H E L G O I ~ N D E R MEI~Rt~SUNTERSUCHUNGI~N Helgol~inder Meeresunters. 45, 301-316 (1991}
Interactions in soft bottom benthic, communities:
quantitative aspects of b e h a v i o u r in the surface deposit feeders Pygospio elegans (Polychaeta) and
Macoma balthica (Bivalvia}*
Thomas Brey
Alfred- Wegener-Institut ffir Polar- und Meeresforschung;
ColumbusstraBe, D-W-2850 Bremerhaven, Federal Republic of G e r m a n y
ABSTRACT: The surface deposit feeding species Pygospio elegans and Macoma balthica are dominant members of many sandy bottom communities of northern boreal regions. The feeding mode of both species and the tube-building of P. elegans are assumed to affect community structure by interactions with other species. The weight of tubes of P. elegans varied between 2 and 13 g DW/
100 cm 2 at the two stations investigated and during the year, which is equivalent to 230-1500 cm of tubes per 100 cm 2 of sediment surface. Sediment stability may be affected directly or indirectly by the amount of tubes present. M. balttn'ca shows a hnear relation between the maximum size of particles which can be inhaled and animal length. In Kiel Bay, particles > 0.5 mm are out of the range of this species. In summer, the potential feeding area (PFA,) of a P. elegans population at one station in Kiel Bay was 1.8 times the available surface area. The PFA of three different populations of M. balthica in Kiel Bay exceeded the available surface area by factors of 2.6, 2.7, and 3.2. These findings indicate strong intra- and interspecific competition for food. Additionally, the feeding of both species may strongly affect the recruitment of benthic species via pelagic larvae. Experiments are proposed to evaluate the significance of the investigated behavioural aspects for community structure.
I N T R O D U C T I O N
I n t r a - a n d i n t e r s p e c i f i c i n t e r a c t i o n s a n d t h e i r s i g n i f i c a n c e for c o m m u n i t y s t r u c t u r e in soft b o t t o m s h a v e b e e n t h e s u b j e c t of m a n y p u b l i c a t i o n s d u r i n g r e c e n t y e a r s (Bell &
Coull, 1980; B l a c k & P e t e r s o n , 1988; B l a r i c o m , i 9 8 2 ; B o n s d o r f f et al., 1986; G a l l a g h e r et al., 19831 H u n t et aE, 19871 L e v i n , 19811 L u c k e n b a c h , 1987; O l a f s s o n , 1989; P e t e r s o n , 1979; Reise, 1983; W h i f l a c h & Z a j a k , 19851 Wilson, 1983b; W o o d i n , 1981; a n d m a n y others).
A n y i n t e r a c t i o n d e p e n d s o n t h e e f f e c t of o n e a n i m a l o n a n o t h e r a n d v i c e v e r s a , i.e.
i n t e r a c t i o n s a r e b a s e d o n c e r t a i n a s p e c t s of t h e life style or b e h a v i o u r of t h e a n i m a l s . In m o s t cases, w e k n o w t h e m e c h a n i s m s w h i c h c a u s e a p o s i t i v e or n e g a t i v e e f f e c t of o n e a n i m a l o n a n o t h e r , e.g. p r o t e c t i o n , p r e d a t i o n , t e r r i t o r i a l i s m , o c c u p a t i o n of s p a c e , or d i s t u r b a n c e . H o w e v e r , w i t h r e s p e c t to soft b o t t o m b e n t h i c c o m m u n i t i e s , o n l y a f e w a u t h o r s h a v e e x a m i n e d t h e " q u a n t i t y " of a c e r t a i n b e h a v i o u r w h i c h a n i n t e r a c t i o n m a y
" A W I Publication No. 393
9 Biologische Anstalt Helgoland, Hamburg
b e b a s e d on, e.g. the m o v e m e n t of a m e i o b e n t h i c predator (Watzin, 1985); the s e d i m e n t t u r n o v e r of a p o p u l a t i o n of s e d i m e n t feeders (Cadee, 1976 a n d r e f e r e n c e s therein), or the pore w a t e r transport rates of a p o p u l a t i o n of t u b e b u i l d i n g p o l y c h a e t e s (Aller, 1980).
T h e aim of this p a p e r is to e v a l u a t e some aspects of b e h a v i o u r w h i c h are p o t e n t i a l sources of i n t e r a c t i o n s in two surface deposit f e e d i n g species, the t u b e b u i l d i n g polychaete t~gospio elegans (Clapar~de) a n d the bivalve Macoma balthica (L.), which are both very c o m m o n i n shallow s a n d y s e d i m e n t s of the n o r t h e r n b o r e a l regions. In both species, f e e d i n g is a s s u m e d to affect other a n i m a l s which hve at t h e s e d i m e n t surface, either via d i s t u r b a n c e a n d competition for food or via p r e d a t i o n (see e.g. H i n e s et al., 1989; Olafsson, 1989; Wilson, 1981). I have tried to quantify the p o t e n t i a l f e e d i n g a r e a at the s e d i m e n t surface, i.e. the area w i t h i n the r a n g e of the t e n t a c l e s (P. elegans) or the i n h a l e n t siphon (M. baltln'ca) of the animals. Additionally, I i n v e s t i g a t e d t h e particle size selection of M. balthica, w h i c h m a y play a n i m p o r t a n t role for the s u c c e s s f u l r e c r u i t m e n t of species with p e l a g i c larvae (see H i n e s et al., 1989) a n d the a m o u n t of s e d i m e n t which is b o u n d in the t u b e s of P. elegans, w h i c h are a s s u m e d to affect pore w a t e r t r a n s p o r t a n d s e d i m e n t stabihty.
METHODS
Samples were t a k e n at two stations, the s u b t i d a l station " G a b e l s f l a c h " (GF) i n Kiel Bay ( m e d i u m / f i n e sand, 12 m w a t e r depth) a n d the intertidal station " W e s t e r h e v e r " (WH) in the G e r m a n W a d d e n Sea {fine sand), d u r i n g 1986 to 1988 {Fig. 1). S p e c i m e n s for laboratory e x p e r i m e n t s were s a m p l e d at the station GF with a 0.1 m 2 V a n V e e n grab or a 0.09 m 2 box corer. All other s a m p l e s were t a k e n b y h a n d {station GF: diver} o p e r a t e d corers (27 cm 2, 10 cm s a m p l i n g depth), fixed in a s e a w a t e r solution of 0.4 % f o r m a l d e h y d e a n d 3 % Kohrsolin (see Brey, 1986}, s t a i n e d with Bengal rose, a n d s i e v e d t h r o u g h 0.25 m m in the laboratory.
Pygospio elegans -
potential feeding area ( P F A )At the station G F a 40 x 30 crn P V C tray w a s filled with a 4 c m layer of natural sediment, w h i c h h a d b e e n sieved through 1 r n m previously in order to r e m o v e larger animals. O n top of this sediment layer I put unsieved sediment from the u p p e r 3-5 c m of the content of two grabs. Afterwards, the tray w a s filled with seawater. After two days, the specimens of
P. elegans
in the tray had re-established their tubes a n d w e r e easily recognizable by the area around each tube, w h i c h w a s swept clean of all fine detritus particles. T h e size of 50 (June 86) a n d 60 (July 87) randomly selected P F A s w a s measured.Pygospio elegans-
tubesT h e a m o u n t of tubes of/9.
elegans
w a s investigated at severa] dates at both stations.T h e tubes w e r e collected from core samples, dried at 8 0 ~ a n d weighed. In the laboratory, specimens of P.
elegans
w e r e allowed to build tubes in 15-ml glass tubes filled with azoic sediment a n d placed in acirculating
sea water system at 12 ~ A n i m a l s a n d their tubes w e r e m e a s u r e d a n d weighed.I n t e r a c t i o n s i n s o f t b o t t o m b e n t h o s 303
8 ~ 1 0 ~ 1 2 ~ 14 ~
9 I ~J
)iiiiiiii!iiiiii!iiiiiiii i
k i
r I
" iiiii!i ii;i iii!iiiiiii!!'
::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: 1=
y o ~ 8 S e
..:::::::::::::::::::::::::::!):~:~:::::~:~:~:~:~:~.:~:~::...:...:~i~i::~!~i~i~!~i~i:~!~!~!:i~i~iii~
' @ . 5
,,~",e ~ -t " ..i:~:i:i:i:?:i:!:'i:i:!:i :i:i:~:~:!:i:i:!:!:!:!:i:i:~:i:!:!:!:!:!:i:i:~:i:i:?:!:~:!:!:!:!:!:!:i:i:i:??~:~:~:~:~:i:~!i!i:. :
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===================== ::::::::::~!!:! :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::!:i; ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: r . . . .-.,-...,,..-..,~ ... ,,-~.-~ ... . ... :.: ... :.: ... : ... :.x.: ... : ... t
... ~ ~ ~ - . ~ - ~ :
8 = E 1 0 ~ 1 2 = 1 4 ~
Fig'. 1. Location of stations G a b e l s f l a c h (GF, star) in Kiei Bay a n d W e s t e r h e v e r (WH, s q u a r e ) in t h e G e r m a n W a d d e n S e a
M a c o m a b a l t h i c a - p o t e n t i a l f e e d i n g a r e a (PFA)
Single s p e c i m e n s w e r e m e a s u r e d to the lower 0.1 ram, p l a c e d in a t r a y filled with 6 cm n a t u r a l s e d i m e n t a n d k e p t at 12~C. T h e , a n i m a l s b u r i e d t h e m s e l v e s v e r y rapidly a n d started to suck m a t e r i a l from the surface with their i n h a l e n t siphon. T h e a r e a w h i c h h a d b e e n s w e p t c l e a n was m e a s u r e d after 2 a n d 5 h in a first set of e x p e r i m e n t s , a n d after 24 h i n a s e c o n d set.
I~r b a l t h i c a - p a r t i c l e s e l e c t i o n
Self & J u m a r s (1988) stated that M. balthica does not select particles of a c e r t a i n size with its i n h a l e n t siphon, b u t there m a y b e a n u p p e r hmit of particle size w h i c h is r e l a t e d to the size of the animal. T h e m a x i m u m size of particles in the m a n t l e c a v i t y of p r e s e r v e d s p e c i m e n s (4-17 m m length) from the station GF w a s m e a s u r e d u n d e r t h e stereo microscope a n d correlated with a n i m a l length.
RESULTS
P y g o s p i o e l e g a n s - p o t e n t i a l f e e d i n g a r e a (PFA)
T a b l e 1 shows the results of these experiments. The a v e r a g e PFA of P. e l e g a n s was 57 m m 2 {June 86) a n d 91 m m 2 (July 87), respectively. F i g u r e 2 shows the f r e q u e n c y distribution of PFA in July 87. T h e m i n i m u m distance b e t w e e n two t u b e s w a s b e l o w 4 m m i n both experiments. Direct observations s h o w e d that P. e l e g a n s is a b l e to p u t the greater part of its b o d y out of the tube, if the r a n g e of the tentacles is n o t sufficient to
Table 1. The average feeding area of Pygospio elegans from the station GF in two laboratory experiments. Min. dist.: Minimum distance between two tubes; S.D.: Standard deviation
Date N m -2 Feeding area
Min. dist. Mean radius S . D . Average area
(mm) (mm) (mm 2)
19 June 86 4200 3.5 4.26 0.9 57
15 July 87 3800 3,9 5.46 2.0 91
reach a certain spot at the s e d i m e n t surface. Furthermore, I could not o b s e r v e a n y sign of aggressive reactions d u r i n g e n c o u n t e r s of t e n t a c l e s of different s p e c i m e n s .
T h e l e n g t h of the s p e c i m e n s u s e d for f e e d i n g area m e a s u r e m e n t s w a s n o t deter- m i n e d , b u t there is a l e n g t h f r e q u e n c y distribution of P. e l e g a n s a v a i l a b I e from the t u b e s a m p l e s (see below) of July 87 (Fig. 3). A s s u m i n g that the relation b e t w e e n a n i m a l l e n g t h a n d radius of PFA is linear, the p a r a m e t e r s of the c o r r e s p o n d i n g e q u a t i o n m a y b e e s t i m a t e d r o u g h l y from the smallest a n d largest l e n g t h a n d radius v a l u e s (length:
1 r a m - 9 m m ; radius: 1.5 m m - 9 . 5 ram):
RpF A ~ 0.5 -[" 1.0 • Lpygospio; ( m m - m m ) (1)
Interactions in soft bottom b e n t h o s 305
0 50 100 150 200 250
Feeding Area (ram z)
300
Fig. 2. Frequency distribution of potential feeding areas of Pygospio elegans sampled at station GF (10 July 87, N = 60)
60
50
40' ..Q E 30"
z 2O
0
m
6 8
Length (ram)
10
Fig. 3. Length frequency distribution of Pygospio elegans at station GF (10 July 87, N = 289)
P y g o s p i o e l e g a n s - t u b e s
T h e i n d i v i d u a l t u b e b u i l d i n g was i n v e s t i g a t e d b y laboratory e x p e r i m e n t s . F i g u r e 4 shows the relation b e t w e e n l e n g t h of P. eIegans a n d total l e n g t h ( i n c l u d i n g b r a n c h e s ) of the c o r r e s p o n d i n g tubes. T h e r e is a l i n e a r relationship b e t w e e n t u b e l e n g t h a n d a n i m a l l e n g t h :
L T u b e = 2,275 + 3.567 x
Lpygospio;
( m m - m m )r 2 = 0.671, N = 127 (2)
8O
70'
60"
"~ sc
}
E ' ~ ..JZ 3c
2 O
. . . .
' l
TL = 2.275 + 3.567 * PL; r = 0.819, N = 127 o
o
o o o
o o
^ 8
o o o ~ i~o o
~176176
0 2 4 6 8 10 1 2 14
P y g o s p i o L e n g t h ( m m )
Fig. 4. Relation between animal length and tube length in Pygospio elegans (animals from station GF)
By m e a n s of the relation b e t w e e n t u b e l e n g t h a n d t u b e dry w e i g h t D W T u b e ---- - 2 . 1 5 2 + 0.864 x LTube; ( m g - m m )
r 2 = 0.927, N = 94 (3)
a n d the relation b e t w e e n Pygospio l e n g t h a n d Pygospio dry w e i g h t log(DWpygospio) = - 2 . 3 5 6 + 1.750 x log (Lpygospio); ( m g - m m )
r 2 = 0.992, 21 l e n g t h classes, 187 s p e c i m e n s (4) the e m p i r i c a l relation b e t w e e n P. elegans w m g h t a n d t u b e w e i g h t w a s e s t a b l i s h e d (see Fig. 5): DWTube = 7.532 + 97.388 X DWpygospio; ( m g - m m )
r 2 = 0.677. N = 127 (5)
Figure 6 s h o w s t h e a b u n d a n c e ( N / t 0 0 cm a) of P. elegans, the total a m o u n t of tubes (g DW/100 cm2), a n d the a v e r a g e t u b e w e i g h t p e r i n d i v i d u a l at the two stations G F a n d W H d u r i n g the y e a r (combination of d a t a from 1987 a n d 1988). In g e n e r a l , the c h a n g e s of t h e s e p a r a m e t e r s in time are similar at both stations. A b u n d a n c e is l o w in w i n t e r a n d spring, rapidly i n c r e a s i n g t o w a r d s summer. T h e a m o u n t of t u b e s is low d u r i n g w i n t e r a n d spring, b u t h i g h in s u m m e r a n d autumn. T h e a v e r a g e w e i g h t of t u b e s p e r i n d i v i d u a l h o w e v e r , is h i g h e s t in F e b r u a r y a n d March, d e c r e a s i n g towards s u m m e r . F i g u r e 7 s h o w s the vertical distribution of t u b e s in the s e d i m e n t at both stations in July. At station WH t h e tubes r e a c h d o w n to 6 cm depth, w h e r e a s at station GF t h e r e are no t u b e s b e l o w 4 c m depth.
Macorna balthica - p o t e n t i a l f e e d i n g a r e a ( P F A )
Figures 8 a n d 9 s h o w t h e relation b e t w e e n the s q u a r e of l e n g t h a n d PFA in M.
balthica after 2 h, 5 h, a n d 24 h, respectively. T h e c o r r e s p o n d i n g r e g r e s s i o n e q u a t i o n s are:
I n t e r a c t i o n s i n soft b o t t o m b e n t h o s 307
7oi T - D W = 7.523 + 97.38a m P-DW; r=0.823, N = 127 . . . o 6 O
5 0
~ 3 0 ~ 1 7 6 o o o o
o o 0 8 ~ 8
20. o oo ~ - ~ ~ o o
8 o
10
O 0 ,05 .I 1 5 .2 .25 .3 .35 .4
Pygospio D W (mg)
Fig. 5. Relation between animal dry weight and tube dry weight in Pygospio elegans {animals from station OF)
PFA2h = - 1 2 . 9 1 9 + 1.863 x
LMacoma2;
( m m 2 - m m )r 2 = 0.654, N = 30 (6)
P F A s h = - 6 9 . 5 5 9 + 4.354 x
LMacoma2;
( m m 2 - m m )r 2 = 0.740, N = 30 (7)
PFA24 h = 689.716 + 31.398 •
L~4~coma2;
( m m 2 - m m )r 2 = 0.694, N = 51 (8)
Macorna b a l t h i c a -
p a r t i c l e s e l e c t i o nT h e m a n t l e c a v i t y of 72 s p e c i m e n s of M.
balthica
w a s e x a m i n e d a n d t h e l a r g e s t p a r t i c l e f o u n d (i.e. a s a n d grain) w a s m e a s u r e d . T h e r e l a t i o n b e t w e e n m a x i m u m p a r t i c l e d i a m e t e r (D) a n d a n i m a l l e n g t h is (cf. Fig. 10):Max. D = 0.094 + 0.020 •
nMacoma2;
( m m - m m )r 2 = 0.626, N = 72 (9)
D I S C U S S I O N
P y g o s p i o e l e g a n s -
t u b e sT h e effects of p o l y c h a e t e t u b e s o n s e d i m e n t stability, p o r e w a t e r c h e m i s t r y , a n d t h e c o m m u n i t y h a v e b e e n d i s c u s s e d i n t e n s i v e l y s i n c e t h e 1960's. F i e l d o b s e r v a t i o n s a n d e x p e r i m e n t s i n d i c a t e d t h a t t h e p r e s e n c e of t u b e - b u i l d i n g p o l y c h a e t e s s t a b i l i z e s t h e s e d i m e n t a n d facilitates t h e s e t t l e m e n t of o t h e r s p e c i e s (see e.g. S a n d e r s et al., 1962~
Pager, 1964; N e u m a n n & Scoffin, 1970; R h o a d s et al., 1978; Reise, 1 9 8 I ; G a l l a g h e r et al.,
E r 0 0 Z 0
0 r~
@4
E
0 0 0
r ~
. Q
~6
_Q
E 450
300
150
7.5
5.0
2.5
60
40
20
I I
! I
I
T
I
I I
I
I I I
I I I
I I
I
I I I
12.85
J F M A M J J A S 0 N D
Fig. 6. Seasonal variation in abundance, total amount of tubes and average tube weight per individual in Pygospio elegans at the stations G F (circles) and W H (dots). N u m b e r of samples/date:
12 (GF) and 15 (WH). Vertical bars: 95 % confidence limits of m e a n
Interactions i n soft bottom benthos 309
20 40 ~ 20 40 60 80
+
2
4
c m
Sta. W H Sta. GF
Fig. 7. Vertical distribution of tubes of
Pygospio elegans
at the stations WH (21 July 87) and GF (10 July 87)1983}. E c k m a n n et al. (1981) s h o w e d that t u b e s do not increase s e d i m e n t stability d u e to a r e d u c t i o n of the t u r b u l e n t flow along the s e d i m e n t water interface, w h e r e a s Ffihrb6ter &
M a n z e n r i e d e r (1987} could d e m o n s t r a t e that t u b e s do act as a physical protection a g a i n s t erosion u n d e r certain circumstances. As far as we k n o w today, the stabilizing effect of t u b e s is most likely to b e a more indirect one. T u b e - b u i l d e r s c h a n g e the chemical composition of the pore w a t e r a n d affect the vertical transport rates of o x y g e n a n d n u t r i e n t s b y the irrigation of their t u b e s (Aller, 1980, 1983). This m a y e n h a n c e b i o m a s s a n d p r o d u c t i o n of bacteria, diatoms a n d f i l a m e n t o u s algae, which c a n stabilize the s e d i m e n t b y m u c u s secretions (see e.g. Aller & Aller, 1986; H o l l a n d et al., 1974; G r a n t et al., 1986; Reichardt, 1986}.
With respect to
Pygospio elegans,
the a v e r a g e a m o u n t of t u b e s f o u n d w a s 3.51 g DW/100 cm 2 at GF a n d 6.78 g DW/100 cm 2 at WH. With Eq. (3), these v a l u e s l e a d to a total l e n g t h of all t u b e s of 410 cm a n d 780 cm b e l o w 100 cm 2 of s e d i m e n t surface. This is e q u i v a l e n t to a n a v e r a g e t u b e w e i g h t of 26 m g / I n d . (133 Ind./100 cm 2) a n d 20 m g / I n d . (333 Ind./100 cm2), a n d to a n a v e r a g e t u b e l e n g t h of 31 ram/Ind, a n d 23 m m / I n d . , respectively. With respect to the m a x i m u m a m o u n t of t u b e s f o u n d i n a single core (8.8 g DW/100 cm 2 at GF a n d 18.8 g DW/100 cm 2 at WH}, the m a x i m u m total l e n g t h of t u b e s was a b o u t 1020 cm (GF} a n d 2180 cm (WH) per 100 cm 2, a n d a v e r a g e t u b e l e n g t h a m o u n t e d to 45 m m a n d 48 ram, respectively. As s h o w n i n Figure 6, the total a m o u n t of t u b e s is h i g h e s t in s u m m e r a n d a u t u m n , w h e r e a s the a v e r a g e t u b e w e i g h t p e r i n d i v i d u a l1200" o F ( 2 ) = - 1 2 . 9 1 9 + 1 . 8 6 3 * L z ; r = 0 . 8 0 9 , N = 3 0 F ( 5 ) = . 6 g . 5 5 9 § 4 . 3 5 4 * L2 ; r = 0 . 8 6 0 , N = 3 0 O
1 0 0 C , a o o a
600!
~ o D
~ 6 0 C
o o o
,e 400" o o ~ o o
o 13o o o o
a a ~ oo o ~ , ~ o
2 0 0 " o o
o o
50 I O0 150 200 250
Length 2
Fig. 8. Relation between potential feeding area and animal length in Macoma balthica after 2 h and 5 h of exposition
1 4 0 0 0 '
1 2 0 0 0 '
I0000~
80001
eooc
4 0 0 0
2 0 0 0
F ( 2 4 ) = 6 8 g . 7 1 6 * 3 1 . 3 9 8 " L2 ; r = 0 . 8 3 3 , N = 5 1 o
0 0
o
9 o o
o o ~ o o
o o ~ o o o
o o
o o ~
o o o o
50 100 150 200 250 300
Length 2
Fig. 9. Relation between potential feeding area and animal length in Macoma balthica after 24 h of exposition
is lowest d u r i n g this period. T h e s e opposite t r e n d s are clearly r e l a t e d to t h e c h a n g e s i n r e c r u i t m e n t with time. D u r i n g s u m m e r there is a c o n t i n u o u s h i g h r e p r o d u c t i o n rate, a n d the p o p u l a t i o n s consist of a few large a n d m a n y small s p e c i m e n s (see Fig. 3), w h e r e a s d u r i n g w i n t e r there are only a few large i n d i v i d u a l s present. The h i g h v a l u e s of a b u n d a n c e a n d t u b e w e i g h t i n N o v e m b e r at station W H are d u e to the e x t r a o r d i n a r y mild a u t u m n 1987 that led to a p r o l o n g e d r e c r u i t m e n t p h a s e i n P. elegans.
Aller (I980) i n v e s t i g a t e d a p o p u l a t i o n of H e t e r o m a s t u s ffliformis at a m u d d y sedi- m e n t site. According to the a v e r a g e a b u n d a n c e of H. filiformis (35 Ind./100 cm 2) a n d
Interactions in soft b o t t o m b e n t h o s 311
.6
D = 0 . 0 9 4 § 0 . 0 2 0 * L; r = o . 7 g ! , N = 7 2 o o
.5 o
o o E
v a 0 CO 0
~' .4 o ~,
0 0 o
o o
.~ .~ o o
~ . o o CO
o o o
o o o o o
~- .2 o o o o o
Length ( m m )
Fig. 10. Relation between length of h,lacoma balthica and the maximum diameter of particles found in the mantle cavity
a v e r a g e t u b e l e n g t h (15 cm), total l e n g t h of all t u b e s w a s a b o u t 530 cm/100 cm 2, w h i c h is w e l l in the r a n g e o b s e r v e d for P. elegans. W i t h r e s p e c t to the u p p e r 5 cm of t h e s e d i m e n t only, the a m o u n t of t u b e s is m u c h h i g h e r in t h e P. elegans p o p u l a t i o n s t h a n in the H.
filiformispopulation. Therefore, the effect of P. elegans on p o r e w a t e r t r a n s p o r t r a t e s m a y b e e v e n s t r o n g e r t h a n t h a t of H. ffliformis in the u p p e r s e d i m e n t layer.
H o w e v e r , a few calculations s h o w t h a t only a p a r t of all t u b e s f o u n d in a s a m p l e m a y b e i n h a b i t e d . T h e direct m e a s u r e m e n t s of a n i m a l l e n g t h in J u l y 87 at station G F (Fig. 3) g a v e a n a v e r a g e l e n g t h of P. elegans of 3.7 mm. A c c o r d i n g to E q u a t i o n s (2) a n d (3), this c o r r e s p o n d s to a n a v e r a g e w e i g h t of 11.2 m g D W of t u b e s p e r animal. T h e field samples, h o w e v e r , y i e l d e d on a v e r a g e a t u b e w e i g h t of 15.5 m g DW at station G F (Fig. 6). T h a t m e a n s t h e r e a r e a b o u t 30 % m o r e t u b e s in the n a t u r a l e n v i r o n m e n t t h a n a r e p r e d i c t e d b y the e m p i r i c a l relations. This d i f f e r e n c e m a y b e d u e to e m p t y t u b e s , w h i c h h a v e lost their i n h a b i t a n t e i t h e r b y e m i g r a t i o n ( F a u c h a l d & J u m a r s , 1979; Wilson, 1981, 1983b) or b y p r e d a t i o n (Poxton et al., 1983; DeVlas, 1979; Woodin, 1984), a n d w h i c h still exist for a certain time.
P y g o s p i o e l e g a n s - p o t e n t i a l f e e d i n g a r e a ( P F A )
T h e total P F A of the P. elegans p o p u l a t i o n at station G F in J u l y 87, w h i c h can b e e s t i m a t e d via the l e n g t h - f r e q u e n c y distribution (Fig. 3) a n d t h e r e l a t i o n b e t w e e n l e n g t h a n d r a d i u s of P F A [Equation (1)], is a b o u t 184 cm2/100 cm 2, i.e., if all a n i m a l s u t i h z e their p o t e n t i a l r a n g e , t h e r e is an o v e r e x p l o i t a t i o n of t h e a v a i l a b l e a r e a b y a factor of 1.8. This i m p l i e s i n t r a s p e c i f i c c o m p e t i t i o n for food, if food is not a v a i l a b l e in excess. T a g h o n et al.
(1980) could s h o w that the g r e a t e r p a r t of a P. elegans p o p u l a t i o n c h a n g e s its f e e d i n g m o d e from d e p o s i t f e e d i n g to s u s p e n s i o n f e e d i n g if the w a t e r c u r r e n t i n c r e a s e s a b o v e 10 cm/sec, w h i c h w o u l d r e d u c e d i r e c t competition. H o w e v e r , also in spionids, w h i c h are a b l e to c h a n g e their f e e d i n g m o d e , i n t r a s p e c i f i c c o m p e t i t i o n for food m a y affect g r o w t h
a n d r e p r o d u c t i o n , as d e s c r i b e d b y W i l s o n (1983a) for P. elegans a n d b y Z a j a k (1986) for Polydora ligni. A d d i t i o n a l l y , a c c o r d i n g to Boehlich & B a c k h a u s (1987), S c h w e i m e r (1976) a n d S t r u v e - B l a n k (1982) currents a b o v e 10 c m / s e c do not occur v e r y f r e q u e n t l y at station G F d u r i n g s u m m e r , so it is most l i k e l y that the a n i m a l s a r e d e p o s i t f e e d i n g most of the time.
T h e o v e r e x p l o i t a t i o n of the a v a i l a b l e surface a r e a b y a P. elegans p o p u l a t i o n (as w e l l as b y o t h e r d e p o s i t f e e d i n g spionids) m a y h a v e serious effects on the r e c r u i t m e n t of o t h e r species, b e c a u s e s e v e r a l d e p o s i t f e e d i n g s p i o n i d s are k n o w n to p r e d a t e on s e t t l i n g l a r v a e of o t h e r s p e c i e s (e.g. W e i n b e r g , 1984; W h i t l a t c h & Zajak, 1985; Wilson, 1981; T a m a k i , 1985). As soon as t h e total PFA of one of those p o p u l a t i o n s is a b o v e the a v a i l a b l e area, t h e r e are no r e f u g e s left for settling larvae. R e p r o d u c t i o n via b r o o d i n g in m a n y d e p o s i t f e e d i n g s p i o n i d s p e c i e s m a y b e a s t r a t e g y to o v e r c o m e the p r o b l e m Of c a n n i b a l i s m .
M a c o r n a b a l t h i c a - p a r t i c l e s e l e c t i o n a n d p o t e n t i a l f e e d i n g a r e a ( P F A ) H y l l e b e r g & G a l l u c c i (1975) a n d Self & J u m a r s (1988) could not find a n y p a r t i c l e s e l e c t i o n prior to i n h a l a t i o n in Aft. nasuta a n d M. balthica, r e s p e c t i v e l y . A c c o r d i n g to Self
& J u m a r s (1988), the selection of food t a k e s p l a c e in the m a n t l e cavity w i t h r e s p e c t to d i a m e t e r a n d specific w e i g h t of the particles. The r e j e c t e d p a r t of the i n h a l e d m a t e r i a l is p u s h e d out t h r o u g h the i n h a l e n t siphon. H o w e v e r , a p a s s i v e size s e l e c t i o n of the m a t e r i a l to b e i n g e s t e d t a k e s place, if the s e d i m e n t c o n t a i n s p a r t i c l e s w h i c h a r e a b o v e the d i a m e t e r of the i n h a l e n t siphon. The l i n e a r r e l a t i o n b e t w e e n a n i m a l l e n g t h a n d m a x i - m u m d i a m e t e r of p a r t i c l e s in the m a n t l e cavity (Fig. 10) shows d e a r l y t h a t this m u s t b e the c a s e at station GF. At this station, p a r t i c l e s a b o v e 0.5 m m are out of the r a n g e of the w h o l e p o p u l a t i o n .
T h e total PFA of a M. balthica p o p u l a t i o n can b e e s t i m a t e d b y a l e n g t h - f r e q u e n c y d i s t r i b u t i o n a n d E q u a t i o n (8). F i g u r e 11 s h o w s the l e n g t h - f r e q u e n c y d i s t r i b u t i o n a n d the c o r r e s p o n d i n g l e n g t h - P F A distribution of the p o p u l a t i o n from station G F (N = 909 Ind./
m2; total P F A = 27 230 cm2/m2), a n d from two stations w h i c h w e r e s a m p l e d d u r i n g p r e v i o u s r e s e a r c h p r o g r a m s in Kiel Bay: station S c h l e i m f i n d e (6 m d e p t h , N = 2322 Ind./
m2; total P F A = 26 090 cm2/m 2) a n d station S c h S n b e r g (12 m d e p t h , N = 598 Ind./m2;
total PFA = 31 850 cm2/m2). At all stations, the total PFA is far a b o v e t h e a v a i l a b l e a r e a (factor 2.6, 2.7, 3.2), w h i c h i n d i c a t e s strong intraspecific c o m p e t i t i o n for food. H o w e v e r . like P. elegans, Aft. balthica is a b l e to switch from d e p o s i t f e e d i n g to s u s p e n s i o n f e e d i n g , if the w a t e r c u r r e n t e x c e e d s a certain limit (Rasmussen, 1973; Olafsson, 1979). Therefore, c o m p e t i t i o n m a y not b e as strong as i n d i c a t e d b y the figures above. H o w e v e r , the fact that total P F A is similar at the t h r e e stations, a l t h o u g h t h e l e n g t h - f r e q u e n c y d i s t r i b u t i o n s are q u i t e different, m a y b e a hint on a n u p p e r limit of total PFA for M. balthica, at l e a s t in Kiel Bay. T h e r e g u l a r distribution of t h e r e l a t e d tellinid s p e c i e s Tellina tenuis, w h i c h has b e e n o b s e r v e d b y H o l m e (1950) on a n i n t e r t i d a l s a n d flat, m a y b e a s t r a t e g y of m i n i m i z i n g o v e r l a p of i n d i v i d u a l PFA.
T h e i n h a l a t i o n activity of M. balthica is a s s u m e d to b e the m m n s o u r c e of t h e n e g a t i v e effect of this b i v a l v e on r e c r u i t m e n t a n d g r o w t h of the o w n l a r v a e a n d l a r v a e of o t h e r s p e c i e s (Bonsdorff et al.. 1986; Brey, 1989~ H i n e s et al., 1989; Olafsson, 1989), e i t h e r b y d i s t u r b a n c e a n d c o m p e t i t i o n for food, or b y i n h a l a t i o n a n d rejection of l a r v a e , w h i c h m a y h a v e l e t h a l effects (see M i l e i k o v s k y , 1974). Most of the p e l a g i c l a r v a e of m a r i n e
Interactions in soft bottom b e n t h o s 313
4 0 0 -
2 0 0
F 2
c m ,10
A
~49S
B C
" 8 " 1 2 " 1 ~ ' 2 b 4 8 12 16 ' 4 ' ~ . . . 8 12 16
)
Fig. 11. Length-frequency and PFA-frequency distributions of three different populations of Macorna balthica in Kiel Bay. A: Schleimiinde, 6 m water depth, N = 2322 Ind./m 2. B: Gabelsflach,
12 m water depth, N = 909 Ind./rn 2. C: Sch6nberg, 12 m water depth, N = 598 Ind./rn 2
i n v e r t e b r a t e s are b e t w e e n 0.2 a n d 0.5 m m (see e.g. Muus, 1973) w h e n t h e y settle, i.e.
w e l l w i t h i n the size r a n g e of particles w h i c h can b e i n h a l e d by adult h4. balthica. Fast g r o w t h m a y b e one s t r a t e g y for l a r v a e to e s c a p e this "/V/acoma-bottleneck" s e n s u Bell &
Coull (1980).
To sum up, the results of the p r e s e n t study indicate that t u b e - b u i l d i n g activities a n d p o t e n t i a l f e e d i n g a r e a s m a y be i m p o r t a n t structuring factors in the i n v e s t i g a t e d s a n d y b o t t o m c o m m u n i t i e s . H o w e v e r , m o r e i n t e n s i v e i n v e s t i g a t i o n s h a v e to b e c a r r i e d out to e v a l u a t e the s i g n i f i c a n c e of t h e s e factors, for e x a m p l e :
- M e a s u r e m e n t s similar to those of Aller (1980), c o m b i n e d with i n v e s t i g a t i o n s of the g r o w t h of b a c t e r i a a n d m i c r o a l g a e a r o u n d tubes, could p r o v e t h e i n d i r e c t positive effect of the t u b e - b u i l d i n g of P. e l e g a n s on s e d i m e n t stability.
- T h e potential f e e d i n g areas of p o p u l a t i o n s of b o t h P. e l e g a n s a n d /V/. balthica are a s s u m e d to b e b e t t e r indicators for the strength of intra- a n d interspecific interactions t h a n are p u r e a b u n d a n c e values. T h e q u e s t i o n of w h e t h e r or not u n a f f e c t e d a r e a s b e t w e e n i n d i v i d u a l PFAs act as r e f u g e s for freshly settled l a r v a e a n d j u v e n i l e s m a y b e t e s t e d by an e x p e r i m e n t that c o m p a r e s the effects of a p o p u l a t i o n w i t h total PFA <
a v a i l a b l e surface a r e a with the effects of a p o p u l a t i o n with total PFA > a v a i l a b l e surface area.
- T h e q u e s t i o n of w h e t h e r c o m p e t i t i o n for food or i n h a l a t i o n of l a r v a e is the m o r e i m p o r t a n t factor in t h e interactions b e t w e e n adult h4. balthica a n d settling l a r v a e m a y
b e a n s w e r e d b y a f i e l d e x p e r i m e n t t h a t c o m p a r e s t h e e f f e c t s of d i f f e r e n t s i z e c l a s s e s of M. balthica o n t h e r e c r u i t m e n t of d i f f e r e n t s p e c i e s .
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