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specific patterning in neural maps, interpretable with the same type of machinery used to decode cortical representation of spatial images.

Much previous work on dynamic perception has shown that for the brain time and space are not processed separately, but can influence each other strongly [19]. The new study [8] points to another example of the interaction between the two dimensions, showing that time analysis can depend on spatial position. Einstein’s stunning insight that revolutionised physics a century ago was that space and time are in some sense ‘the same stuff’ and can be treated in the same way. Perhaps a similar conceptual leap is needed to understand space-time in the brain. While this line of thinking is clearly highly speculative, we have suggested that the effects of saccades on temporal judgements may be a relativistic-like consequence of rapidly shifting receptive-fields at the time of saccades, that also cause spatial compression [20] (schematically illustrated in Figure 2). It remains to be seen whether this approach will provide a useful framework to study spatial and temporal neural events.

References 1. Treisman, M. (1963). Temporal

discrimination and the indifference interval. Implications for a model of the ‘internal clock’. Psychol. Monogr. 77, 1–31.

2. Gibbon, J. (1977). Scalar expectancytheory and Weber’s Law in animaltiming. Psychol. Rev. 84, 279–325.

3. Buonomano, D.V., and Karmarkar, U.R. (2002). How do we tell time? Neuroscientist 8, 42–51.

4. Buhusi, C.V., and Meck, W.H. (2005).What makes us tick? Functional andneural mechanisms of interval timing.Nat. Rev. Neurosci. 6, 755–765.

5. Keele, S.W., Pokorny, R.A., Corcos, D.M., and Ivry, R. (1985). Do perception and motor production share common timing mechanisms: a correctional analysis. Acta Psychol. (Amst.) 60, 173–191.

6. Nagarajan, S.S., Blake, D.T., Wright, B.A., Byl, N., and Merzenich, M.M. (1998). Practice-related improvements in somatosensory interval discrimination are temporally specific but generalize across skin location, hemisphere, and modality. J. Neurosci. 18, 1559–1570.

7. Westheimer, G. (1999). Discrimination of short time intervals by the human observer. Exp. Brain Res. 129, 121–126.

8. Johnston, A., Arnold, D.H., and Nishida, S. (2006). Spatially localised distortions in event time. Curr. Biol., this issue.

9. Roelofs, C.O.Z. (1951). Influence of different sequences of optical stimuli on the duration of a given interval of time. Acta Psychol. (Amst.) 8, 89–128.

10. Verstraten, F.A.J., Kanai, R., Hogendoorn, J.H.A., and Paffen, C.L.E. (2005). Visual motion expands perceived time. Perception 34 (sup), 111.

11. Morrone, M.C., Ross, J., and Burr, D. (2005). Saccadic eye movements cause compression of time as well as space. Nat. Neurosci. 8, 950–954.

12. Ross, J., Morrone, M.C., and Burr, D.C.

(1997). Compression of visual space before saccades. Nature 384, 598–601.

13. Burr, D.C., Morrone, M.C., and Ross, J. (1994). Selective suppression of the magnocellular visual pathway during saccadic eye movements. Nature 371, 511–513.

14. Melcher, D. (2005). Spatiotopic transfer of visual-form adaptation across saccadic eye movements. Curr. Biol. 15, 1745–1748.

15. Leon, M.I., and Shadlen, M.N. (2003). Representation of time by neurons in the posterior parietal cortex of the macaque. Neuron 38, 317–327.

16. Janssen, P., and Shadlen, M.N. (2005). A representation of the hazard rate of elapsed time in macaque area LIP. Nat. Neurosci. 8, 234–241.

17. Buonomano, D.V., and Merzenich, M.M. (1995). Temporal information transformed into a spatial code by a neural network with realistic properties. Science 267, 1028–1030.

18. Eagleman, D.M., Tse, P.U., Buonomano, D., Janssen, P., Nobre, A.C., and Holcombe, A.O. (2005). Time and the brain: how subjective time relates to neural time. J. Neurosci. 25, 10369–10371.

19. Burr, D.C. (2000). Motion vision: are ‘speed lines’ used in human visual motion? Curr. Biol. 10, R440–R443.

20. Morrone, M.C., Ross, J., and Burr, D.C. (2006). Keeping vision stable: rapid updating of spatiotopic receptive fields may cause relativistic-like effects. In Problems of Space and Time in Perception and Action, R. Nijhawan, ed. (Cambridge: CUP).

1Department of Psychology, University of Florence, Italy. 2Faculty of Psychology, San Raffaele University, Milan, Italy.

DOI: 10.1016/j.cub.2006.02.043

Krill Migration: Up and Down All Night

A new study showing Antarctic krill sink when their stomachs are full has provided indirect evidence that krill undergo multiple daily vertical migrations. Such behavior could make a significant contribution to carbon sequestration by the deep oceans.

Kerrie M. Swadling

Many small pelagic animals undertake extensive daily vertical migrations, sometimes travelling hundreds of meters to and from the food-rich surface layers of the ocean. The classic paradigm has organisms ascending to the upper layers at night to feed and returning to deeper waters during the day to avoid visual predators, predominantly fish. It has long been assumed that they make only one round trip every 24 hr [1].

While the vertical migration of populations can be monitored by sampling with nets and other devices, uncovering the movements of individuals has been more problematic. Indirect evidence — analysis of gut contents — has suggested that animals move in and out of the feeding zone, as individuals collected from deep waters at night often contain prey that are only present in surface waters [2]. Direct verification of this, however, has been lacking. A new

study [3] has provided tantalising evidence that one of the most numerically and ecologically important small pelagic species, Antarctic krill (Euphausia superba, Figure 1), undertake more than one vertical migration per day. As they reported recently in Current Biology, by examining the swimming behavior of tethered krill, Tarling and Johnson [3] have shown that individuals actively reposition themselves lower in the water column when their stomachs are full.

Antarctic krill are negatively buoyant and so must swim continuously to remain in the surface layers; if they stop swimming, they sink. Fortunately they can exert some control over their rate of descent by adopting a parachute mode, in which they fan out their swimming legs and open their feeding baskets, to decrease

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Figure 1. Swimming krill (Euphausia superba) capturing food particles.

Photograph courtesy of R. King.

their sinking rate. In their experiments Tarling and Johnson [3] showed that krill with moderately full stomachs adopt the parachute mode significantly more often than those with empty stomachs. From the time it takes krill to digest their stomach contents and their sinking rate when parachuting, Tarling and Johnson [3] calculated that krill might make as many as three vertical migrations per night, during which they descend up to 43 meters while digesting and then re-ascend to feed.

These observations provide important support for a theory known as the hunger/satiation hypothesis, which says that animals make short intermittent forays into surface waters where there is high food concentration, but also an increased risk of predation, and then return to intermediate depths to reduce risks while they digest [1]. If krill are making multiple forays into their foraging grounds it seems the trade-off between the extra energy required to swim back to the surface and that saved by reducing activity while digesting is considerable. Perhaps more compelling, modelling of copepods making similar forays suggests that predation risk is lowered by up to 50% [4]. Such a potential benefit would suggest that other species exhibit similar behavior, yet at present we have no direct evidence for the

prevalence of multiple daily vertical migrations.

The implications of Tarling and Johnson’s [3] study are significant and raise intriguing questions. Many large-scale models of ocean biogeochemical processes tend to take a simplistic view of the behavior of marine populations and rarely, if ever, address the behavior of individuals. Yet if models are going to provide realistic results then understanding these behaviors is imperative. Two issues arise directly from consideration of multiple daily vertical migrations. What are the implications for biogeochemical flux? And what effects could these movements be having on small-scale physics and chemistry of the water column?

Faecal pellets of zooplankton are an important part of the carbon cycle in the southern ocean, with a large proportion being retained and remineralised in the upper ocean [5]. However, large pellets, such as those of Antarctic krill, sink faster than smaller ones and may sink up to 500 meters per day [6]. These pellets can be important transporters of carbon to the deep ocean where the carbon can be sequestered for many hundreds of years [7]. The contents of sediment traps deployed in the southern ocean at greater than 500 meters are often dominated by the pellets of krill and large copepods [8]. Tarling and

Johnson [3] calculated that, through their multiple daily vertical migrations, krill could be indirectly responsible for transferring as much as 6% more carbon than previously estimated to the deep ocean sediment. If other members of the pelagic community are undertaking similar hunger-driven vertical migrations, the addition to the carbon flux below the mixed layer could be considerable.

Surprisingly, the phenomenon of multiple daily vertical migrations could also have a major influence on ocean hydrodynamics [9]. At night, krill tend to be distributed diffusely within 15 to 30 meters of the surface and they are concentrated below 50 meters during the daytime [10,11]. It has been calculated that the movements of krill aggregations could make a significant contribution to turbulent mixing in coastal surface waters [9]. In spring the bloom of phytoplankton in surface waters rapidly depletes macronutrients, especially nitrogen. It is possible that krill­induced mixing causes nutrient­poor surface waters to mix with deeper nutrient-rich waters, and so promotes the continual bloom of phytoplankton on which krill rely for reproduction and growth in summer. This biological turbulence, while not as important as physical mixing, could have significant impacts on water chemistry and biology at scales of 10 to 1000 meters [9]. If true, then it seems clear that multiple daily vertical migrations would only enhance the extent and role of animal-induced mixing.

The laboratory experiments performed by Tarling and Johnson [3] provide an insight into the behavior of one important small pelagic species. What is needed now is field verification of the movements of individuals and populations over 24 hr, and the inclusion of this behavior into biogeochemical and other models.

References 1. Pearre, S., Jr. (2003). Eat and run? The

hunger/satiation hypothesis in vertical migration: history, evidence and consequences. Biol. Rev. Camb Philos Soc 78, 1–79.

2. Pearre, S., Jr. (1973). Vertical migration and feeding in Sagitta elegans Verrill.

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Ecol. 54, 300–314. 3. Tarling, G.A., and Johnson, M.L. (2006).

Satiation gives krill that sinking feeling.

4. Curr. Biol. 16, R83–R84. Leising, A.W., Pierson, J.J., Cary, S., and Frost, B.W. (2005). Copepod foraging and predation within the surface layer during night-time feeding forays. J. Plankt. Res. 27, 987–1001.

5. Beaumont, K.L., Nash, G.V., and Davidson, A.T. (2002). Ultrastructure, morphology and flux of microzooplankton faecal pellets in an east Antarctic fjord. Mar. Ecol. Prog. Ser. 245, 133–148.

6. Cadée, G.C., González, H., and Schnack­

Stem Cells: SpeciNiches in the Wor

Recent work has shown that compodirectly activate a homeodomain trathe cell fate that provides niche funnematode Caenorhabditis elegans.

Samantha Van Hoffelen and Michael A. Herman

The location, identity, fate and control of stem cell populations are active areas of research spanning many disciplines of biology. Stem cells, whether embryonic, adult, somatic or germ-line precursors reside in specific compartments known as ‘stem cell niches’ [1,2]. The concept of the ‘niche’ is far from new, however, the molecular characterization of such microenvironments is still very much under study. Lam et al. [3] have recently reported work which sheds new light on the signaling pathway leading to specification of the germline stem-cell niche in the nematode Caenorhabditis elegans.

Stem-cell niches are defined by their ability to maintain proliferative pools of cells. By secreting factors and providing an organized architecture, niches maintain cells in mitosis while promoting self-renewal. Niches have been described in many tissues, including the hematopoietic system, skin, intestinal epithelium, neural tissue and reproductive system in animals, and roots in plants [4].

Cell–cell adhesion between niche cells and stem cells, accompanied by secreted niche

Schiel, S.B. (1992). Krill diet affects faecal string settling. Polar Biol. 12, 75–80.

7. Broecker, W.S., and Peng, T.H. (1982). Tracers in the Sea. Lamont Doherty Geological Observatory, Columbia University, 690 pp.

8. Suzuki, H., Sasaki, H., and Fukuchi, M. (2003). Loss processes of sinking fecal pellets of zooplankton in the mesopelagic layers of the Antarctic marginal ice zone. J. Oceanogr. 59, 809–818.

9. Huntley, M.E., and Zhou, M. (2004). Influence of animals in turbulence in the sea. Mar. Ecol. Prog. Ser. 273, 65–79.

fying Stem-Cell m

nents of the Wnt signaling pathway nscription factor so as to specify

ction to germline stem cells in the

factors, control the fate of stem cells while allowing cells that migrate from the niche to differentiate (or undergo meiosis in the case of the germline). In the Drosophila testis, germline stem cells form a ring around differentiated cells known as the hub [5]. Upd signals from hub cells activate the JAK/STAT pathway, causing germline stem cells to divide, each giving rise to a germline stem cell and a gonialblast, which subsequently differentiates. Only the cells adjacent to the hub receive Upd signals and undergo self-renewal; so that the hub cells provide the niche for the Drosophila male germline [6].

Other systems are not so well defined and the exact identity, much less the specification, of their niche cells is not well characterized. In C. elegans, the distal tip cell and its extended processes provide a niche for proliferating germ cells. Although it is not yet clear how stem-like the C. elegans germline stem cells really are, it is clear that they remain undifferentiated in the distal tip cell niche and give rise to differentiated progeny; giving them stem-like qualities that are worthy of study.

C. elegans exists as either hermaphrodite or male sexes. In a hermaphrodite, the distal tip cells

10. Godlewska, M. (1996). Vertical migrations of krill (Euphausia superba Dana). Pol. Arch. Hydrobiol. 43, 9–63.

11. Zhou, M., and Dorland, R.D. (2004). Aggregation and vertical migration behavior of Euphausia superba. Deep­sea Res. II 51, 2119–2137.

Tasmanian Aquaculture and Fisheries Institute and School of Zoology, University of Tasmania, Private Bag 49, Hobart, Tasmania 7001, Australia.

DOI: 10.1016/j.cub.2006.02.044

lie at the end of each arm of the U-shaped gonad (Figure 1A). In a male, there is one distal tip cell at the end of their single-armed gonad. The adult hermaphrodite germline has a ‘mitotic region’ at the distal end of the gonad and a more proximal ‘transition zone’. Germline stem cells undergo self­renewal in the mitotic region, which extends about 20 cell diameters along the distal­proximal axis. Just beyond that lies the transition zone, where the germline nuclei begin differentiating and start to enter the early stages of meiotic prophase [7] (Figure 1B).

The distal tip cell and its processes maintain contact with germline stem cells in the mitotic region and express LAG-2, a Delta-like ligand, which binds to GLP-1, a Notch-like receptor expressed by the germline stem cells. GLP-1/Notch signaling activates the Pumilio family RNA binding proteins FBF-1 and FBF­2. These proteins regulate the stability of the gld-1 mRNA that encodes an RNA binding protein that represses the translation of factors required to maintain mitosis, like GLP-1 [7].

Regulation of mRNA stability or translation within germline stem cells is conserved in C. elegans and Drosophila germlines [6]. Removal of the distal tip cell, or loss-of-function of lag-2 or glp-1, causes the germline stem cells to enter meiosis prematurely, thereby loosing their stem-cell identity. Conversely, constitutive GLP-1 signaling blocks entry into meiosis and causes overproliferation of the germ cells. The distal tip cell (and its processes) provides an environment that is required for