role in the productivity of the Southern Ocean. Exopolysaccharide (EPS) is a major component of the exopolymer secreted by many marine bacteria to enhance survival and is abundant in sea ice brine channels, but little is known about its function there. This study investigated the effects of temperature on EPS production in batch culture by CAM025, a marine bacterium isolated from sea ice sampled from the Southern Ocean. Previous studies have shown that CAM025 is a member of the genus Pseudoalteromonas and therefore belongs to a group found to be abundant in sea ice by culture-dependent and -independent techniques. Batch cultures were grown at -2 degrees C, 10 degrees C, and 20 degrees C, and cell number, optical density, pH, glucose concentration, and viscosity were monitored. The yield of EPS at -2 degrees C and 10 degrees C was 30 times higher than at 20 degrees C, which is the optimum growth temperature for many psychrotolerant strains. EPS may have a cryoprotective role in brine channels of sea ice, where extremes of high salinity and low temperature impose pressures on microbial growth and survival. The EPS produced at -2 degrees C and 10 degrees C had a higher uronic acid content than that produced at 20 degrees C. The availability of iron as a trace metal is of critical importance in the Southern Ocean, where it is known to limit primary production. EPS from strain CAM025 is polyanionic and may bind dissolved cations such at trace metals, and therefore the presence of bacterial EPS in the Antarctic marine environment may have important ecological implications.

Life in frozen veins: Coping with the cold

Every autumn a fundamental transition occurs in the surface waters of polar oceans. Millions of square kilometres of surface waters freeze to form an ice layer that varies from a few centimetres through to several metres thick, and which effectively separates the ocean from the atmosphere above. Ice made from seawater is a porous, semi-solid matrix permeated by a labyrinth of brine channels and pores, and within these a diverse microbial assemblage, including viruses, Archaea, bacteria, flagellates and unicellular algae can thrive. These assemblages can reach such high abundances that the ice becomes a rich coffee colour. The microbial assemblages are in turn a rich food source for grazing protoplankton and zooplankton, especially in winter when food in the water column is scarce.

Chemical control of colloidal fouling of reverse osmosis systems

Fouling of membranes by colloidal organic and inorganic particles continues to be documented as the most common and challenging obstacle in attaining stable continuous operation of reverse osmosis (RO) and ultrafiltration (UF) systems. Much current research is being conducted on physical parameters to mitigate such fouling. The focus has been on membrane synthesis and element design; microfiltration and ultrafiltration pretreatment; electromagnetic devices; correlation with physical factors such as Silt Density Index, zeta potential and critical flux; technique of direct observation of fouling process through a membrane; and classification of macromolecular organics for correlation with fouling characteristics. We report initial successes with chemical control of colloidal fouling. Through screening with a large number of observable coagulations of natural colloids, we have developed a group of proprietary anticoagulants and dispersants that would, at less than 10 ppm dosage to the RO feedwater, control various classes of colloidal foulants. Case studies of the control of humic matter, elemental sulfur and colloidal silicate in problematic RO systems that became stabilized are briefly presented. We conclude that a great need and potential exists in economically controlling the myriads of fouling interactions of colloidal particles during concentration within the brine channels of RO membrane elements. Low dosages of antifoulants can in many cases obviate the need for installation and maintenance of pretreatment unit or operations designed to remove such colloidal foulants from the process stream.

Atmospheric CO2 balance: The role of Arctic sea ice

Climatic changes in the Northern Hemisphere have led to remarkable environmental changes in the Arctic Ocean, including significant shrinking of sea‐ice cover in summer, increased time between sea‐ice break‐up and freeze‐up, and Arctic surface water freshening and warming associated with melting sea‐ice, thawing permafrost, and increased runoff [Carmack, 2000; Morison et al., 2000; Semiletov et al., 2000; Serreze et al., 2000]. These changes are commonly attributed to the greenhouse effect resulting from increased carbon dioxide (CO2) concentration. The greenhouse effect should be most pronounced in the Arctic where the largest air CO2 concentrations and winter‐summer variations in the world for a clean background environment were detected [Conway et al., 1994; Climate Monitoring and Diagnostics Laboratory Data Archive, http://www.cmdl.noaa.gov/info/ftpdata.html]. Some increased seasonal variation may be a consequence of increasing summer CO2 assimilation by plants in response to higher temperature and longer growing season [Keeling et al., 1996]. Here we show that sea‐ice melt ponds and open brine channels form an important spring/summer air CO2 sink that also must be included in any Arctic regional CO2 budget; both the direction and amount of CO2 transfer between air and sea during the open water season may be different from transfer during freezing and thawing, or during winter when CO2 accumulates beneath Arctic sea‐ice.

Critical behavior of transport in sea ice

Geophysical materials such as sea ice, rocks, soils, snow, and glacial ice are composite media with complex, random microstructures. The effective fluid, gas, thermal, and electromagnetic transport properties of these materials play an important role in the large-scale dynamics and behavior of many geophysical systems. A striking feature of such media is that subtle changes in microstructural characteristics can induce changes over many orders of magnitude in the transport properties of the materials, which in turn can have significant large-scale geophysical effects. For example, sea ice, which mediates energy transfer between the ocean and atmosphere, plays a key role in global climate, and serves as an indicator of climatic change, is a porous composite of ice, brine and gases. Relevant length scales range from microns and millimeters for individual brine structures, to centimeters and meters for connected brine channels across floes, to hundreds of kilometers across an ice pack. Sea ice is distinguished from many other porous composites, such as sandstones or bone, in that its microstructure and bulk material properties can vary dramatically over a relatively small temperature range. The fluid permeability of sea ice ranges over six orders of magnitude for temperatures between 0°C and −25°C. Moreover, small changes in brine volume fraction around a threshold value of about 5%, corresponding to variations in temperature around a critical point of about −5°C, control an important transition between low and high fluid permeability regimes. Below this critical temperature, the sea ice is effectively impermeable, while for higher temperatures the brine phase becomes connected over macroscopic scales, allowing fluid transport through the ice. This transition has been observed to impact a wide range of phenomena such as surface flooding and snow–ice formation, enhancement of heat transfer due to fluid motion, mixing in the upper ocean, melt pool persistence, surface albedo (ratio of reflected to incident radiation) and other optical properties, growth and nutrient replenishment of algal and bacterial communities living in sea ice, and remote sensing of the sea ice pack from space. Recently, we have shown how continuum percolation theory can be used to understand the critical behavior of fluid transport in sea ice. Here we review this application of percolation theory to sea ice, and briefly discuss electromagnetic transport in sea ice, in particular how the geometry and connectivity of the brine microstructure determine its effective complex permittivity.

Ecology of Southern Ocean pack ice

Around Antarctica the annual five-fold growth and decay of sea ice is the most prominent physical process and has a profound impact on marine life there. In winter the pack ice canopy extends to cover almost 20 million square kilometres--some 8% of the southern hemisphere and an area larger than the Antarctic continent itself (13.2 million square kilometres)--and is one of the largest, most dynamic ecosystems on earth. Biological activity is associated with all physical components of the sea-ice system: the sea-ice surface; the internal sea-ice matrix and brine channel system; the underside of sea ice and the waters in the vicinity of sea ice that are modified by the presence of sea ice. Microbial and microalgal communities proliferate on and within sea ice and are grazed by a wide range of proto- and macrozooplankton that inhabit the sea ice in large concentrations. Grazing organisms also exploit biogenic material released from the sea ice at ice break-up or melt. Although rates of primary production in the underlying water column are often low because of shading by sea-ice cover, sea ice itself forms a substratum that provides standing stocks of bacteria, algae and grazers significantly higher than those in ice-free areas. Decay of sea ice in summer releases particulate and dissolved organic matter to the water column, playing a major role in biogeochemical cycling as well as seeding water column phytoplankton blooms. Numerous zooplankton species graze sea-ice algae, benefiting additionally because the overlying sea-ice ceiling provides a refuge from surface predators. Sea ice is an important nursery habitat for Antarctic krill, the pivotal species in the Southern Ocean marine ecosystem. Some deep-water fish migrate to shallow depths beneath sea ice to exploit the elevated concentrations of some zooplankton there. The increased secondary production associated with pack ice and the sea-ice edge is exploited by many higher predators, with seals, seabirds and whales aggregating there. As a result, much of the Southern Ocean pelagic whaling was concentrated at the edge of the marginal ice zone. The extent and duration of sea ice fluctuate periodically under the influence of global climatic phenomena including the El Niño Southern Oscillation. Life cycles of some associated species may reflect this periodicity. With evidence for climatic warming in some regions of Antarctica, there is concern that ecosystem change may be induced by changes in sea-ice extent. The relative abundance of krill and salps appears to change interannually with sea-ice extent, and in warm years, when salps proliferate, krill are scarce and dependent predators suffer severely. Further research on the Southern Ocean sea-ice system is required, not only to further our basic understanding of the ecology, but also to provide ecosystem managers with the information necessary for the development of strategies in response to short- and medium-term environmental changes in Antarctica. Technological advances are delivering new sampling platforms such as autonomous underwater vehicles that are improving vastly our ability to sample the Antarctic under sea-ice environment. Data from such platforms will enhance greatly our understanding of the globally important Southern Ocean sea-ice ecosystem.

Effects of gravity on pattern formation in directional growth of ice crystals

Side-view images of interfacial patterns and solute concentration distribution during directional growth of ice crystals in a water-KCl solution were obtained by Moire phase shift interferometry. It was shown that the vertical distribution of solute concentration changed drastically due to the effect of gravity depending on the growth cell thickness h. In the case of h=900 μm, the concentrated solution near the growth interface subsided and flowed ahead much further than the diffusion length of solute. In thinner cells, the subsided dense solution extended the brine channel under the lower side of the interface instead of flowing ahead, and caused the formation of an asymmetrical wedged pattern. In the thinnest cell of h=135 μm, although the turbulence in the solution was quite small, the symmetry of the interfacial pattern was still broken. The results indicate that the three-dimensional analysis considering the effect of gravity is essential for the interfacial patterns even in the thin growth cell.

A mesocosm study of physical-biological interactions in artificial sea ice: effects of brine channel surface evolution and brine movement on algal biomass

The impact of changing physico-chemical boundary conditions in sea ice on biological processes was investigated during a 20-day-long simulated freeze-melt cycle in an 180-m3 mesocosm filled with artificial seawater and addition of a mixed Arctic sea-ice community. Ice formation started at Tair of –15°C with a growth rate of 0.7–1.2 mm h–1 for 10 days. The last 10 days (Tair of=–5°C), ice thickness remained around 20 cm. Ice temperature gradients inside the ice were linear and determined brine salinities. Brine was collected by means of centrifugation and its volume ranged from 5 to 30% of total ice volume. Surface areas of interconnected brine channels were determined with two similar techniques and maximum values ranged between 1.5 and 4.8 m2 kg–1ice. Measurements determined with a modified method varied considerably and differed by a maximal factor of 2.0–6.5. Brine channel surfaces increased during the experiment as a result of the warming of the ice. The inoculated algal community was dominated by flagellates <10 µm. The low diatom biomass increased in the ice after the air temperature rise with rates comparable to field data (µ=0.2–0.3 day–1). Comparison with brine salinities points towards the hypothesis of vertical brine stability being a controlling factor for ice algal growth. We infer from brine channel surface measurements that persistence of brine channel surfaces during spring might be an important prerequisite for the commencement of net diatom biomass accumulation. Advantages and limitations of mesoscale mesocosms as alternatives in ice biological work are discussed.

Chlorinity/salinity distribution patterns in experimental granular sea ice

AbstractThis work investigates the possibility of brine-channel formation and development during the freezing of granular ice from a loose frazil-ice suspension in an NaCl solution at set-water concentration. Three experiments were performed at various constant growth rates in a purpose-built vessel with computer-controlled thermal driving. High-resolution chlorinity measurements are used as a proxy for the bulk salinity of the samples. These show clear brine-segregation processes in the ice, with very high salinity gradients for the fast (10 mm h−1) to medium (2 mm h−1) freezing rates, provided that a suitable sampling scale is adopted. Weak segregation was found at the low freezing rate (0.5 mm h−1). The spatial distribution of the bulk salinity fits the visual appearance of brine channels in the ice adequately, in both horizontal and vertical sections. In a similar way to columnar-congelation sea ice, the number of brine channels significantly decreases with growth rate, but the density of channels is systematically lower in the granular ice than that found at equivalent freezing rates in the columnar ice. This is attributed to the lower geometrical constraints on brine transport in the granular medium. Contrasts between brine-channel geometry and density at different growth rates are discussed in light of the "mushy-layer" concept adapted to sea-ice growth from the solidification of alloys

Primary production in the upper sea ice

Observations and expenments were conducted on fast ice in McMurdo Sound, Antarctica, to investigate seasonal changes in primary production in the upper sea ice intenor. In November and early December 1995, a dense phytoflagellate assemblage developed in the brine channels and pockets at a snow-free site. Primary production was calculated from I4C measurements of primary productivity in brine samples combined with estimates of the proportion of the ice volume occupied by brine. On 4 December 1995, when the dinoflagellate PolareUa glacialis dominated, eshmated daily production peaked at 12.4 mg C m-' in the upper 50 cm of ice. On this date, brine temperature was --3°C and brine salinity was -60. By mid-December, daily production d e c h e d by 77 %, but chlorophyll-specific rates of photosynthesis remained high. The decline in production coincided with encystment of P. glacialis and nutnent deplehon, the former triggered by the latter. Pnmary production continued to decrease during December and January. On 9 January 1996, when ice temperatures were --1°C and brine salinlty was -20, there was a brief bloom of small pennate diatoms in the upper ice interior, but chlorophyll-specific rates of photosynthesis were low and estimated daily production was < l mg C m-' Based on I4C uptake and brine volume, algal production in the upper 50 cm of sea ice was 181 mg C m-* for the season (mid-November through mid-January). Increases in phytoflagellate biomass in the upper 90 cm of ice for this same period indicated that produchon was 2 5 6 mg C m-'. Bnef early season blooms of cryoand halo-tolerant phytoflagellates accounted for most of the primary production in the upper sea ice interior.

Implications of brine channel geometry and surface area for the interaction of sympagic organisms in Arctic sea ice

Dynamic temporal and spatial changes of physical, chemical and spatial properties of sea ice pose many challenges to the sympagic community which inhabit a network of brine channels in its interior. Experiments were conducted to reveal the influence of the internal surface area and the structure of the network on species composition and distribution within sea ice. The surface of the brine channel walls was measured via a newly developed method using a fluorogenic tracer. These measurements allowed us to quantify the internal surface area accessible for predators of different sizes, at different ice temperatures and in different ice textures. Total internal surface area ranged from 0.6 to 4 m2 kg−1 ice and declined with decreasing ice temperature. Potentially, 6 to 41% of the area at −2°C is covered by micro-organisms. Cooling from −2 to −6°C drastically increases the coverage of organisms in brine channels due to a surface reduction. A combination of brine channel frequency measurements with an artificial brine network experiment suggests that brine channels ≤200 μm comprise a spatial refuge with microbial community concentrations one to two magnitudes higher than in the remaining channel network. The plasticity of predators to traverse narrow passages was experimentally tested for representative Arctic sympagic rotifers, turbellarians, and nematodes. By conforming to the osmotic pressure of the brine turbellaria match their body dimensions to the fluctuating dimensions of the brine channel system during freezing. Rotifers penetrate very narrow passages several times their body length and 57% their body diameter. In summary, ice texture, temperature, and bulk salinity influence the predatory–prey interactions by superimposing its structural component on the dynamic of the sympagic food web. Larger predators are excluded from brine channels depending on the architecture of the channel network. However, extreme body flexibility allows some predators to traverse structural impasses in the brine channel network.

Multidisciplinary ice tank study shedding new light on sea ice growth processes

A multidisciplinary ice tank study, involving an international team of scientists, is shedding new light on sea ice growth processes. New findings have been made on brine distribution, on the fate of dissolved organic compounds during ice growth, and on sediment entrainment through frazil ice formation. For example, it was found that there is considerable brine migration out of and into brine channels during a freeze‐melt‐refreeze cycle. The study, known as INTERICE II, also is providing quantitative data for future modeling and analyses.

Linkages between salinity and brine channel distribution in young sea ice

A high‐resolution study of bulk salinity was undertaken on laboratory‐grown sea ice to determine the extent of spatial variability of salinity and whether this was associated with brine channel structures. Ice samples at two different stages of development were compared with respect to physical state, brine distribution, and brine channel structures. Cold, growing sea ice is shown to have steep bulk salinity gradients and a highly variable brine distribution that is closely linked to the location and morphology of brine channels. Areas of high bulk salinity are found to correspond directly to the positions of the brine channels, and these areas are surrounded by ice that shows significant brine depletion. It is observed that redistribution of the brine occurs as the ice cover goes through warming and melting phases. The redistribution is attributed to an increase in porosity and pore connectivity permitting migration of brine through the ice. The distribution of brine in “warm” ice is shown to be more homogeneous and independent of brine channel structures. A number of mechanisms exploiting the enhanced mobility of brine in warm sea ice are used to explain the changes in the salinity profiles and brine distribution.

Turbellaria (Archoophora: Acoela) from Antarctic sea ice endofauna - examination of their micromorphology

Acoel Turbellaria constitute a regular component of the metazoa populating Antarctic sea ice (sea-ice endofauna). Two species were collected, which differ in colour, size, shape and egg spawning season. They do not resemble any known pelagic species. Their small body diameter of less than 300 μm allows them to penetrate deeply into the network of brine channels. Their vertical distribution within one ice floe was positively correlated with the accumulation of algal biomass; maxima for both parameters were found in the bottom 5 cm of the floe. The method by which the Turbellaria invade the sea ice is not clear. At present we have no indication that they pass through a pelagic or benthic stage in their life-cycle. As the Turbellaria were found to populate sea ice in areas with water depths ranging from 370 to 4450 m, the presence of benthic phases in their life-cycle, either free-living or epizooic, is not very probable. We suggest that the Turbellaria either use migrating invertebrates as a vector for their propagation or pass through a pelagic stage in their life-cycle.

Winter production of sea ice algae in the western Weddell Sea

Short- and long-term series of observations were carried out during the US–Russian Ice Station Weddell no. 1 (ISW-1) Expedition, 1992, in the western Weddell Sea. The goal of in situ observations was to assess the winter biological dynamics within the 1-year and newly formed sea ice. It was shown that at the initial stage of ice formation, there is a period of mechanical harvesting of plankton cells from the sea water. In this period, the biomass of ice algae was 10–20 times lower, in terms of chlorophyll a concentration, than that of the underlying phytoplankton. A remarkable increase in chlorophyll a content begins when the ice is 30–40-cm thick and environmental conditions are more favourable for algal growth. As a rule, reproduction of algae in the newly formed ice takes place within the lower layer of ice and close to the skeletal layer, where sea water with high nutrient concentrations is transported to the cells through brine channels during oscillation processes. By contrast, the highest concentrations of chlorophyll a in the 1-year ice were found within the upper layers. It was shown that chlorophyll a concentrations produced by the sea ice algae within both the young and the 1-year sea ice were always remarkably higher than chlorophyll a concentrations in the sea water below the ice. These results also indicate that winter production by ice algae in the extensive Antarctic sea ice zone should be considered an important factor in future biological models of the Southern Ocean.

POPULATION DYNAMICS OF MICROALGAE IN THE UPPER LAND‐FAST SEA ICE AT A SNOW‐FREE LOCATION

The population dynamics of interior ice microalgae were investigated at a snow‐free site on annual land‐fast sea ice in McMurdo Sound, Antarctica, during the austral spring and summer of 1995–96. A dynamic successional sequence was observed with life history transformations playing an important role. During late November and early December (austral spring), cryo‐ and halotolerant dinoflagellates and chrysophytes bloomed in brine channels within the upper ice. At this time, competition and grazing pressure are low because of the inability of most marine species to grow under the extreme environmental conditions found in the upper ice during the austral spring. In November and December, dinoflagellates, chrysophytes, and prasinophytes contributed an average of 66%, 44%, and &lt;1% of the phytoflagellate biomass, respectively. Both the dinoflagellates and the chrysophytes encysted in December, with cyst formation most intense just prior to surface melt and flushing of the ice. The cysts appear to be an adaptation for survival and dispersal in the plankton during ice decay and/or overwintering in the sea ice. In January (austral summer), when ice temperatures were similar to those in the water column, pennate diatoms replaced flagellates as the photosynthetic dominants in the upper sea ice. The upper land‐fast sea ice undergoes dramatic seasonal changes in light availability, temperature, brine salinity, and inorganic nutrient availability. Ephemeral blooms of cyst‐forming phytoflagellates exploit this habitat in the austral spring, when both inorganic nutrients and light are available but temperatures &lt;−2° C and brine salinities elevated.

Natural convection during solidification of an alloy from above with application to the evolution of sea ice

We describe a series of laboratory experiments in which aqueous salt solutions were cooled and solidified from above. These solutions serve as model systems of metallic castings, magma chambers and sea ice. As the solutions freeze they form a matrix of ice crystals and interstitial brine, called a mushy layer. The brine initially remains confined to the mushy layer. Convection of brine from the interior of the mushy layer begins abruptly once the depth of the layer exceeds a critical value. The principal path for brine expelled from the mushy layer is through ‘brine channels’, vertical channels of essentially zero solid fraction, which are commonly observed in sea ice and metallic castings. By varying the initial and boundary conditions in the experiments, we have been able to determine the parameters controlling the critical depth of the mushy layer. The results are consistent with the hypothesis that brine expulsion is initially determined by a critical Rayleigh number for the mushy layer. The convection of salty fluid out of the mushy layer allows additional solidification within it, which increases the solid fraction. We present the first measurements of the temporal evolution of the solid fraction within a laboratory simulation of growing sea ice. We show how the additional growth of ice within the layer affects its rate of growth.

Upward flushing of sea water through first year ice

Abstract Observations in both the ice and slush layers suggest that sea water intrudes into the snow layer following a snow storm. Ice temperature values recorded at 1 cm below the snow‐ice interface show that the upward flux of sea water is of short duration. This is followed by a period of intense brine drainage characterized by the migration of a salty brine layer, with salinities up to 42 psu. These results suggest that a snow storm induces a complete (upward) flushing of the brine channel network and major modifications of snow and ice characteristics. Melt rates and downward brine fluxes were calculated using salinity measured in a 40 cm deep box placed on the ice‐water interface, which isolated a 50 × 50 cm area of sea ice from ocean mixing processes. In this semi‐isolated environment, observed salinity changes allowed us to determine melt water fluxes and brine drainage or flushing even though ice thickness measurements did not show any significant change. Melt rates up to 21 cm/month and equivalent growth rates up to 32 cm/month were measured.

Acoustic scattering in an elastic medium as it relates to sea ice and the determination of effective moduli

The scattering of sound in ice is a complex problem which depends upon the material properties of the ice as well as the frequency of the sound source. Because of the complexity of the problem, a simplified approach to acoustic wave scattering is taken. The ice is modeled as a two‐phase medium in which fluid‐filled cylindrical cavities (brine channels) are embedded in an attenuating elastic matrix. An analytical solution is obtained for the acoustic scattering of longitudinal waves from an infinitely long cylinder. The solution is extended for low‐ka wave propagation in order to determine effective moduli (Lamé parameters and density) and effective wave speeds. An experiment is conducted in the lab to determine the scattering cross section versus angle for a single cylindrical scatterer for frequencies of 160 and 200 kHz. In addition, effective moduli are determined as a function of cavity concentration. The experimental and theoretical results are presented. [Work supported by ONR.]

Morphology and morphogenesis of Thigmokeronopsis antarctica nov. spec, and T. crystallis nov. spec. (Ciliophora, hypotrichida) from Antarctic Sea Ice

Summary The morphology and morphogenesis of Thigmokeronopsis antarctica nov. spec, and T. crystallis nov. spec, discovered in brine channels of Antarctic sea ice, were investigated using protargol silver impregnation. Both species are characterized by a left postoral ciliary field which is composed of much fewer longitudinal files than in T. jahodai, the type species of the genus. In addition, the new species have unique morphogenetic features, viz. all marginal and dorsal primordia originate de novo and the macronuclear nodules fuse incompletely during cell division. Thigmokeronopsis antarctica is distinguished from T. jahodai by shortened midventral rows, 1 left postoral file and 3–4 buccal cirri; transverse cirri are perhaps absent. Thigmokeronopsis crystallis has long midventral rows, usually 5 slightly irregular left postoral files, 1 buccal, and 8 transverse cirri. Morphogenesis is similar in both species, i.e. an anarchic field originates close to left midventral cirri and produces the opisthe's adorai membranelles, undulating membranes, and fronto-ventral-transverse (FVT) cirri; the proter's anarchic field forms near the paroral membrane and splits into primordia for undulating membranes and adorai membranelles; the FVT anlage originates independently close to the anterior left midventral cirri; all primordia form apparently de novo and all parental cirri are replaced. Reorganization resembles the development of the proter.