Vertical movements and distribution of planktonic larvae of the serpulid polychaete Spirobranchus polycerus (Schmarda); effects of changes in hydrostatic pressure

JOURNAL OF EXPERIMENTAL MARINE BIOLOGY AND ECOLOGY

ELSEVIER

J. Exp. Mar. Biol. Ecol. 176 (1994) 87-105

Vertical movements and distribution of planktonic larvae of the serpulid polychaete Spirobranchus polycerus (Schmarda); effects of changes in hydrostatic pressure Joan R. Marsden Depmtnzrnt

qf Biology, McGill University, 1205 Ave. Dr. Penfield. Montreal. H3A IBI, Cmadcr and The Bellairs Research Institute qf McGill Universit~~,St. James. Barbados

(Received 3 December 1992; revision received 8 September; accepted 21 October 1993)

Abstract Vertical distribution, in a 60 cm water column, was recorded for 1, 2, 3, 4 and 6 day larvae of the tropical serpulid Spirobranchus polycerus (Schmarda), in the dark and under low level overhead illumination. All age classes of larvae were distributed unevenly under both light conditions. One-day larvae concentrated at both the bottom and the top of the water column; 2- and 3-day larvae concentrated at the top, as did 4-day larvae in the dark. Four-day larvae in the light and h-day larvae under both light regimes were equally concentrated at the top and the bottom. Upward and downward vertical displacement rates were measured for swimming l-day larvae; sinking speed was measured for anaesthetized larvae. One day larvae swim up or down more frequently than horizontally. One-day larvae respond to an increase in hydrostatic pressure with an increase in the percent of larvae moving downward; they respond to a decrease in pressure with an increase in the percent moving upward. The same overall change in pressure was applied at three rates; in all cases the change in the percent of larvae moving up or down was significant at the p < 0.05 level. Maximum calculated rates of change in pressure experienced by larvae swimming up and down are close to the lower end of the range of experimentally applied

rates of change of pressure. The results are discussed in terms of earlier models for the effects of changes in hydrostatic pressure on decapod larvae. Key words: Hydrostatic pressure; Planktonic larvae; Serpulid polychaete; Vertical distribution

1. Introduction

In general, the distribution in the ocean of planktonic invertebrate larvae is regarded as a function of two factors, the physical, hydrodynamic action of water masses and the behavioural, locomotory responses of individual larvae to environmental cues (Mackas et al., 1985; Jackson, 1986; Stancyk & Feller, 1986; Richards & Lindeman, 1987). 0022-0981/94/$7.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0022-0981(93)E0154-Q

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Most studies on invertebrate larvae attempting to evaluate the relative roles of these two factors have been concerned with the responses of settling larvae at the end of planktonic life. There is an extensive literature, including both laboratory and field studies, on the substratum preferences of settling larvae (see review by Pawlik, 1992). Studies on settling barnacle and polychaete larvae have emphasized the importance of tides (Levin, 1986; Kingsford et al., 1991), internal waves (Shanks, 1986) planktollic zonation (Grosberg, 1982) and near-bottom currents (Eckman, 1983; Butman, 1987; Lagadeuc et al., 1990) in effecting shoreward movement to appropriate sites for recruitment. Recent, elegantly designed experiments using laboratory flumes to test settling stages of soft-bottom polychaetes and molluscs have made progress in identifying the roles of hydrodynamic forces and larval behaviour in habitat selection at the time of recruitment (Eckman, 1983; Butman, 1986, 1987, 1989; Butman et al., 1988a,b; Grassle & Butman, 1989; ~ullineaux & Butman, 1991; Butlnan & Grassle, 1992; Grassle et al., 1993; Snelgrove et al., 1993). The presettlement life of a planktonic larva may, however, last for weeks or more, and our understanding of both the nature and the causes of the distribution of presettlement stages is very limited. The patchy natural distribution of planktonic larvae has been regarded as indicative of both the prevailing effect of hydrodynamic forces and of selectively directed locomotion by individual larvae (Banse, 1986; Silva & O’Dor, 1988: LeFevre & Bourget, 1992; Tremblay & Sinclair, 1992). The retention of larval populations in bays, lagoons or estuaries has been attributed, in part, to larval movements that keep them out of currents moving out to sea (Burton & Feldman, 1982; Cronin, 1982; Cronin & Forward, 1982, 1986; Mathivat-Lallier & Cazeaux, 1990; Mullineaux & Butman, 1991). Die1 vertical migrations by decapod (Cronin & Forward, 1986; Hobbs & Botsford, 1992) and polychaete (Bhaud, 1969; Daro, 1973) larvae have been regarded as active excursions from one water layer to another that may result in transport by currents in directions favourable to larval survival. Although laboratorybased experiments have provided information on the capacities of the early stages of decapod and polychaete planktonic larvae to respond to light, salinity, gravity and water-borne chemicals under simplified, artificial conditions (Latz & Forward, 1977; Forward & Cronin, 1979; Forward et al., 1984; Marsden, 1984, 1986, 1987, 1988, 1990, 1991; Marsden et al., 1990; Marsden & Meeuwig, 1990) such studies offer little insight into the deployment of these responses in nature. On the other hand, observations on patterns of vertical distribution in the laboratory and experiments on the effects of changes in hydrostatic pressure, (Sulkin, 1984; Forward, 1989; Forward & Wellins, 1989) have yielded a convincing hypothesis explaining static and changing vertical distributions of planktonic decapod larvae. Such larvae normally move largely up and down and in doing so experience a change in hydrostatic pressure that acts as a stimulus for a change in direction of movement. Larvae of different species or at different stages of development may be responsive to different ranges and rates of pressure change. Light adaptation may have a modulating effect. It is argued that a feedback mechanism of this sort could serve to maintain a larval population within a given vertical distance or cause an upward or downward displacement of the population (Cronin & Forward, 1986; Forward, 1989). The passibility that this model could be applied to other taxa receives anecdotal support from

J.R. Mursden /J. Exp. Mar. Biol. Ecol. 176 (19941 87-105

89

these studies: (1) Banse’s (1986) account of polychaete and echinoderm larvae distributed as though they were neutrally buoyant in discrete, subsurface water masses, (2) observations on the effects of vertical swimming by a sand-beach isopod (Warman et al., 1991), and (3) video records of Eupo&mia larvae moving up and down within a horizontally moving layer of water (Duchene & Nozais, 1992). Control of the vertical distribution of a larval population by up and down movements of individual larvae assumes the absence of water turbulence strong enough to negate larval locomotion. The degree of turbulence to which early larval stages are normally exposed is largely undocumented. Oceanographic studies recording rates of water movement seldom report on the species or developmental stages of larvae present. Presettlement larval populations have been reported both in water masses below the wind-mixed surface layer (Banse, 1986; Tremblay & Sinclair, 1992) and in both surface and deeper layers (Lefevre & Bourget, 1992). Recent calculations on the dissipation of turbulent energy in the sea indicate that wind-generated turbulence declines significantly at about 10 m below the surface (MacKenzie & Leggett, 1991; Agarawal et al., 1992; MacKenzie & Leggett, 1993). Miliekowsky (1973) reports very low rates of vertical water displacement in rapidly moving tidal currents. Although more information is needed, available evidence does not reject the possibility that the Sulkin-Forward model, control of vertical position by responding to small scale changes in hydrostatic pressure, may be both feasible in the open sea and applicable to taxa other than decapod crustacea. The following laboratory study examines the behaviour of presettlement planktonic larvae of the serpulid polychaete Spirobranchus polycevus [Schmard~z), a species with a planktonic larval phase of about 2 wk. Vertical distribution under two light regimes is reported for larvae l-6 days old. One-day larvae are studied in terms of sinking rate, rate of vertical displacement while swimming, direction of swimming and response to a change in hydrostatic pressure. The aim of this study is to test for a significant response to pressure change by larvae of a serpulid polychaete at an early stage of development. Observations are compared with the decapod model provided by Sulkin (1984) and Forward (1989), with comments on the possible significance of behavioural differences between the two taxa.

2. Materials

and methods

2.1. Larval cultures Adults of the two-spined morphotype of S. polycerus (Marsden, 1992) were collected at low tide at Round Rock on the southeast coast of Barbados from March through June, 1993. Adult worms were maintained in running seawater for not more than 4 days. Mature animals were removed from their tubes and placed in seawater in individual containers. Gametes were shed spontaneously and larval cultures were started by fertilizing a mixture of the ova available on any one day with a mixture of available spermatozoa. Usually 3-4 individuals of each sex were involved. Eggs were fertilized

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U-105

in 500 ml beakers. About 1 h after fertilization the supernatant fluid in each beaker was decanted and fresh seawater was added up to the 500 ml mark. Temperature (26 27 ‘C) was controlled by setting the beakers in circulating seawater. Each culture was fed ad tibitum every second day with a mixture of isochrysis gntbana (T strain) and Dunaliella sp. Fertilized eggs of S. pol~~vus reach the morula stage at about 6 h. Morulae are sparsely ciliated and swim fitfully, rising from the bottom and sinking back again. Within the next 6 h the larva differentiates into a pla~ktotropl~ic trochophore with a functional digestive tract. One-day larvae, 22-25 h oId, are small trochophores, about 100 ,nm long (excluding cilia); 2-day larvae are Iarger, around 150 pm Iong. 3y 3 days the body has lengthened posteriorly and the metatrochal ring of swimming cilia is present. Larvae remain metatrochophores, increasing in length until they are about X-9 days old. At this time the larva is over 300 pm long and the first three setigers are becoming apparent in the lost-~ruchal region. At lo-12 days a distinctive red spot is present at the posterior tip of the body, there is a collar rudiment and tentacfes are beginning to form. Larvae at this stage occasionally settle and form tubes on glass in the laboratory.

AI1 data sets tested negatively for normality (Lilfiefors parametric methods were used in all statistical analyses. 2.3.

test).

Consequently

non-

VeMml distribution

The vertical distribution of larvae was examined in 1000 ml glass graduated cyfinders which provided a water column 60 cm high. In preparation for a larval count, a 500 ml larval culture was decanted into a cylinder which was then topped up with seawater to the 1000 ml level. The cylinder was left to equilibrate for 15 min under testing conditions. Larval concentration, counted in 2 ml aliquots from each culture, varied from 5 to 20/ml. Larval counts were made in an air cunditioned faboratory (T = 27- 28 “C) under two light regimes. Under the first regime larvae were exposed to diffuse, overhead itlumination provided by fluorescent lights some 6 feet above the laboratory bench where the graduated cylinders rested on a dark background. In this situation the irradiance level in seawater, tested 2-4 cm below the surface, was 150 x 10’49 . cm2 . s --1(mean value of 10 counts). S. &ycerus larvae are sensitive to white light at this Level ofirradiance (Marsden, 1990) Irradiance was measured using an infegrating quantum scalar &radiance meter (DSI-140 Biospherical Instruments Inc.) with an equal quantum response to light in the 400-700 nm range. Under the second light regime larvae were allowed to equilibrate in the dark in a light-free room in the same laboratory (same temperature conditions). Counts were made using horizontal illumination provided by a projector lamp and passed through a Kodak no. 25 wratten filter (590 nm and above). Larvae of S. ~o~~~e~~ have been shown to be insensitive to light in this wave length range (Marsden, 1990) All experiments were carried out in the afternoon, between 12 noon and 5 pm.

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Larvae were counted using a 10 x hand lens, with a field of view 19 mm in diameter, against a black background. Counts were made at the top of the water column (= level A) (upper margin of the hand lens positioned just under the meniscus), at the bottom of the water column ( = level C) (lower margin of the hand lens positioned at the base of the water column) and at the 500 ml level ( = level B) (500 ml line crossing the middle of the field of view). Larval number was assessed by counting larvae along each of five scans, at successively lower levels, across the field of view. Counts were made first at level A, next at level C and finally at level B. This proceedure was carried out three times in the same sequence. Total time involved was about 20 min. Values used in statistical analysis were means of the three counts at any one level. Counts were made on 10 larval cultures for each of the following age classes: l-day (22-25 h), 2-day (45-51 h), 3-day (69-74 h) and 4-day (93-99 h) and on 6-day (140-146 h) cultures. A Kruskal-Wallis one factor test was applied to the data for each age class to determine if larval abundance varied with level in the cylinder. When appropriate, a non-parametric version of Tukey’s test (Zar, 1984) was used to make pairwise comparisons of abundance at the three levels. 2.4. Direction

qf movement

Direction of movement was determined by observing larvae swimming across a horizontal line (the 500 ml line on the graduated cylinder) bisecting the field of view of the hand lens. Larvae were observed against the background design shown in Fig. 1. Larvae crossing the line on a track 45” or less above or below the horizontal were considered to be moving “along”. All others were considered to be moving “up” or “down”, Ten counts were made for l-day larvae, two on each of five different cultures, all equilibrated for 15 min under overhead illumination. To make a count the direction of movement of each larva crossing the line was recorded during a single scan across the field of view. The scan was then repeated until 25 larvae had been examined. The results were expressed as percent of larvae counted that were ascending, descending or moving horizontally. A Kruskal-Wallis test on arcsine transformed data was used to determine if larvae moved with equal frequency in all three directions.

Fig. 1. Design used in assessing

the angle of direction

of movement

for larvare

crossing

a horizontal

lint.

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2.5.

J.R. Marsden /J. E-up. Mar. Biol. Ecol. 176 11994) 87-105

Sinking

rate

One-day larvae were anaesthetized by immersion for 2 min in 107; rate was measured by adding anaesthetized larvae, rinsed in seawater, water column in a 1000 ml graduated cylinder and recording the time larva sank from the 900 ml mark to the 890 ml mark, a distance of 3 covered from the anaesthetic after about 10 min in seawater.

MgCt,. Sinking to the top of the elapsed as each mm. Larvae re-

Measurement of swimming speed is complicated by variability in the swimming mode of S. polycrrus larvae. Larvae swim, both upward and downward, by moving forward along an undulating course while spiraling about the long axis of the body. The amplitude of undulation varies. In the most direct mode larvae procede in a consistent direction while undulating up and down slightly (amplitude of undulation about 100 pm). In less direct modes the undulations are larger (amplitude up to 1.5 mm) and may take the form of circular loops. The trajectories of larvae moving in this way tend to follow a meandering course with frequent changes of direction. Vertical displacement rate was measured by timing the movement of l-day larvae between two markings on the wall of the graduated cylinder, one 3 mm above the other. Observations were made against the background shown in Fig. 1 for 20 larvae moving up and for 20 larvae moving down. Larvae moving along or changing direction, from up to down or vice versa, within the 3 mm testing distance, were not included. 2.7. Pressure change The results of pressure change were observed in clear, plastic, flat-sided culture bottles, 75 ml capacity, 6.5 cm high (Fig. 2) with a plastic screw cap. The centre of the screw cap was cut away and a 25 cm length of tygon tubing, 1 cm outside diameter, was fitted into the hole such that it ended at shoulder level inside the chamber when the cap was screwed tight. Cap and tubing were cemented together with 5-min epoxy glue. A second opening at shoulder level permitted the insertion of the sensor of a digital vacuum gauge (Vacuubrand, DVR-1; Fisher Scientific}. A 1 m long cord was tied through a small hole near the free end of the tubing. A larval culture was stirred to effect an even distribution and then decanted into the testing chamber, filling it completely. The tubing was filled with seawater and, while submerged in a water table, the cap and chamber were screwed together, thus preventing inclusion of air. Experiments were carried out with the chamber supported by a burette holder on a stand. The free end of the tygon tube was positioned by passing the cord over a bar at the top of the stand (40 cm above the chamber shoulder) and securing it on a screw lower on the stand. Experiments were conducted with the free end of the tube in two positions: one in which the total height of the water column above the shoulder of the chamber was 10 cm (position A) and another in which the height was 20 cm (position B). A change in pressure was effected by pulling in or letting out the cord attached to the free end of the tube. Since 1 cm water = 1 mbar, the hydrostatic pressure generated

J.R. Marsden 1 J. Exp. Mar. Biol. Ecol. 176 (1994) 87-105

‘iv --

--

_---

93

1Ocm

1 -

chamber-4?Lhand lens

Fig. 2. Apparatus used in the pressure change experiments. was moved from position A to position B or vice versa.

To affect a change

in pressure

the tygon tube

at shoulder level in the chamber, by the water in the tube, was 10 mbar under the 10 cm high column and 20 mbar under the 20 cm column. These pressures were checked using the vacuum gauge. A horizontal black line was drawn at mid-level on both faces of the chamber. The design in Fig. 1 was added to the line on the back of the chamber. The hand lens was mounted such that the horizontal line on the front of the chamber bisected the field of view. At the start of an experiment the tube was secured in either position A or B and the system was allowed to equilibrate for 10 min. At the end of this period the viewer scanned the horizontal line, in both directions, recording the number of larvae moving up and down (at an angle greater than 45 “). This count was used as a control. The pressure was then changed by moving the tube from A to B or vice versa over a preselected period of time. When the pressure change was completed the number of larvae crossing the line was counted again. A set of 20 preliminary experiments, testing for (1) a response to pressure change and (2) the timing of the response were carried out on l-day larvae. In these experiments the pressure was increased over an interval of 5 s, by pulling on the cord while keeping an eye on a running stop watch. The number of larvae moving up and down across the line was counted, using a single 2-directional scan, in both the control count and at 5, 15 and 25 s after the change in pressure was completed. The Wilcoxon paired sample test was used to determine if there was a significant difference between the

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control and experimental counts, expressed as arcsine transformations of the percent of larvae moving upward. In a second set of experiments undertaken following positive results from the preliminary set, pressure was changed over three periods of time which provided three different rates of change. The time periods used were 15, 10 and 5 s, corresponding to rates of change of pressure of 0.66, 1 and 2 mbar- s-‘, respectively. Although every attempt was made to move the end of the tube at an even pace, manual control of this operation was far from perfect. Consequently, although it can be said that a range of rates of pressure change was used the limits of the range cannot be regarded as precise. In this set of experiments, counts were made in the IO-25 s interval after the completion of the pressure change, i.e. over a 15 s time span, making possible three 2-directional scans of the horizontal line and higher absolute counts than were obtained in the first set of experiments. Control counts were also based on three 2-directional scans. For experiments on the effect of increasing pressure, equilibration and control counts were carried out with the tube at the 10 cm level. For experiments on the effect of decreasing pressure the starting level was 20 cm. Not more than four experiments, all starting at the same pressure level, were carried out on one sample from one larval culture. The time required was never more than 45 min. The temperature in the testing chamber was recorded at the beginning and end of each set of experiments. The maximum observed temperature change was 1 “C (29-30 “C). A single larval culture was used for not more than three samples. All experiments were conducted on 22-25-h-old (l-day larvae). All experiments were carried out in the afternoon (12 noon to 5 pm) under overhead illumination at a temperature of 26-27 ‘C. Ten experiments were carried out for each rate of pressure change for both increasing and decreasing pressure. The Wilcoxon paired sample test, on arcsine transformations of the percent of larvae ascending or descending, was used to test for a significant difference between control and experimental counts.

3. Results 3. I.

Vertical ~~stp~buti~~

Kruskal-Wallis one factor tests show that larvae in all five age classes are unevenly distributed in a 60 cm water column, both in the dark and under overhead illumination (p< 0.05 in all cases) (Table lA,B). At 1 day the number of larvae at level C is greater than it is at level B under both light regimes (Tukey’s test: p-c 0.05 in all cases; Table 1C). Values for level C are higher than those for levels B and A but the difference between A and C is significant only at the pc 0.1 level. At 2 and 3 days larvae concentrate at the top of the container. At this stage the number of larvae at level A is greater than it is at both levels B and C, under both light regimes (Tukey’s test: p < 0.05 in all cases; Table 1C). At 4 days the number of larvae at levei A is greater than the numbers at levels B and C under red light (Tukey’s test: ~(0.05, Table lC), but under overhead illumination there is no significant difference between numbers at A and C (Tukey’s test: p> 0.05, Table IC). At 6 days the number of larvae

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Table 1 (A) Median values for counts of number of 1,2,3,4 and 6-day larvae at levels A, B and C under illumination of 590 nm and above (dark counts) and under overhead illumination. (B) Kruskal-Wallis statistic and associated level of probability for a one-factor test for a significant difference between numbers of larvae at levels A, B and C, for each age class of larva under both light regimes. (C) Results of Turkey’s test for a significant difference in numbers of larvae in pairwise comparisons of levels A, B and C under each light regime

Age

Dark counts

Overhead

illumination

days A

B

C

38.0 112.0 122.5 52.5 41.5

19.0 10.0 10.0 7.0 9.5

58.5 22.0 36.5 17.5 23.5

A

B

C

39.5 113.0 90.0 39.0 50.0

23.5 14.0 20.5 13.0 7.5

150.0 36.0 45.0 20.0 18.5

(4 1 2 3 4 6

K-W statistic

K-W statistic

P

P

(B) 1 2 3 4 6

6.894 17.677 22.420 17.804 14.437

13.273 17.620 17.678 12.673 27.840

< 0.05

0 Overhead

Dark count A

B

C

A

B

233,436 2, 3

1

C

A

A B C

illumination

2, 334, 6 2,334

1

_

B C

at level A is greater than the number at level B under both light regimes (Tukey’s test: p 0.05 in all cases, Table 1C). The Wilcoxon paired sample test was used to compare, for each larval age class, the percent of the total count found at level A under the two light regimes. The same comparison was made for the percent at level C. There was no significant difference between counts made in darkness and under overhead illumination, at any one level, for any larval age class (p>O.O5 in all cases). Larvae concentrate at the top and/or bottom of the container in layers ranging from about 2-30 mm deep. At the base (or top) of this layer larval concentration declines abruptly. Within the layer of concentration larvae move both up and down at various angles. Some appear to contact the underside of the surface film or the glass base of the cylinder. When this happens they do not stop but

J.R. ~arsden

96 Table 2 Direction

of movement

/ J. Exp. Mar. Biol. Ecol. 176 (1994) 87-105

of l-day larvae;

percent

moving up, down and along

Trial

UP

Down

Along

1 2 3 4 5 6 7 8 9 10

56 52 28 52 40 24 36 40 40 60

20 20 48 32 48 12 28 44 32 20

24 28 24 16 12 4 36 20 28 20

Median

40

32

22

Range

24-60

20-72

4-36

Table 3 Vertical displacement rates for swimmming 45-90” to the horizontal Trial

l-day

larvae,

in mm. S-I,

for larvae moving

UP

Down

Rate

Rate

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 I6 17 18 19 20

3.3 3.3 2.9 2.9 2.5 2.5 5.0 3.0 4.0 2.5 2.0 0.6 2.0 1.2 I.5 0.9 1.0 1.5 1.0 1.5

2.5 3.3 2.0 1.8 3.3 0.7 2.0 3.5 3.5 2.0 0.7 0.4 0.8 1.0 1.0 1.0 1.5 1.0 0.9 0.9

Median

2.25

1.0

Range

0.6-5.0

0.4-3.5

at 80-90”

and

change direction without any apparent change in speed of movement. In the mid-levels of the container l-day larvae have been observed to move up or down, without changing direction, for distances varying from 2 to 9 mm; 3-day larvae have been seen to move upward for distances of up to 45 cm. 3.2. Direction of movement One-day larvae do not move up, down and along (Table 2) with equal frequency (Kruskai-Wallis one factor test statistic = 11.52; p~O.005). There is no significant difference between the percent of larvae counted that move up and down (Tukey’s test, p < 0.1) but the percent of larvae moving along is si~ific~tly less than the percent moving up or down (Tukey’s test, ~~0.05 in both cases). 3.3. Vertical displacement rates for swimming larvae Vertical displacement rates (Table 3) for l-day larvae swimming both up and down vary from 0.4 to 5 mm * s - 1 depending on the angle of swimming direction with respect to the vertical. There is no significant difference between rates for upward and downward movement (Wilcoxon paired sample test: p>OO5). Among upwardly moving larvae 84% of those crossing the starting line completed the 3 mm course without changing direction. Among downwardly moving larvae the proportion was less, 53 %. 3.4. Sink&g rate Sinking rates of anaesthetized l-day larvae are shown in Table 4. These rates are significantly lower than both downward and upward swimming rates (Wilcoxn paired sample test: p < 0.05 in both cases).

Table 4 Sinking rates of l-day

anaesthetized

larvae Trial 1 2 3 4 5 6 I 8 9 10

0.75 0.35 0.35 0.6 0.75 0.75 0.38 0.6 0.38 0.43

Median

0.63

Range

0.35-0.75

98

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80

60

40

20

1

1 0

0

10

20

30

time (seconds) Fig. 3. Median value {+ range) for the percent of larvae ascending at 5-10, 15-20 and 25-30 s after a pressure change of 1.05 mbar. C = control; * = value significantly different from the control (Wilcoxon paired sample test on arcsine transformed data: p
iO0

/

,

80

60

I

T C-4 40

20

0

0

1

2

3

rate of pressure change. mbar/second Fig. 4. Median values (+ range in one direction) for percent of larvae ascending in pressure at four rates. C = control count; E = experimental counts.

IO-25 s after an increase

J.R. Marsden /J. Exp. Mar. Bid. Ecol. 176 (1994) 87-105

99

3.5. Effects ofchange in pressure In the prelimin~y experiments, a si~ificant increase in the percent of larvae moving upward occurred at 15-20 s after the change in pressure was complete (Wilcoxon paired sample test: p < 0.02). The increase took the form of a wave of upwardly moving larvae, most of them crossing the horizontal line between 15 and 20 s after the change in pressure. There was no significant change in the percent of larvae ascending at 5-10 and 25-30 s after pressure change (Fig. 3). Consequently, in the experiments on effects of rate of pressure change, counts were made between 10 and 25 s after the change in pressure was complete. The effects of three different rates of pressure increase are shown in Fig. 4. Median values for experimental and control counts, expressed as the percent of larvae ascending, were compared using the Wilcoxon paired sample test on arcsine transformed data. All three rates of pressure increase resulted in a significant (p
100

80

20

0 0

1 rate

2

3

of pressure change

Fig. 5. Median values (= range in one direction) for percent of larvae descending in pressure at four rates. C = control counts; E = experimental counts.

lo-25

s after a decrease

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4. Discussion The observation that l-day larvae of S. polycerus are usually moving either up or down is consistent with reports on oyster larvae (Cragg & Gruffydd, 1975; Hidu & Haskin, 1978), decapod larvae (Sulkin, 1973; Forward & Wellins, 1989) and larvae of the polychaetes Eupolymnia nebulosa (Duchene & Nozais, 1992) and Arenicola cristata (per. obs.). In the case of 1-6-day S. polycerus larvae, in a 60 cm column of water in the laboratory, the consequence of this activity is an uneven vertical distribution, both in the dark and under low level overhead illumination, a situation that cannot be directly attributed to a response to light. The uneven vertical distributions and the up and down movements of S. polycerus larvae are consistent with a feedback response to pressure change (Sulkin, 1984) and with the concept (Forward, 1989) of an asymmetrical depth regulatory window leading to vertical displacement of the larval population. According to the decapod model the absence of any significant difference between the concentration of l-day larvae at the bottom and top of a graduated cylinder suggests that in nature larval populations at this age maintain a constant vertical distribution. On the other hand, 2- and 3-day larvae concentrate near the top of the cylinder, a distribution consistent with an upward movement of the larval population. The bimodal distribution shown by all larval cultures in the laboratory implies that the upper and lower limits of the water column are preventing vertical expansion of the population, and that in nature such larvae occupy a column taller than 60 cm. Four-day larvae concentrate at the surface in the dark but not under overhead illumination, suggesting some modulating effect of light. However, only one level of irradiance was used in this study and it is low relative to mean noonday irradiance levels measured in waters around Barbados: 1276x 1014q~cm2.s-’ at 1 m and 829x 1014q.cm2.sm’ at 4 m (Tomascik, pers. comm.). A set of irradiance levels approximating those in nature should be used in any test for an effect of light on the pressure response. Six-day larvae are equally concentrated at the top and bottom under both light regimes implying that larval populations at this age maintain a constant vertical distribution. The maximal vertical displacement rate for l-day S. polycerus larvae (5.0 mm. s-l), is close to the upper end of the range for maximal swimming speeds (direction unspecified) quoted by Chia et al. (1984) (data from Konstantinova, 1966) for polychaete trochophores (0.5-5.2 mm. SC’). The range for l-day S. polycerus larvae encompasses the range of speeds reported for horizontal swimming by l-4 day-larvae of S. gigunteus (0.4-3.3 mm* s-‘) (Marsden, 1984). The overall change in pressure used in this study, 10 mbar, corresponds to a vertical displacement of 10 cm. Since l-day larvae have been observed to travel vertically for up to only 0.9 cm without a change in direction, the threshold level for change in overall pressure may be considerably less than the 10 cm used here. All the experiments with l-day larvae on the effects of decreasing and increasing pressure indicate a change in the proportion of larvae moving up or down in response to a change in pressure. The three rates of pressure change applied (M 0.66-2 mbar s -‘) correspond to a range achievable by larvae moving up or down at z 6-20 mm. s -‘. The maximum rate of vertical displacement measured for l-day larvae is 5 mm * s -I, equivalent to a pressure

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change of 0.5 mbar ’ s-‘, close to the slowest rate of pressure change used in this study. Threshold levels for rates of pressure change may be expected to be lower than this rate. measurement of threshold levels for both parameters of pressure change will be important to any evaluation of the role of pressure response in the swimming behaviour of S. polycerus. If response to pressure change controls the vertical distribution of the larval population then a certain minimum proportion of that population must be responding. The definition of upward and downward swimming as movement at 45” or more to the horizontal is arbitrary. based on the protocol used by Forward & Wellins (1989) and the actual proportion of the S. pofqceuus population that is responding to change in pressure is unknown. Threshold levels for rates of pressure change for this species could be used, together with rates of vertical displacement, to calculate the percent of the population that is moving fast enough to experience threshold levels. Another pertinent estimate is the proportion of larvae that swim far enough in one direction to achieve the threshold level of overall pressure change. Observations on l-day- larvae, at all levels in the water column, show that among larvae moving up or down, some change direction repeatedly within a vertical distance of l-3 mm whereas others continue on an upward or downward course for up to 9 mm. This suggests either that larvae differ widely in threshold levels of pressure change (Sulkin, 1990), or that some are changing direction in response to some other stimulus. The interpretation of the experimental results raises other questions such as: what is the behavioL~r of a larva when its vertical movement in response to pressure change is physically interrupted? why does the angle of ascent or descent vary? how is larval swimming behaviour affected by experimental conditions, such as restricted space, crowding, still water? The results of this study indicate a directional swimming response to a change in hydrostatic pressue by l-day larvae of S. poiycenu but the part played by this response in the complex and varied swimming behaviour of these animals is not clear. Although the results of this study suggest that the stimulus controlling the direction of vertical movement may be similar for crustacean and S. pofycerus larvae, the nature of the behavioural response is not the same. S. pofycerus larvae move downward not by sinking passively but by swimming actively, suggesting that an effective stimulus results in a change in geotactic sign without any change in level of activity. This difference may relate to several aspects of S. yolycevus biology. The young S. polycems larvae studied here are smaller than crustacean larvae (a diameter of 0.1 mm vs 0.5 mm or greater) (this paper; Schembri, 1982) and consequently experience greater drag ( = higher Reynolds numbers) (Vogel, 1981; Emlet & Strathmann, 198.5). Serpulid larvae may aiso have an overall higher buoyancy because of the relatively low density of the posterior end of the body (Marsden & Anderson, 198 1; Marsden & Hassessian, 1986; this paper) and because of the absence of an exoskeleton which must contribute significantly to the specific gravity of a crustacean zoea. Consequently, crustacean larvae, being effectively heavier, may achieve the rate of downward movement required for a change in geotactic sign by passive sinking, whereas the serpulid larva, being lighter and subject to greater drag, may sink too slowly to achieve the threshold rate of change in pressure. In this study sinking rates of l-day larvae were found to be marginally slower than minimal downward displacement rates for swimming larvae. A more precise

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technique for controlling the experimental rate of change in pressure is needed to resolve this problem. A second possibility concerns the disturbance created by sinking or swimming animals: chary swimming moves water close to the body and causes less disturbance at a distance than does passive sinking (Vogel, 1981). A small, vulnerable planktonic larvae presumably gains security by broadcasting as few signs of disturbance as possible and a downward swimming larva may be less noticeable to predators than a sinking larva. Finally, the ciliary activity that results in swimming also creates the feeding current in a serpulid larva (Strathmann, 1978). Possibly the energy budget for these animals makes it impossible for them to spend approximately one-half their time (i.e. time involved in descent) without feeding. This paper offers evidence for a cyclical change in geotactic response, mediated by changes in hydrostatic pressure, in young planktonic larvae of a serpulid polychaete. The evidence suggests, but does not confirm, a depth control system similar to that demonstrated for crustacean larvae by Forward & Wellins (1989) and Forward (1989).

5. Acknowledgements Research reported in this paper was supported by an Operating Grant to the author from the National Science and En~neering Research Council of Canada. The author is grateful for the use of research facilities at the Bellairs Research Institute of McGill University in Barbados.

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