Javiera Martinez

Degree project for Master of Science (One Year) in

Biology 45 ECTS

Department of Marine Ecology

University of Gothenburg

399

Feeding and growth in the ctenophore Pleurobrachia pileus

1

Abstract

The abundance and carnivorous habits of Pleurobrachia pileus have been extensively

reported, but little is known about its feeding behavior per se. Feeding and growth of P.

pileus as a function of prey concentration, prey type and ctenophore size were estimated in

laboratory conditions and extrapolated to field conditions in spring. Functional response as

given by clearance rates as a function of prey densities fitted both logarithmic and linear

models. No saturation level was reached within the range of laboratory and field prey

densities. Feeding was enacted even at very low prey densities, when metabolic demands

apparently were not fulfilled, deriving in negative growth. Clearance rates were fitted to an

exponential model (F = 0.2D1.9; R2 = 0.97) after excluding the effect of the container

volume in large animals. (>10 mm). Filtration rates also depended on prey type; both in the

laboratory and in the field. Highest dependence was found when P. pileus fed on

Centropages typicus and lowest when feeding on Temora longicornis. Feeding of P. pileus

(upon several prey types) showed only a slight change in a 24 h period. Both the lack of a

diel rhythm, no saturation in feeding, and no prey type preferences suggest that P. pileus

uses an opportunistic strategy for feeding upon a supposedly patchy prey in a transient

environment. Estimation of feeding impact from field samples suggested that impact is

occasional and meaningful in only one out of nine instances. P. pileus seems to have

physiological and behavioral potential to take advantage of large aggregations of

zooplankton due to its unlimited, continuous and generalist feeding and growth habits.

2

Introduction

The ability of the ctenophores to substantially affect the dynamics of copepods population

has been well documented (Kremer, 1979; Deason, 1982; Sullivan & Reeve, 1982;

Buecheur & Gasse, 1998). Despite this, little is known about the role of ctenophores and

other gelatinous zooplankton species in the energy flow in the marine ecosystems, and the

role of gelatinous zooplankton in the marine food webs is still widely debated (Greene et al

1986). A complete understanding of energy flux through the food web requires detailed

knowledge of the bioenergetics of the organism.

Pleurobrachia pileus is a gelatinous planktonic carnivore (Ctenophora, Tentaculata,

Cydippida) and is an exclusive tentacle feeder (Reeve and Walter, 1978). Copepods seem

to be main prey item for ctenophores. Even though several studies have investigated

feeding in Pleurobrachia sp. there are still uncertainties about their feeding impact on

different prey types and therefore their role in the food web.

Also, actual values for clearance rates have probably been underestimated due to the

influence of container volume (Gibbons, 1992). Most studies have focused on its

abundance, distribution and feeding impact. A few studies have evaluated the feeding and

growth of P. pileus (Reeve & Walter, 1978; Båmstedt, 1998). No systematic investigation

of specific growth rate has been made in P. pileus in either laboratory or field.

P.pileus is considered an important planktivorous predator in coastal waters of the north-

east Atlanthic (Franz & Gieskes 1984; Williams and Collins, 1985) the north west Atlantic

and the Black sea (Mutlu et al., 1994). Its seasonal distribution of P.pileus shows a

fluctuating pattern throughout the year with strong peak abundances one or two times per

year. P. pileus typically occurs in seasonal bloom concentrations in shallows areas, but

peak abundance is reported both for early summer (Van der Veer & Sadde, 1984; Williams

& Collins 1985) and autumn. In the Gullmar fjord the ctenophore P. pileus may be very

abundant during winter and early spring and the impact could be two or three times higher

below the pycnocline if the ctenophores prefer the upper 20 m (Gullström, 1998). No

further studies have been done in P. pileus in the Swedish coast.

3

The role of Pleurobrachia pileus in the carbon flux in coastal waters depends not only on

the predation impact. Information is also needed on energetic parameters such as growth

and respiration and how they relate to factors that are fluctuating in the natural habit as fx.

food availability.

The general aim of this work is to understand the role of ctenophore predation in the

pelagic community of this semi enclosed embayment. More specific goals were to estimate

feeding and growth in the laboratory as a function of prey concentration, prey type and,

ctenophore size, to understand ctenophore feeding behaviour and estimate predation impact

in the field.

Material and Methods

Field sampling

Plankton sampling was carried out in the western part of the Gullmar fjord during spring

2007 (March 7- July 11). Oblique WP-2 90 µm hauls between 10-30 m depth were made

every two weeks to follow the abundance and size of ctenophores. A Bongo 450 net and a

hand net were used to collect live animals for feeding experiments.

Maintenance of experimental animals

Ctenophores (P. pileus) were kept in aquaria at Kristineberg Marine Research Station. Soon

after collection organisms were transferred into 10 l buckets and brought to the laboratory

where they were kept in filtered sea water (15º C, 34 psu) with light aeration. The

individuals were starved for at least 3 days before the beginning of the experiments. When

kept longer, the jellyfish were fed with the copepod Acartia tonsa. The prey organisms (A.

tonsa) were cultivated in the laboratory and fed with cultivated Rhodomonas sp. Wild

4

zooplankton were collected with a 90 µm plankton net and then stored in aerated filtered

sea water (15º C, 34 psu) for no more than two days.

Growth and feeding experiments

To determine the effect of prey concentration on growth of P. pileus, a 3 d incubation was

made where P. pileus was fed with A. tonsa at different concentrations (0-60 prey l-1).

Twelve individuals of similar size ((10.5 ± 0.3 mm diameter)were transferred to 30 l

containers with 3 animals each. Two additional containers were used as controls (20

copepods l-1 and no ctenophores). Conditions (Tº = 15º C, salinity = 34 psu) were kept

constant during the entire experimental period.

Prey were counted in aliquots from beakers with prey suspensions, to obtain desired prey

concentrations for experimental and control containers. After ca. 18-24 h, the P. pileus

individuals were removed from the containers and the remaining prey were filtered into a

200 µm filter and counted. The ctenophores were transferred to a new container and fed

approximately the same prey number every day. At the end of each feeding experiment the

size of the jelly fish (diameter) was measured again. Data were standardized to dry body

weight. The specific growth rate μ, (μ d-1) was determined as: μ = ln(Wt /W0)/t, where W0

and Wt are carbon body contents on days 0 and t respectively. The mean prey concentration

(Cm), clearance rate (F) and ingestion rate (I) were calculated by the following equations:

Cm = exp[ln(C0 × Ct)/2]; F = (V/ t × n) × ln(C0/Ct); I = (Ct – C0)/(n × t), or I = F × Cm ,

where C0 and Ct = prey concentration on day 0 and day t respectively, n = number of

ctenophore, V = volume of water in the aquarium.

Size-dependent clearance rate

In order to estimate clearance rate as a function of predator size, feeding experiments were

carried out for 24 h periods. Experiments were performed with ctenophores ranging

between 3 and 19 mm diameter and prey concentration of 20 ind. l-1 of A. tonsa. In order to

discard container size effect on clearance rates, experiments were carried out in 20-50 l

5

containers for small individuals and in 100 l container for individuals larger than 10 mm.

(Table 1).

Size dependent clearance experiments were also made with wild zooplankton as

prey. A stock-solution of wild zooplankton was prepared with the most abundant prey types

(larger than 200 µm). The concentration of prey was similar in each container.

Prey type-dependent Clearance rate

The effect of prey type on clearance rate was estimated in feeding experiments where the

exponential reduction of prey types were followed over 24 h periods. Four containers with

3 ctenophores each and 2 controls with no ctenophores were used. Their average size was

13 ± 0.7 mm in diameter. At time 0 a known concentration of wild zooplankton from a

stock solution was added. Every 6 h the water of each container was filtered and the

remaining prey counted. Only the four dominant species making the bulk of the

zooplankton were counted.

This experimental design had to assume, as usual, that filtering rate was constant and the

stock-solution was homogeneous throughout the experiments. Clearance rates of P. pileus

feeding upon different prey were estimated for individual prey types as:

F = S*V/ n t

where S = slope of the fitted e line in a plot of concentration versus time, V = volume of the

container (30 l), n = the number of ctenophores in the container (2), t = duration of the

experiment (24 h).

6

Conversion factors

P. pileus body wet weight (WW, mg) was estimated as a function of diameter (D, mm) as:

WW = 0.682D2.522 (Mutlu & Bingle 1999). Carbon content was 3.4% of dry weight

(Hoeger 1983). C (ng) content of A. tonsa was estimated as a function of the prosome

length (µm) from W=1.11 x 10-5 L2.92 (Berggreen et al., 1998)

Results

Growth and feeding on Acartia tonsa

Specific growth rates (µ, d-1) were computed as a function of prey concentration. Maximum

growth rates (ca. 0.07 d-1) were reached at 121 µg C l-1 corresponding to 40 ind l-1 (Acartia

tonsa, adults and copepodites) (Fig. 1). It should be noticed that zero growth rate was

reached at low prey concentration (ca. 20 ug C l-1) and even negative growth rates (-0.04 d-

1) were detected when food concentrations reached zero.

Ingestion and clearance rates were computed from the same experiments.

The clearance rates decreased as a function of prey density (Fig. 2a). At low prey

concentrations clearance rates decreased more abruptly than at higher prey concentrations

(> 50 ug Cl-1).

Ingestion rates increased linearly with prey density and satiation was not reached over the

range of prey densities when A. tonsa was used as prey (Fig. 2b).

7

2- Size-dependent clearance rate

Clearance rates were dependent of the container volume. Pooled clearance data of P. pileus

larger than 10.5 mm from experiments performed in 20, 30, 50 l were compared with

experiments done in 100 l containers. Large animals showed significantly depressed

clearance rates (Mann Whitney U test ; p < 0.001), when experiments were performed in 50

l container or less (Table 1). Data (excluding those for large animals in small containers)

were fitted to an exponential model and showed a significant increase of clearance rates

with ctenophore size (Fig. 3a). Consistently, when data are plotted in a log log relationship,

clearance rates showed a significantly straight linear relationship with ctenophore dry

weight (Fig 3 b).

3- Clearance rates in a natural assemblage of prey

Clearance rates of P. pileus fed wild mixed zooplankton dominated by cladocerans, Acartia

clausii (length 1.1 mm), Centropages typicus (length 1.7 mm) and Temora longicornis

(length 1.6 mm) were followed during a 24 h cycle. The reduction of the four types of prey

was always exponential. The highest reduction rates was for Cladocerans with F = 5.032e -

0.041t R2=0.95, followed by C. typicus F =3.99e -0.041t R2= 0.55, mean while A. clausii F =

10.81e -0.041t R2 = 0.25 and T. longicornis F = 2.82e -0.031t R2 = 0.22

The corresponding clearance rates (Table II) obtained from the slopes showed that the

highest values correspond to Cladocerans and C. typicus meanwhile A. clausii and T.

longicornis. showed similar values between them but values three times lower compared

with the species of copepods mentioned above.

8

Clearance rates of P. pileus fed wild mixed zooplankton increased as a function of

individual ctenophore size and prey type (Fig. 4a). The y intercept varied little compared to

the overall range of clearance rates. Instead of slight differences in the exponent,

remarkable differences in the clearance rate-ctenophore size relationships were found. The

highest clearance rates were associated with ctenophores feeding upon C. typicus.

Intermediate rates were associated with A. clausii, and lowest rates with T. longicornis.

When clearance rates are expressed as a function of ctenophore dry weight (W, mg), a

linear relationship in a log-log plot was evident (Fig. 4b).

4- Predation impact Abundance, biomass, growth and predation impact were estimated from field samples.

The abundance of P. pileus during the sampling period showed a peak on 16 April (0.3 ind.

m-3) and a gradual decrease until June 12th when they disappeared completely.

The average body size of the ctenophore population increased with time to reach 26 mm on

April 16th and then dropped dramatically to 8.5 mm on April 27th. In May, body size

increased again to peak at 14.8 mm.

The predation impact of P. pileus upon A. tonsa was computed from field densities and

from individual ingestion rates of P.pileus obtained in the laboratory. Only one relatively

low half-life time value (28.7 d) was obtained on April 16th ,which implied the highest

predation impact on A. tonsa population.

9

Discussion

Growth In this study, maximum specific growth rate for Pleurobrachia pileus of 10 mm size fed on

Acartia tonsa was 0.07 d-1 which was reached at 121 µg C corresponding to 40 A. tonsa l-1.

The coefficient exponential growth can be used as a basis of comparison between different

animals (Kideys et al. 2004) and also to evaluate if the ctenophores are realizing their

growth potential in the field. No maximum specific growth rate has previously been

reported for Pleurobrachia pileus in laboratory. This value is comparable with other studies

made in Pleurobrachia bachei where the specific growth rates were 0.04-0.17 for > 6.5 mm

ctenophore size (Hirota 1972) and 0.09 for 6-10 mm (Reeve & Walter 1976) These results

showed that specific growth rate decrease with individual size. Specific growth values for

large individuals obtained by Reeve agreed with the one in this study. However it has to be

considered that the effect of individual size on growth should be related to maximum

growth for the certain size group, since the growth rate also depend on prey concentration

as shown in the present study. This was not ensured in the studies by Hirota (1972) and

Reeve & Walter (1976).

Feeding

The functional response of P. pileus did not differ significantly from a linear fit, with a very

gentle unbroken slope at extremely low prey densities and no saturation level at high

concentrations. However, expected clearance rate as a function of prey densities for filter

feeders (Omori & Ikeda, 1976) are expected to be maximized and constant at low densities

to fulfill energy requirements and to decrease as prey densities increase. Ingestion rates

should increase abruptly and linearly at low prey densities to reach a maximum asymptotic

level at a threshold high prey density. Present results on this pelagic carnivore did not fit

these general expectations, but were consistent with previous experiments on P. pileus in a

Norwegian fjord by Båmstedt, (1998) who found a linear increase of ingestion with prey

density and no saturation. Reeve et al (1978), whose experiments covered a wider range of

10

prey densities, found that ingestion rates increased ca. linearly at lower prey concentrations

until a maximum (asymptote) beyond which ingestion rates did not increase.

The main implication of this result is that P. pileus feeding activity is fully dependent on

prey density and probably dependent on the area covered by tentacles more than changes in

the extension-retraction rates. This implies that ingestion rates at these reduced prey

densities may barely balance metabolic cost, with the consequence of degrowth (below 20

µgC l-1). In fact, during starvation, feeding rates can be 5 times higher at first. But

individuals can control feeding activity at very low densities, by alternating periods of

activity with prolonged immobility in absence of food (Reeve et al 1978)

The second main implications of a near linear, non-saturated feeding strategy means that P.

pileus is prepared to make use of extremely high prey densities, which is a good fit to

patchy and transient food resources. Båmstedt (1998) also remarked very high predation

rates of P. pileus at high prey concentration which he explained by its capacity to reduced

the time of digestion. Therefore if saturation in feeding would be reached, it would not be

associated with digestion rates, but by limitation of prey catching potential related to size of

tentacular surface or a mechanical saturation due to the cease of fishing to transfer the food

to the mouth suggested by Reeve et al. (1978).

A further implication of unlimited predation rates is that predation is a direct function of

prey abundance and not necessarily of prey size and or carbon content. Båmsted (1998)

stated that predation rates are governed by prey numbers and not by prey biomass. By

extension, this implies that this predator is unselective and opportunistic, like other sit and

wait predators. In this respect, it did not fully fit the profile of an ambush predator, which

would regulate energy imput to pursue a prey, according to expected energy gains.

Effect of the volume of the container

The clearance rates of P. pileus feeding on A. tonsa increased with size of predator as it was

shown earlier (Gibbons and Painting, 1992; Buecher and Gasser, 1998). The clearance rate

(F) described as F= aDb had a much higher b exponent related to the diameter than that

obtained by Buecher & Gasser (1998, 1.48 vs 1.9), which could be attributed to the

container size. In this study the container was 20-100 l versus 5 l. Similar results were

11

obtained (Gibbons & Painting, 1992) for animals of >10 mm which filtered at higher rate in

containers larger than 5, 10 and 20 l. Multiple implications are involved in the dimension of

the container and they may even mask the relevance of animal size (Gibbons and Painting

1992).

Prey type dependence feeding

Clearance rates were directly proportional to predator size. Interestingly, there were

differences in the clearance rates upon the different prey species which were not related to

prey size. In fact P. pileus showed the highest rates upon C. typicus (1.7 mm), intermediate

rates for A. clausi (1.1 mm) and minimum rates for T. longicornis (1.6 mm).

Such differences in clearance upon different species are probably the result of different

prey behavior. In the field, differences in predation would be due to many factors such as

prey availability (abundance of the prey), prey accessibility or rather by the probability of

encountering the existing prey and it escape behavior. This probability of encountering prey

is a direct function of prey swimming activity (Rothschildb & Osborn. 1998)

Swimming speed and size would affect encounter probability (Greene et al, 1986) and

handling time (Båmstedt, 1998). Green (1986) remarked upon the importance of

considering prey composition when estimating clearance rates because of their expected

differences in relative availability and vulnerability of prey species. He expressed

vulnerability as a function of density of prey, its swimming speed and susceptibility.

Therefore differences in prey behavior could explain differences in clearance rates in this

study as calculated by Buechner & Gasser (1998) where C. typicus was “described”

associated to a fast swimmer, A. clausi to an intermediate and T. longicornis to a slow

swimmer (Bishop, 1968) .

P. pileus may be considered a generalist in term of size and behavior of prey and an

opportunistic predator in term of prey density. This behavior is equally emphasized by the

lack of dial cycle in is feeding activity. Clearance rates were only slightly decreased at the

end of the 24 h experiment, which is opposite of most zooplankton animals which have a

12

remarkable diel feeding activity associated commonly to diel vertical migration. This

unchanged feeding activity is consistent to a sit and wait opportunistic carnivore.

Predation impact in the field

P. pileus predatory impact over zooplankton community varies widely between 0.002-

9.2% (Hirota, 1974; Bishop, 1967; Miller & Dann, 1989; Bamsted, 1998; Chandy &

Greene, 1995) which has been attributed (beside differences in the method to estimate the

rates) to the layer chosen to estimate the impact, being the mixed layer 2-3 times higher

than below the pycnocline (Gullström, 1998). The estimates of impact in the present study

was noticeable only 1 out 9 times, although in this occasion the impact had the potential to

reduce half of the A. tonsa population The impact was dependent not so much on the

ctenophore density, which was low and rather constant compare to other studies (Frank,

1986; Greeve 1971), but to the ctenophore size and its changing growth.

Conclusion

P. pileus clearance rates were dependent of predator size, prey type and container volume.

No saturation level was reached for ingestion rates within the range of prey densities used

in the laboratory or in the field. Such behavior implies a range of displays extending from

letargy to feeding frenzy, which seems a particular advantageous strategy for

heterogeneous-patchy and transient environments. Changes in predation rates as a function

of prey types was not necessarily explained by predator selectivity of prey sizes, but by

prey behavior, which results in changes in the encounter probability and vulnerability of

prey. P. pileus predatory impact in the field validated laboratory results and could be used

to compare and evaluate impacts of other gelatinous planktonic carnivores and particular of

some of the invader species in the region (e.g. Mnemiopsis spp).

Also other bioenergetic parameters such as specific growth rates were affected by food

availability and such information is valuable when evaluating the role of P. pileus in the

carbon flux in coastal waters.

13

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and ctenophore summer occurrence off the Norwegian north-west coast. Sarsia 83:169-181.

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production. Mar Biol 99:341-352.

Buecher, E., Gasser, B. (1998). Estimation of predatory impact of Pleurobrachia rhodopis

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laboratory experiments. J Plankton Res 20:631-651.

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Chandy, S. T., Greene, C. H. (1995). Estimating the predatory impact of gelatinous

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the ambush entangling predator Pleurobrachia bachei. Ecology 67:1493-1501

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Gyllenberg, G., Greve, W. (1979). Studies on oxygen uptake in ctenophores. Ann Zool

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Møller, L. F., Riisgård, H. U. (2007b). Feeding, bioenergetics and growth in the common

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16

Table I. Effect of the container volume (V, l) over filtration clearance rates (F, l d -1) and

body size (D, mm) of Pleurobrachia pileus. Clearance rates of individual larger than 10

mm are significantly depressed in container volumes less than 100 l (Mann Withney U test

P < 0.001).

D (mm) F (l d-1) V (l)

3.3 1.2 20

4.6 5 20

7 9.1 20

7.3 8.8 20

8 9.3 20

8.3 10.5 50

11.3 13.2 20

12 16.1 50

13.6 16.9 50

14.6 13.3 30

19 15.7 30

10.7 17 100

12.3 19.2 100

16.3 35 100

17

Table II. Pleurobrachia pileus. Clearence rates feeding upon four different prey types. The

clearance rate were obtained from the slopes in (Cladocerans: F = 5.032e -0.04lt;

Centropages sp.: F =3.99e -0.041t; Acartia sp.: F = 10.81e -0.041t; Temora sp.: F = 2.82e -0.031t

Prey Clearance

( l d-1 ind-1 )

Acartia sp. 5.07

Centropages sp. 14.8

Cladocerans 14.7

Temora sp. 4.6

Table III. Field data of Pleurobrachia pileus from surveys in the Gullmarfjord between

March and July of 2007. N = abundance; Wc = biomass per m3; D = polar lengh of P.

pileus; Find = clearance rate obtained from the equation in the size-dependent experiment;

t½ = half time for Acartia tonsa in the field. ∞ indicates t½ > 30 d

Date N (ind m3)

Wc mg C m3

D (mm)

Find (l d-1)

t ½

07-Mar 0.2 0.1 3.4 1.9 ∞ 14-Mar 0.1 0.3 6.8 6.7 ∞ 16-Apr 0.3 23.8 26.0 89.5 28.7 27-Apr 0.2 1.2 8.5 10.5 ∞ 07-May 0.1 0.4 10.0 14.3 ∞ 28-May 0.02 0.5 14.8 30.4 ∞ 12-Jun 0 0 0 0 0 09-Jul 0 0 0 0 0 11-Jul 0 0 0 0 0

18

Spec

ific

grow

th ra

te ( µ,

d-1

)

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0 20 40 60 80 100 120 140

Prey concentration, C (µg C l-1)

Fig.1. Pleurobrachia pileus Specific growth rate (µ,d-1) as a function of prey

concentration(Acartia tonsa).

19

C

lear

ence

rate

, F (

l d-1

)

02468

101214161820

0 50 100 150 200

(A)

Inge

stio

n ra

te, I

(µg

C d

-1)

0

100

200

300

400

500

600

0 50 100 150 200

(B)

Prey concentration, C (µg C l-1)

Fig. 2. Pleurobrachia pileus. Clearance (A) and ingestion rate (B) as a function of prey

concentration (Acartia tonsa).

y = 2.8433x+58.824 R2= 0.9871

y = 15.7e-0.0036x R2 = 0.93

20

Cle

aren

ce, F

( l d

-1 )

0

5

10

15

20

25

30

35

40

0 2 4 6 8 10 12 14 16 18 20

Diameter, D (mm)

Fig. 3a. Pleurobrachia pileus. Clearance rate as a function of individual diameter when

preying on Acartia tonsa. Closed romboids: data obtained in 30-50 l containers. Open

symbols: data adjusted to experiments in 100 l containers. Triangles are not included in

regression line.

Cle

aren

ce, F

( l d

-1 )

F = 1.64W0.8716

R2 = 0.97

1

10

100

1 10 100

Dry weight, W (mg)

Fig. 3b. Clearance rate of Pleurobrachia pileus as a function of dry weight when feeding

on Acartia tonsa.

F = 0.2D1.9 r2 = 0.97

21

Cle

aren

ce, F

( l d

-1 )

0

2

4

6

8

10

12

14

16

18

4 5 6 7 8 9 10 11 12

Diameter, D (mm)

Fig. 4a.., Clearance rate as a function of Pleurobrachia pileus size (D) preying upon wild

zooplankton.

Cle

aren

ce, F

( l d

-1 )

0.1

1

10

100

1 10 100

Dry weigh, W (mg)

Fig.4b. Clearance rates of Pleurobrachia pileus as a function of its size ( biomass) in a log-

log scale, feeding upon prey types.

ο Centropages sp.

■ Acartia sp.

▲ Temora sp.

F = 1.67W0.85

R2 = 0.81

F = 0.96W0.92

R2 = 0.78

F = 0.30W1.0

R2 = 0.75

ο Centropages sp. ■ Acartia sp. ▲ Temora sp.

F = 1.67W0.85 R2 = 0.81 F = 0.96W0.92 R2 = 0.78 F = 0.30W1.0 R2 = 0.75