Radiolaria Polycystina
Provenance and collection of materials

Most of the surveys on extant polycystine radiolarians published to date are based on samples of their skeletons preserved in the surface sediments, rather than on plankton samples. Sediment samples have some advantages over water-column materials, but also several important shortcomings (see page RaPo. 4 Sedimentary versus water-column materials).

A variety of sediment coring and grabbing devices have been used throughout the years for analyses of the polycystines from the upper centimeters of the sediments (Kennett, 1982). Gravity and Kasten corers are among the simplest, permitting retrieval of up to a few meters of sediment at a time from practically any depth. Piston corers have been used widely due to their ability to recover long sedimentary sequences, up to 20-30 m in length. However, all these devices tend to disturb the sediments, especially the uppermost layer which is of particular importance for the analysis of Recent assemblages. Box corers (rectangular, shallow, ca. 1 m coring boxes which ensure complete closure of water-flow passages after sampling and before leaving the seabed, thus minimizing sample washout during ascent) are preferable for retrieval of the top layer of the sediments. However, the fact that box core samples usually lack the thin uppermost phytodetrital film characteristic of most sediments (Billett et al., 1983) suggests that the bow-wave of the device is strong enough as to wash away any mobile particles before hitting the bottom. Multicorers, an arrangement of several short coring tubes mounted on a rigid frame seem to overcome this problem successfully as they have been shown to collect phytodetritus, as well as significantly higher numbers of macrobenthic specimens than box corers (Bett et al., 1994).

Plankton samples for radiolarian studies are usually collected with nets. However, this group, as well as a few other microzooplanktonic taxa, pose serious methodological difficulties. Indeed, they are too small (around 20-30 to 300 µm) to collect effectively with standard zooplanktonic nets (100 to 300 µm in pore size), yet too scarce in most areas to yield adequate catches with water-bottles or low-powered pumps. Thus, fine-meshed nets have to be employed, which significantly complicates not only the concentration of the radiolarians (due to the concomitant retrieval of other organisms, some of which, like the diatoms, cannot be fractioned out later; see Swanberg and Eide, 1992), but also because net clogging jeopardizes subsequent estimations of the volume of water filtered (Tranter and Smith, 1968; Boltovskoy, 1981b). In order to avoid clogging by smaller particles thus ensuring better estimates of the volume of water filtered and larger sample-sizes, meshes ranging between 60 and up to 100 µm are traditionally used for polycystine studies in the water-column. It should be stressed, however, that both absolute quantitative estimates of radiolarian abundance, and the proportions of at least some species and developmental stages may be seriously biased in these collections: Boltovskoy et al. (1993a) reported that in sediment trap materials from the tropical Atlantic shells below 40-60 µm represent roughly 50% of the overall polycystine fauna.

Estimates of radiolarian abundances in the water-column must be performed with flow-metered nets; clogging of the meshes, in particular of those with small pores, makes assessment of the volume of water filtered based on distance towed and mouth diameter extremely unreliable (Tranter and Smith, 1968; Boltovskoy, 1981a, 1981b). Thus, whenever unflow-metered nets are employed, such as those derived from Tucker's (1951) opening-closing mechanism (e.g., the Multinet, based on Bé's, 1962, design; the MOCNESS, Wiebe et al., 1976; the RMT 1+8, Baker et al., 1973), it is strongly recommended that evaluation of radiolarian concentrations be avoided (species proportions, on the other hand, are in principle unaffected in these samples).

For assessment of the delicate colonial forms, whose abundances are very seriously underestimated by net tow sampling (Dennett et al., 2002), as well as for studies of feeding, growth, metabolism, etc. of live individuals, specimens are collected by divers (e.g., Swanberg, 1979), or by means of very short and slow plankton tows, thus ensuring a better preservation of the protists (Matsuoka, 1992).

Sediment trap techniques have undergone major improvements in the last years, thus constituting a very useful tool for the collection of polycystine materials (US GOFS, 1989; Lange and Boltovskoy, 1995). Simple sediment traps consist of a concentrating cone or funnel which tapers into a collecting jar; the array, which can have either one or several traps, is moored to the bottom or drifts with the current suspended from a buoy at the surface. Time-series models are deployed at different oceanic locations for periods up to a year or more, and are provided with a mechanism which replaces the collecting cup at predetermined intervals thus yielding a detailed record of the changes in the amount and type of flux throughout several seasons (Honjo and Doherty, 1988; Lange and Boltovskoy, 1995).

Sediment-trap materials have some important advantages over planktonic collections. Sample-size is usually much larger in sediment traps than in plankton nets, with fluxes as high as 200,000 shells per m^2 per day having been recorded in the equatorial Atlantic (Boltovskoy et al., 1996; see also Table 3 in Boltovskoy et al., 1993a). Seasonal plankton collections are composed of a sequence of snapshots which represent but an insignificant proportion of the total time elapsed between tows, and may therefore not only under- or overestimate mean protist abundances (e.g., Bé et al., 1985), but also yield "atypical" specific assemblages. Time-series sediment trap samples, on the other hand, integrate over preselected depth and time ranges, thus averaging the overlying plankton over restricted periods which yield adequate chronological resolution to allow pinpointing the relative importance of limited offsets of the yearly cycle. Furthermore, since seasonal variations in total mass flux are usually closely coupled with primary production in the upper mixed layer (Honjo et al., 1982, 1988; Deuser et al., 1983, 1990; Wefer, 1989), comparison of total flux vs. radiolarian numbers and specific makeup can furnish first hand information on indicators (and paleoindicators) of the biological productivity of the associated water masses.

Sediment trap materials, however, also have some shortcomings. Because of limitations associated with the hydrodynamic properties of particle accumulation in the traps, these devices are most effective when deployed at depths in excess of 500-700 m (US GOFS, 1989; Lange and Boltovskoy, 1995). As a result, they integrate the flux from several biologically dissimilar layers (e.g., Kling, 1979; Kling and Boltovskoy, 1995). Furthermore, sinking skeletons intercepted at these depths may not adequately reflect their standing stocks at the surface, nor their specific composition. Boltovskoy and Alder (1992) concluded that, in the Weddell Sea, over 90% of the polycystines that inhabit the upper 400 m are destroyed (probably due to fragmentation by grazing) before reaching 400-900 m of depth. Subsurface advection of shells produced at higher latitudes and integration of low protist abundances over large depth intervals may be responsible for the fact that, in the eastern equatorial Atlantic, polycystine assemblage compositions recorded in plankton samples at 0-300 m are totally different from those recovered in traps at 800-2000 m (Boltovskoy et al., 1996;comparison of abundances).

It should be borne in mind that the yields of sediment trap samples are not amenable to direct comparisons with those of plankton samples: while the former are an expression of the downward flux, which in turn is associated with productivity and preservation, quantitative plankton samples give information on standing stock only. Hence, compositional differences may not only reflect advection, destruction by grazing, etc., but also biological traits of the species considered. Thus, a scarce species with high reproduction, mortality and output rates may be rare in the plankton but abundant in the underlying sediment trap (Kling and Boltovskoy, 1995; Boltovskoy et al., 1996).

As with other zooplanktonic groups, analyses of radiolarian vertical distribution patterns are usually performed with the aid of vertically stratified plankton tows (e.g., Renz, 1976; Kling, 1979; Dworetzky and Morley, 1987; Kling and Boltovskoy, 1995; Abelmann and Gowing, 1997). However, because their identification is based on the siliceous skeleton which preserves after the death of the cell, in order to discriminate live vs. dead protists in the subsurface layers the cytoplasm is often stained with rose Bengal, Sudan black B, or eosin (Petrushevskaya, 1971b; Swanberg and Bjørklund, 1986; Abelmann and Gowing, 1997). Although this technique can furnish some clues on the living depth ranges of the species, it does not provide unequivocal information because of uncertainties associated with the speed of decomposition of the protists' cytoplasm. Boltovskoy and Lena (1970), for example, concluded that specimens of several planktonic Foraminifera still contained protoplasm in their shell 98 days after death. Bernhard (1988) compared estimates of the proportions of presumably live benthic Foraminifera as indicated by rose Bengal and Sudan black B staining and by ATP assay, concluding that stained protoplasm was present in individuals up to 4 weeks after actual death of the cell. These lapses are significantly longer than the time it takes a radiolarian shell to reach the sea-floor (Takahashi and Honjo, 1983).

Unless special cytological studies are required (e.g., Petrushevskaya, 1986), plankton and sediment trap samples can be preserved in 4-5% formaldehyde; the addition of picric acid to the solution enhances the preservation of the colonies, yet acidification should be avoided if the calcareous plankton is to be saved from dissolution (see the chapter Foraminifera).