ABELSON_ AVIGDOR_ DANIEL WEIHS_ AND YOSSI LOYA Hydrodynamic by mifei

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									164                                                                          Notes

-,      P. N. SALKELD, B. L. BAYNE, E. GNAIGER, AND D.                          TANDE, K. S., AND D. SLAGSTAD.         1985. Assimilation
    M. LOWE. 1985. Feeding and resource allocation in                              efficiency in herbivorous aquatic organisms-The      po-
    the mussel Myths edulis: Evidence for time-averaged                            tential of the ratio method using 14C and biogenic
    optimization.  Mar. Ecol. Prog. Ser. 20: 273-287.                               silica as markers. Limnol. Oceanogr. 30: 1093-1099.
LAIWRY, M. R., R. P. HASSE-I-I-,V. FAGERNESS,J. DOWNS,                          YENTSCH, C. S., AND D. W. MENZEL. 1963. A method
    AND C. J. LORENZJZN. 1984. Effect of food acclima-                             for the determination    of phytoplankton  chlorophyll
    tion on assimilation efficiency of Calanus pacificus.                          and pheophytin by fluorescence. Deep-Sea Res. 10:
    Limnol. Oceanogr. 29: 36 l-364.                                                 221-231.
PARSONS, T. R., Y. MAITA, AND C. M. LALLI. 1984. A
    manual of chemical and biological methods for sea-
    water analysis. Pergamon.
SHUMAN, F. R., AND C. J. LORENZEN. 1975. Quantitative
                                                                                                      Submitted: 15 October 1992
    degradation of chlorophyll    by a marine herbivore.                                                 Accepted: 25 May 1993
    Limnol. Oceanogr. 20: 580-586.                                                                        Amended: 7 July 1993


Limnol. Oceanogr., 39( 1). 1994, 164-169
0 1994, by the American    Society of Limnology   and Oceanography,   Inc.



Hydrodynamic impediments to settlement of marine propagules, and
adhesive-filament solutions
        Abstract- “Protruding bodies,” such as kelp stems,                       site selection, passive transport of propagules
     seagrasses, filiform algae, artificial reefs, and engi-                     to the substratum, or both (Butman 1987; Eck-
     neered structures, constitute substrata for prolifer-
     ation of benthic communities of great ecological and                        man 1990; Mullineaux        and Butman 1990).
     economical importance. Unfortunately,       very little is                  Many studies dealing with settlement have
     known of hydrodynamic aspects of settlement in such                         shown the significance of various environ-
     habitats. Based on flow-tank experiments and the-                           mental factors, such as chemical cues, sub-
     oretical considerations,   we discuss hydrodynamic
     interference with settlement of larvae on protruding-
                                                                                 stratum heterogeneity, and flow pattern, on dif-
     body habitats. We suggest that larvae may overcome                          ferential settlement (Jumars and Nowell 1984;
     these interferences by producing mucous threads up                          Chabot and Bourget 1988; Morse 1990). Stud-
     to 100 body lengths in size. These adhesive threads                         ies that deal with flow pattern effects on set-
     enable propagules of suspension feeders to settle in                       tlement consider “planar” substrata, but none
     environments of high food-particle flux and low sed-
     imentation rate. The results suggest that hydrody-                          of them approaches the problems of settlement
     namic impediments to encounter play a major role                            on “protruding bodies” (for definitions, see Fig.
     in determining the spatial distribution      of benthic                     I). Coral reefs, like other benthic environ-
     species on protruding bodies by favoring propagules                         ments, consist of various protruding-body
     of species with such adhesive devices.
                                                                                habitats dominated by species that are ex-
                                                                                tremely rare in nearby planar habitats and vice
                                                                                versa (Ableson and Loya unpubl.).
   Differential settlement by propagules of ben-
                                                                                    Protruding-body    substrata differ dramati-
thic organisms may play an important role in
                                                                                cally from planar substrata in their flow re-
determining the survi vorship of settled indi-
                                                                                gimes. Planar substrata (characterized by high
viduals, adult spatial distribution,   and com-
                                                                                boundary-layer Reynolds numbers), due to the
munity structure (e.g. Gaines and Roughgar-
                                                                                adverse pressure gradient that is developed on
den 1985; Mullineaux        1988). Differential
                                                                                their surface, induce fully turbulent,       thick
settlement is thought to be a result of active
                                                                                boundary layers characterized by eddies and
                                                                                sweeps (Fig. 1, A). Protruding bodies, because
Acknowledgments                                                                 of the reduction of the cross-sectional area for
   We thank P. Jumars, J. Eckman, M. Patterson, M. Ilan,                        the flow to pass through and the resultant, fa-
and two anonymous reviewers for comments on the manu-                           vorable pressure gradient, induce accelerating,
script. We are indebted to Mark Patterson who provided                          laminar, thin boundary-layer       flows (Fig. 1,
us the scheme of the flow tank. Special thanks to Y. Be-
nayahu, Y. Shlesinger, D. Veil, and M. Dahan for help in                        B,C). The hypothesis proposed here is that the
obtaining the coral larvae. The MBL at Eilat provided                           flow regime characterizing      protruding-body
hospitality and lab facilities.                                                 substrata exerts hydrodynamic        impediments
                                                            Notes                                                       165

                                               2Um




                                                                    Fig. 2. Plan view of the nozzle-diffuser       flow tank,
                                                                showing the two acrylic cones (expansion angle of 11”; 2.5
                                                                 x 10 x 34 cm high) functioning as a nozzle (n) and a
                                                                diffuser (d), the closed flume (f) bounded by acrylic walls
                                                                (76.5 x 10.6 x 10.8 cm deep), returning reservoir (r; 20
                                                                 x 20 cm in diam), and acrylic tubing (1.6-cm i.d. x 220
                                                                cm long) connecting the returning reservoir to the nozzle
                                                                and diffuser. Water is recirculated by an electric bilge pump
                                                                (p; 50 liters min-’ capacity) located in the reservoir, and
    Fig. 1. Schematic representation of a planar substra-       DC power supply (s) used to set the flow velocity. Arrows
tum (A) and a protruding body (B) and its cross-section         indicate flow direction.
(C) and the nearby flow regimes. Planar substrata are de-
fined as hard- or soft-bottom areas of spacious and flat-
tened or depressed surfaces, generally horizontal; protrud-
ing bodies are defined as substrata of high slenderness ratio   of the cones. Flow velocities were determined
(i.e. height to width of the body plane normal to the flow)     from analysis of the motions of fine Pliolite
extending above the seabed with most of their surfaces          VT-particles (Goodyear Co.) recorded on vid-
vertical. Planar substrata induce fully turbulent, thick        eotape. Larvae were added to the tank through
boundary layers (6) characterized by eddies and sweeps.
The flow pattern induced by protruding bodies (in high          the return reservoir (Fig. 2) and had a choice
Reynolds numbers) is of two zone types: accelerated lam-        of various settling substrata. In addition to the
inar, thin boundary-layer    flow (L) in upstream turns and     tank walls which were available as planar sub-
retarded turbulent flow in downstream wakes (W). U,,,-          strata, we placed flattened pebbles and coral
free-stream flow; broken lines-the        outer edge of the
boundary layers; broken arrows-streamlines;         solid ar-
                                                                skeletons in the trough, as well as upright acryl-
rows- flow velocity.                                            ic cylinders and skeletons of coral branches
                                                                which served as protruding bodies. During the
                                                                experiments, larval behavior was detected by
to settlement of larvae on their surfaces. These                direct observations and by videotaping. Each
impediments prohibit settlement of larvae that                  experiment lasted 3-10 d, at the end of which
do not have a mechanism to overcome them.                       the tank walls and the various bodies were
   Flow-tank experiments were conducted to                      surveyed by dissecting microscope to count
determine the role of hydrodynamic processes                    settled larvae.
and larval behavior in the settlement of four                      The larvae of S. caliendrum and A. dae-
species of coral larvae in different flow regimes.              dalea attached and metamorphosed exclusive-
Seriatopora caliendrum and Alveopora dae-                       ly on substrata characterized by decelerating
dalea inhabit planar substrata, and Litophyton                  (dilhtser) and nonaccelerating flow (trough walls
arboreum and Dendronephthya hemprichi                           and separation zones that are sites on the
dominate protruding-body        substrata (Abelson              trough’s bottom either where the boundary
and Loya unpubl.). The experiments were con-                    layer separates from the wall or in the down-
ducted in a “nozzle-diffuser”    recirculating flow             stream wake of bodies; Table 1, Fig. 3A,B).
tank designed to create steady, non-uniform                     Larvae of L. arboreum and D. hemprichi set-
flows in a wide velocity range (Fig. 2). Two                    tled preferentially in accelerating flows (nozzle
velocity ranges were established: 0.5-l 2 cm                    and protruding bodies); no settlement was ob-
s-l (Exp. 1 and 3) and 2-50 cm s-l (Exp. 2)                     served in decelerating flows (Table 1; Fig.
where 0.5 and 2 cm s-l are the average free-                    3C,D). Direct observations of larval behavior
stream velocities in the closed flume and 12                    during the experiments clarified the causes for
and 50 cm s-l the velocities in the narrow base                 the significant differences in settlement pat-
166                                                                Notes

   Table 1. Distribution of larval settlement of four coral species among different substratum categories. Upper numbers
of each substratum-category   row indicate the observed frequencies of settled larvae; lower numbers indicate the expected
frequencies based on the relative surface area of each category. G critical value at P = 0.00 1 is 18.467. Alveo. -Alveoporu
daedalea; Seria. -Seriatopora       caliendrum; Dendro. - Dendronephthya hemprichi; Litoph. - Litophyton arboreum.
                                            Alveo.                     Seria.                    Dendro.                    Litoph.
                            Area
                             cw        Exp. 1        Exp. 2   Exp. 1   Exp. 2   Exp. 3   Exp. 1            Exp. 2    Exp. 1           Exp. 2

Protruding   bodies          12          0             0       0           0     0        51                18       287               42
                                         4.8          12.6      1.9     21       3.4      11.6               2.16     56.76            12.6
Nozzle                       21          0             2       0         0       0       23                27          92              39
                                         8.4          22       3.4      36.8     5.9     19.53             12.18       99.33           22
Trough                       43          5            20       1        84      10       17                 4          0                4
                                        17.2          45.1     6.9      75.2     5.2     40.92             25.52     203.4             45.1
Separation   zone               3       34            74       2        82       0         2                 9         94              20
                                         1.2           3.2     0.5       5.3     0.8       2.8               1.7       14.2             3.2
Diffuser                     21          1             9      13         9      18         0                 0          0               0
                                         8.4          22.1     3.4      36.8     5.9      19.5              12.2       99.33           22.1
Total of settled larvae                40            105      16       175      28        93                58        473             105
G-value                               211            406      37       611.5    53       131               135      1,271             218




terns. The settlement pattern of L. arboreum                           the body is reduced; second, after encounter,
and D. hemprichi is a consequence of larval                            the accelerated flow interferes with larval at-
rejection of decelerating flow zones and their                         tachment and establishment. In the encounter
high ability to encounter and attach to sub-                           phase, the quantities of larvae that are trans-
strata in accelerating flows. The larvae of these                      ported to the protruding body surface are dras-
species were observed to deposit in deceler-                           tically reduced due to decline in the effective-
ating and nonaccelerating flow zones, as did                           ness of two important transport mechanisms -
larvae of other species, but most of them, after                       turbulent transport and gravitational     deposi-
brief exploration, actively detached from the                          tion. The first transport mechanism operates
substratum and swam back into the flow. Like-                          via eddies or sweeps to enhance particle de-
wise, they were observed being caught by the                           position (Sumer and Oguz 1978; Abelson et
protruding bodies, at distances from the body                          al. 199 1) and propagular settlement (Charack-
where larvae of the other two species were un-                         lis 198 1; Mullineaux         and Butman 1990).
able to encounter it. For larvae of A. daedaZea                        Sweeps characterize planar substrata and do
and S. caliendrum, in contrast, the protruding                         not exist in the vicinity of protruding bodies.
bodies and nozzle greatly hampered settle-                             Turbulent eddies, however, which might exist
ment; hence, their settlement pattern is a result                      in the vicinity of protruding bodies, are less
of their inability to obtain access to these zones.                    effective as transport mechanisms due to the
Theoretically, larvae gain access to protruding                        laminar boundary layer of the body which in-
bodies through the wake zone of the body by                            hibits their penetration to the body’s surface.
active swimming. In our experiments, how-                                  The efficiency of the second mechanism-
ever, no case of such active settlement was                            gravitational     deposition-is    dependent on
observed. Our observations have shown that                             substratum orientation relative to the gravi-
encounter events of S. caliendrum and A. dae-                          tational vector (Rubenstein and Koehl 1977;
dalea larvae with the protruding bodies are                            LaBarbera 1984; Shimeta and Jumars 199 1).
extremely infrequent. Moreover, in those cases                         Thus, in cases of many vertically oriented sur-
where physical contact took place, larvae were                         faces of the protruding bodies, gravitational
instantaneously swept off the body surface by                          deposition is negligible. Due to the size and
the currents.                                                          density of the larvae, the main mechanism
    Obstructions of the flow regime act on set-                        transporting larvae to protruding bodies is di-
tling larvae during two stages of settlement.                          rect interception, for which the encounter ef-
First, the probability of larval encounter with                        ficiencies of larvae are extremely low due to
                                                                                                                                         Notes                                                167

           A            Seriatopora caliendrum                                                                               n      rxp.1
                                                                                                                                                  the small size of the larvae and the large size
 2                                                                                                                           la exp.2
                                                                                                                             El exp.3
                                                                                                                                                  of the bodies (Rubenstein and Koehl 1977;
                                                                                                                                                 LaBarbera 1984; Shimeta and Jumars 199 1).
                                                                                                                                                     The establishment phase, which is crucial
                                                                                                                                                 for settlement, requires a finite time to achieve
                                                                                                                                                 firm attachment (e.g. Jumars and Nowell 1984;
                                                                                                                                                 Denny 1988). Such a residence time is much
                                                                                                                                                 harder to attain in the case of protruding bod-
                no*z,s           protruding    bodieo      reparation          trough     walls                  diffuser
                                                                                                                                                 ies, as compared with the seabed, due to a
                        accelerating                              nonaccelerating                         decelerating                           combination of high shear stresses (stemming
                        flow                                      flow                                    flow
                                                                                                                                                 from the relatively high velocity gradients in-
                                                                                                                         n       exp.l
                                                                                                                                                 duced by the protruding bodies) and two forces
                                                                                                                         0       exp.2           that sweep the larvae away.
                                                                                                                                                     The first is drag forces that are much higher
                                                                                                                                                 than those on the seabed due to the higher flow
                                                                                                                                                 velocities. Protruding bodies experience flow
                                                                                                                                                 velocities higher than those of seabed envi-
                                                                                                                                                 ronments because of their positions higher in
                                                                                                                                                 the benthic boundary layer. Also, flow in the
                                                                                                                                                 vicinity of the protruding body may be several
               nozzle            protruding    bodiaa     rqmrrtion            trough     V&II)         1        difturer                    ,   times the ambient velocity, depending on the
                        accelerating                           nonaccelerating                           deceleratina
                                                                                                         flow                            -
                                                                                                                                                 cross-section aspect ratio of the body (i.e. the
                                                                                                                                                 ratio between the two diameters of the body’s
                                                                                                                                                 cross-section; for circular cylinders which have
                                                                                                        n
                                                                                                        c3
                                                                                                                 exp.1
                                                                                                                 exp.2
                                                                                                                                                 a ratio of 1, the velocity achieves about twice
                                                                                                                                                 the free-stream velocity).
i
                                                                                                                                                    The second is acceleration reaction forces,
d     O”                                                                                                                                         which are unique for accelerating flows and are
B     40                                                                                                                                         accompanied by drag forces. Protruding bod-
L     30
z                                                                                                                                                ies induce non-uniform         flow, which under
cn    20                                                                                                                                         steady flow conditions       has an acceleration
      10

       0
                                                                                                                                                 component
               nollfs           protruding    bodie      reparation        trough       wat,s     1 ~,,“,i;U~~e;~,;

                        accelerating                         nonaccelerating                                                                                                     &A
                        flow                                 flow                                  flow
                                                                                                                                                                       as=uds*
      ‘E D         Dendronephthya                        hemprichi
‘;;   80                                                                                                     n      exp.1                        u is the velocity and s is distance. The accel-
k
0)    70
                                                                                                             a      exp.2
                                                                                                                                                 eration reaction force acting on a propagule in
:     60                                                                                                                                         such a flow becomes
5     50
B
=     40
      30                                                                                                                                                    I;,   =   V(P~ + kapwb    $.
5
cn    2o
      10
                                                                                                                                                  v/is the propagule volume, &, is the propagule
       0
               “OLZIe          protruding     bodierll   reparation         trough      waIIs      11        diffuser                            density, pwthe water density, and k, the added
                        accelerating
                        flow
                                                             nonaccelerating
                                                             flow
                                                                                                     decelerating
                                                                                                     flow
                                                                                                                                                 mass coefficient which is the ratio of the added
                                                   Flow regime                                                                                   volume of fluid to the volume of the propagule.
                                                                                                                                                 In cases of curvilinear protruding bodies, the
     Fig. 3. Larval settlement of four coral species in dif-
 ferent flow pattern conditions (n-number      of larvae in a                                                                                    flow may accelerate up to several times the
 given experiment). A. n, = 16, n, = 175, n, = 28. B. n,                                                                                         free-stream velocity along the sides of the body.
 = 40, n, = 105. C. n, = 473, n, = 105. D. n, = 93, n, =                                                                                         The vector direction of this acceleration is the
 58. The experiments were conducted in a nozzle-diffuser                                                                                         same as for the drag force. The forces resulting
 flow tank in two flow-velocity  ranges: slow [Exp. 1 (and
 3 in panel A)] and fast (Exp. 2). G-tests for goodness-of-
                                                                                                                                                 from acceleration might be significant in cases
 fit were run for each of the nine experiments and in all                                                                                        of large propagules settling at high accelera-
 cases P KO.001.                                                                                                                                 tion.
168                                                          Notes

                                                                   and other adhesive devices produced by prop-
                                                                   agules of many taxa are used for feeding (Emlet
                                                                   and Strathmann 1985), dispersion (Lane et al.
                                                                    1985), and for high aggregation efficiency
                                                                   among bacteria (Cowen 1992). In our study,
                                                                   however, the common denominator of larvae
                                                                   that produce threads is their ability to settle in
                                                                   protruding habitats, resulting in high abun-
                                                                   dance of their adults in such habitats (Abelson
                                                                   and Loya unpubl.). A related issue for these
                                                                   species is their potential food, which consists
                                                                   of fine particles or organic molecules. Such food
                                                                   particles are distributed        nearly uniformly
                                                                   throughout the benthic boundary layer. Their
                                                                   highest fluxes are obtained in high flow-veloc-
                                                                   ity environments      such as those surrounding
                                                                   protruding bodies.
                                                                      We suggest that the mucous threads and oth-
                                                                   er adhesive devices produced by propagules of
                                                                   various taxa are used as mechanisms to over-
                                                                   come hydrodynamic         interference exerted by
   Fig. 4. Schematic representation of two main catego-            protruding-body      substrata, enabling the prop-
ries of orientation, trajectory, and subsequent retainment         agules to settle on such substrata. We conclude
of “larvae-bearing     threads” by protruding bodies. A. Ori-      that the described hydrodynamic            impedi-
entation of larvae and threads, in which the thread is ex-         ments to settlement may play a major role in
tended into the trapping zone of the body (2rJ determined
by larval radius (r,,). B. Orientation of larvae and threads,      determining the spatial distribution of benthic
in which the local shear flow of the body rotates the larva        species by favoring propagules of species with
and thread so that the thread comes into contact with the          such adhesive devices, while prohibiting         the
body. U,,, - mainstream flow; L-larva;      T-thread;    broken    settlement of larvae that lack them.
arrows-larval      trajectory; solid arrows-flow      direction.
Numbers l-3 describe the sequence of arrival, rotation,                                               Avigdor Abelson’
and attachment of the larva.
                                                                   Department of Zoology
                                                                   Tel Aviv University
   Our observations on larval behavior in the                      Tel Aviv 69978, Israel
flow tank indicate that the capacity of larvae
to overcome the impedence and settle onto
                                                                                                           Daniel Weihs
protruding habitats is directly attributed to                      Faculty of Aerospace Engineering
mucous threads that are secreted by the aboral                     Technion-     Israel Institute of Technology
part of the larvae. The mucous threads are used                    Haifa, Israel
to overcome impedance through the two phases
of settlement. In the first phase, the threads                                                                Yossi Loya
greatly increase the efficiency of larval en-                      Department of Zoology
counter with the substratum over that for lar-                     Tel Aviv University
vae without threads (Fig. 4). In the second
phase, the mucous threads enable an instan-
taneous attachment to the body. The adhesive                       References
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by transient contact in continuous flow and to                         modifications  in stony corals caused by indwelling
                                                                       crabs: Hydrodynamical   advantages for crab feeding.
retain the tethered larvae on the substratum.                          Symbiosis 10: 233-248.
Hence the larva does not require a finite res-
idence time of locally calm flow conditions to
attach.                                                              ’ Present address: Hopkins Marine Station, Stanford
   Previous studies have suggested that mucus                      University, Pacific Grove, California 93950-3094.
                                                                              Notes                                                          169

BUTMAN, C. A. 1987. Larval settlement of soft-sediment                           LANE, D. J. W., A. R. BEAUMONT, AND J. R. HUNTER.
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Limnol. Oceanogr., 39(l),   1994, 169-175
0 1994, by the American     Society of Limnology   and Oceanography,   Inc.



Primary production of prochlorophytes, cyanobacteria, and
eucaryotic ultraphytoplankton: Measurements from
flow cytometric sorting

       Abstract-A partitioning       of ultraphytoplankton                            for prochlorophytes    and 0.2 to 10 fg C celll’ hm ’ for
    primary production among prochlorophytes,          cyano-                         cyanobacteria. Results indicated that the dominant
    bacteria, and eucaryotic algae was made by ship-                                  primary producer was not necessarily the numerical
    board flow cytometric sorting of 14C-labeled cells.                               dominant nor necessarily the group with the highest
    Aggregate primary production was derived from the                                 cell-specific rate of 14Cuptake. Generally, eucaryotic
    sum, over all three ultraplankton groups, of the prod-                            ultraphytoplankton     are dominant because of their
    uct of cell abundance and cell-specific rate of 14C                               high cell-specific rate of 14C uptake and in spite of
    uptake which ranged from 0.03 to 4 fg C celll’ h-l                                their relatively low abundance. Less often, it seems,
                                                                                      procaryotic picoplankton     may dominate in spite of
                                                                                      their low cell-specific rate of 14C uptake because of
                                                                                      their high abundance.
Acknowledgments
   I am grateful to W. G. Harrison for unpublished data
shown in Fig. 3C.                                                                   Primary production, meaning the rate at which
  This work was supported by the following Canadian                              carbon is converted from inorganic to organic
government organizations:   Department   of Fisheries &                          form by photosynthesis, is often measured by
Oceans, Department of National Defense, Canadian Panel
on Energy R&D (PERD), and the interdepartmental “Green                           the rate at which phytoplankton become radio-
Plan.” Other support was provided by the Joint Research                          labeled when supplied with NaH14C0,. Various
Center, Commission of the European Communities.                                  methods exist which allow this production to

								
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