Size and shape ofDaphnia longispina in rock-pools
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Hydrobiologia 145: 259-268, (1987) 259 O Dr W, Junk Publishers, Dordrecht - Printed in the Netherlands
Size and shape of Daphnia longispina in rock-pools
Esa Ranta & Sara Tjossem 1 Department of Zoology, University of Helsinki, P. Rautatiekatu 13, SF-O0100 Helsinki, Finland IPresent address: Section of Ecology and Systematics, Cornell University, Ithaca, NY, U.S.A.
Keywords: Daphnia Iongispina, body size, body shape, small and large rock-pools
The body size and shape of 19. longispina in small and large rock-pools was measured. The mean body length of Daphnia in large rock-pools with vertebrate planktivores was smaller than that of Daphnia in small rock-pools without vertebrates, but the variability in body lengths within pools over the season was as great as that found between pools and predator regimes. We did find that D. longispina in large rock-pools produced one egg at a smaller body length and had fewer eggs per individual than did Daphnia in small rock- pools. D. Iongispina populations also showed different body shapes both as the season progressed and in rock-pools having different predator composition. As expected in Daphnia cyclomorphosis, the core body length decreased during the summer, accompanied by significantly more pointed heads and longer tail spines than in early summer. D. longispina in large rock-pools (with vertebrate predators) have significantly larger exuberances than those in small rock-pools (lacking vertebrate planktivores). The results are discussed in the context of size-selective predation.
Rock-pools on Baltic islands are small fresh- water habitats. Although they are usually too small to support vertebrate planktivores, in the largest of them one can find smooth newts (Triturus vulgaris L.), sticklebacks (Pungitius pungitius L.), and tench (Tinca tinca L.) (Ranta & Nuutinen, 1984, 1985a). Common predators in rock-pools are corix- ids (Arctocorisa carinata Sahlberg, Callicorixa producta Reuter) and a diving beetle species (Potamonectes griseostriatus Degeer) (Lindberg, 1944). In the Tv~irminne archipelago in SW Find- land D. Iongispina O. E Mfiller is found in both small and large rock-pools, i.e., in those without and with vertebrate planktivores (Ranta & Nuuti- nen, 1985b).
Size-selective predation (e.g., Brooks & Dodson, 1965; Zaret, 1980) distinguishes between the prey- size choice by vertebrate and invertebrate plankti- vores. Vertebrate predators show higher electivity
of large zooplankton over smaller ones, while in- vertebrate predators prefer smaller prey. The risk of vertebrate predation can be minimized by reproducing at a smaller size and producing fewer and relatively small-sized eggs. One would expect the opposite trends with invertebrate predators, since increasing body size and exuberances (longer tail spine and antennae, helmeted head) may help minimize planktivory.
Our objective was to examine D. longispina body size and shape in small and large rock-pools (i.e., in rock-pools without and with vertebrate plankti- vores), and in samples taken at different times of the season,
Material and methods
We chose six rock-pools of two size categories and varying predator composition for our study (Table 1). Three small rock-pools (Br~innsk~ir BK
Table 1. Characterization of the rock-pools studied. The first three rock-pools (B = Br~innskhr, L = Lgmgsk~ir) have only invertebrate predators, while the three other pools (L = L~ngsk~ir, I = Issk~ir) have also vertebrate predators (N = smooth newt, T = tench). D. longispina population densities (mean of five tows with standard error, SE, - = not sampled) are indicated for two sampling occa- sions. Chlorophyll A (Chl. a), pH, and conductivity (Cond.) are for 8 August 1984, maximum depth and area of the rock-pools are also indicated.
Population density ind./l Chl. a pH Cond, Maximum mg/m 3
5 July 8 August Depth Area cm m 2
Mean SE Mean SE
Small rock-pools BK 33 9 29 13 0.19 6.4 1.63 45 8 BI9C 101 13 36 7 0.21 5.7 4.55 40 5 LX 9 4 34 10 1.19 6.6 9.62 45 3
Large rock-pools LN 40 16 25 9 1.61 6. t 7.43 100 500 LT 60 5 12 1 0.34 5.7 1.08 150 5000 IN 60 4 I. 10 7.2 4.67 120 600
and BI9C, L~ngsk~ir LX) had only invertebrate planktivores, mostly corixids and diving beetles, and to a lesser extent Chaoborus larvae and Poly- phemus. Two of the large rock-pools (L~mgskfir LN and Isskfir IN) also had smooth newts as predators,' and L~.ngskfir Tench Pond (L~mgsk~ir LT) had tench, a planktivorous fish. In addition to these predators the vertebrate ponds had Notonecta and odonate larvae as potential plankton predators. Daphnia longispina populations in these rock- pools were sampled at different periods of the sea- son in 1983 and 1984 to estimate (1) population size distribution (Fig. 2), (2) the relationship between body length and number of eggs (Fig. 3), and to enable (3) characterization of body shape (Figs 1, 4 and 5).
19. longispina population samples were taken with a 65-t~m mesh net having a 10.5-cm mouth di- ameter, which was towed a 5 times from the bottom to the surface. The Daphnia were collected in small containers, killed with 9570 ethanol, and preserved in 607o ethanol. The samples gave estimates both of population densities (ind./liter) and body size- frequency distributions of D. longispina in differ- ent rock-pools. For each sample we measured (un- der 25 magnification) the body length (character B in Fig. 1) of 200 randomly selected individuals. Measurements were taken to the nearest 0.06 mm. From the same samples 70 gravid females were tak- en to score the relationship between body length
cE l/t D )
Fig. 1. Lateral view of Daphnia longispina and the measures taken for the statistical analyses (A=core body length, B = standard body length, C = head length, D = maximum head height, E = maximum body height, F = tail spine length).
and number of eggs within the carapace (Hebert, 1977).
We used the nonparametric Kruskal-Watlis analy- sis of variance to examine possible differences in D.
No ver tebrates
20 L X 12.7
Q3 ~_ 10
t. J LN 315
lk J LN 12.7
1.O LN 8.8
, I "8 . 8
1.0 2.0 1.0 1.0
Body lengt h (mm)
Fig. 2. Frequency distributions of D. Iongispina body sizes in rock-pools with and without vertebrate planktivores. Different sampling dates and rock-pools are indicated in the graphs (L refers to the island L~mgsk~ir, B to the island Br/innsk~ir, and I to the island Issk~ir; the second letter in vertebrate pools refers to the name of the vertebrate planktivore N = smooth newt, T =tench). The sampling dates for most of the rock-pools were 5 July and 8 August 1984. For two rock-pools, L&ngsk~ir LX and L&ngsk~ir newt pond LN, we also have data from 31 May and 12 July 1983. The lssk~ir newt pond (IN) was sampled only on 8 August 1984. Small triangles above histo- grams indicate median size classes.
No ver tebrates Ver tebrates
e e O O O O e
I I I I I I ! I
l LN 5.7
1 i i I I l I | I I
8 ~ LX 8.8
0 ~ i ...... I I ' I I I I ! I
I LN 8,8 Q
I I I l q I
Q I g
l l I I I
I l I I I I I
t ~ ! T | I
8 ~- B19C 8,8
0 ] , , ~ , T" , t , 1.0 1.5
I LT 5.7
I I I I l I
Io LT 8.8
I I I
I I I
2.0 l ength (mm)
I I I ]
I I I l I
I " ! l F 1
Fig. 3. Relationship between D. longispina body length and number of eggs within the carapace for populations in rock-ponds with and without vertebrate planktivore (Table 2 describes the relationships in statistical terms). Rock-pool codes are as in Fig. 2; sampling dates (5 July and 8 August) refer to 1984.
longispina body sizes on different sampling dates and between rock-pools. To describe the relation- ship between body length and number o f eggs we calculated linear regression equations: e = a + bx,
where e = number o f eggs, a = constant, b = regression slope, and x = body length in mm. For compar ison of the number of eggs produced by a female o f a given size in each of the rock-pools
Core body length
0 .76 0 .78 l ! I i
O~ . . . . . . 0
( - -0
O~- . . . . . . . 0
g ( - - - - - O
O(" . . . . . . . 0 ~ . . . . 0
O~- - - - - 0
-LX 84- 83 -
- LN 84- 83-
- LT -
- IN -
Body he ight
0 .50 0 .54 "~ ! i
( " . . . . . . . 0
( - - - - -0
@ A- . . . . . . . . . . . . . . 0
O( - - - - - -O
O~- . . . . . . . 0
O~- . . . . 0
0 .24 i !
0 - - - - - )0
O-- - - - -
0 - - - - - 9 0
-LX 84- 83 -
o . . . . . . . * -LN 84- o . . . . . . . ~,o 83-
O-~O -LT -
- IN -
0 - - - - - - - -~0
Sp ine length
0.41 0 .45 ............... , T i
O . . . . . . . "~0
0- - -~@
0 . . . . . . . . . . . -~0
0 . . . . . . . . . . . ,~ l l l
Fig. 4. Four D. longispina body-shape characters (see p. 264) for the six rock-pool populations (large rock-pools LN, LT, and IN have vertebrate p[anktivores, small rock-pools BK, BI9C, and LX lack vertebrate planktivores). Open circles refer to early summer samples, dots refer to late summer samples, and arrows show the direction of change (see Fig. 2 for sampling dates). The following differences are statistically not significant: Core body length: BI9C, LT; Body height: LX 1983; Head length: BK, LT; Spine length: LX 1984 (t-tests).
on different sampling dates, the regression equa- tions were solved for a 1.5-mm standard-sized fe- male (Hebert, 1977). In addition, an estimate of the minimum body length needed to produce one egg was calculated as: x = a" x b'e, where a" is the con- stant and b" is the slope of the regression equation and then solving for e = 1. The regression equa- tions were computed for every population on each sampling date.
During sampling we also collected D. longispina with a 100-/~m mesh pond net for body measure- ments of adult animals. These samples were preserved with 60% ethanol for later measure- ments. From these animals we chose 50 adult re-
males greater than 1.0 mm in body length (see Fig. 3). Each animal was placed laterally on a grooved microscope slide and covered by an object class. The prepared animals were than examined at 50 magnification with a projecting microscope. We measured six body characters (Fig. 1) to an ac- curacy of 0.02 mm: (A) body length without head and tail spine, (B) body length without tail spine, (C) head length, (D) head height, (E) maximum body height, and (F) length of the tail spine.
In order to refer to D. longispina body shape in dimensionless terms (James, 1982) and to examine body shape independent of the actual body size, the following seven measurements were derived
sp ine head l ength ~ heig h tallL
B . .
\ \ f~"
head body l ength heig h t~
Canon ica l var iab le 1 (55%)
Fig. 5. A discriminant analysis space (canonical variables 1 and 2; inserted 70 indicates the fraction of total variance explained) for body shape variables in D. Iongispina populations taken from small rock-pools (dots) without vertebrate planktivores (A = Br~innsk~r BK, B = Br~innskfir B19C, C = L~ngskfir LX 1984, D = Lhngsk~ir LX 1983), and from large rock-pools (squares) wilh vertebrate planktivores (E = L~mgsk/ir LN 1984, F = L~ngsk~ir LT, G = L~ngsk~ir LN 1983, H = lssk~ir IN) (open symbols are for 1983 data, closed symbols for 1984 data). Thin arrows point out the direction of the change with time within the year. The thick arrows in the graph corners indicate directions of increasing spine length, head height, head length, and body height, which are the main determinants of the discrimination space.
from the original six: (1) core body length (A/B), (2) body height (E/B), (3) head length (C/B), (4) head height (D/B), (5) tail spine length (F/B), (6) head pointedness (C/D), and (7) head height/body height (D/E). A more thorough justification of the method used is given e.g. by Jolicoeur and Mosi- man (1960), Kerfoot (1980b), and James (1982).
In statistical analyses we first calculated popula- tion and sampling date-specific means, variances, minima, and maxima of the seven shape variables. Pairwise correlations between the variables were computed for all populations and sampling dates, as well as for the combined data. The seven shape measurement data were further used in a dis- criminant analysis, with populations and sampling dates as grouping variables.
D. longispina body lengths (character B in Fig. 1) in the six rock-pools studied ranged from 0.46 mm to 2.79 mm. Although there are statistically signifi- cant differences in body sizes between rock-pools and between sampling dates (Kruskal-Wallis ANO- VA), they are not attributable to differences in planktivore composition in the rock-pools. For ex- ample, the small rock-pool Lgmgsk~ir LX (lacking vertebrate planktivore) has both the largest (31 May 1983) and the smallest (8 August 1984) medians of the body-size histograms (Fig. 2).
Body length (character B in Fig. 1) of D. Ion- gispina females correlates positively with the num- ber of eggs within the brood pouch (Fig. 3) in all the populations. With the single exception of the rock-pool L&ngsk~ir LX (sampled 5 July 1984), the correlations were also statistically highly significant (Table 2). The relationship between female body length and number of eggs produced varies con- siderably between populations and sampling dates. The slopes of the regression equations (Table 2) derived for D. longispina populations do differ from each other (t-tests) for almost all rock-pools on different sampling dates, with the exception of Br~innsk~ir B19C sampled 5 July 1984 (slope 7.19, constant -8.06) and 8 August 1984 (slope 8.50, constant -9.16), where no differences between the two regression equations were found (an analysis of covariances, F = 1.65, df = 1,129, not significant).
We used the regression equations between body length and number of eggs to compare the number of eggs produced by a standard 1.5-ram D. longispi- na female from each sample. The results suggests that D. longispina living in large rock-pools (sup- porting vertebrate planktivores) do have slightly smaller numbers of eggs (average 2.1 eggs) than fe- males living in small rock-pools lacking vertebrate predators (average 2.7 eggs). If the same pattern is considered in the context of size-selective preda- tion, the expectation is that D. longispina living in rock-pools with vertebrate planktivores should be likely to reproduce at a smaller size than D. lon- gispina in rock-pools lacking vertebrate plankti- vores. Computing regression equations as the rela- tionship between number of eggs produced (x-variable) against body length (y-variable) makes
Table 2. Statistics for regression equations describing relationships between number of eggs and core body population samples (1984 data).
length (BL in mm) in the 11
Rock-pool Date # eggs = a + b*BL 100*r 2 n BL 1 egg # eggs (ram) 1.5 mm
Small rock-pools BK 5/7 -12.50+ 11.10 75070 70 1.27 4.2
8/8 -0.99+ 1.81 22070 70 1.35 1.7 B19C 517 -8.06+ 7.19 67% 70 1.32 2.7
8/8 -9.16+ 8.50 6870 61 1.32 3.6 LK 5/7 0.67 + 0.83 6070 67 1.48 1.9
8/8 -5.65 + 5.24 7107o 70 1.34 2.2 mean 1.35 2.7 SD 0.07 1.0
Large rock-pools LN 5/7 -2.68+ 3.15 36% 51 1.31 2.1
8/8 -4.87+ 5.30 517o 70 1.21 3.1 LT 5/7 -2.23 + 2.27 347o 70 1.24 1.2
8/8 -3.72+ 4.55 52070 70 1.I1 3.1 IN 8/8 -1.11 + 1.57 30o70 70 1.38 1.3
mean 1.25 2.1 SD 0.10 0.9
it statist ical ly possible (Sokal & Rohlf, 1981) to f ind the min imum body length needed to produce one egg. The average D. longispina body length to pro- duce one egg in rock-pools with vertebrates was 1.25 mm, while in rock-pools lacking vertebrate ptankt ivores it was 1.35 mm (Table 2). I f the differ- ent sampl ing occasions are accepted as indepen- dent observations, the difference between the two figures is statist ical ly signif icant (Mann-Whitney U- test, z = 1.643, one-tai led P = 0.050).
19. longispina body shape characters showed con- siderable var iat ion between sampl ing dates (Fig. 4). First, both core body length (A /B) and body height
(E/B) were signif icantly smaller in the late summer samples compared to the early summer samples. Second, both head length (C/B) and tail spine length (F/B) increased as the season proceeded (Fig. 4).
The statist ical ly signif icant differences in body shape characters both between sampl ing dates within rock-pools (Fig. 4) and between rock-pools of the two different size categories hinder any pool- ing o f the data at the level of the two different pred- ator regimes. Thus, instead of displaying body shape-specif ic means and variances for each sam- pi ing date and rock-pool populat ion, we appl ied d iscr iminant analysis to al low s imultaneous com- par ison of sample-specif ic var iat ion in a number of measured characters. We took all seven body shape variables (Table 3) into the analysis and kept the 15 samples (Fig. 2) as grouping variables.
Table 3. Correlations between the seven body shape characters of D. Iongispina (pooled data for all samples and populations). See Fig. 1 for the body measures A-F.
Head pointedness (C/D) Core body length (A/B) -542 Head length (C/B) 713 -759 Tail spine length (F/B) 204 -234 Body height (E/B) -272 -205 Head height (D/B) -589 -102 Head height/body height (D/E) I1 -269
268 -266 -141 138 27 393 382 162 -658 427
C/B E/B E/A D/B
The following four variables entered into the dis- criminant functions: head length (C/B), tail spine length (F/B), body height (E/B), and head height (D/B). The discriminant scores for the 15 samples (group means) are shown in Fig. 5. When the sample-specific variation is included, the percent- age of 'correct' classifications was 27% (range 18-55%). This analysis shows that the body shape of D. Iongispina in large rock-pools with vertebrate predators differs from that of D. tongispina living in small rock-pools lacking vertebrate planktivores (Fig. 5).
Since Brooks and Dodson's (1965) size-efficiency hypothesis, the theory of size-selective predation has become well established (reviews in Zaret, 1980; Greene, 1983). Introducing vertebrate planktivores into natural waters shifts the plankton species abundances from initial dominance by large in- dividuals to dominance by smaller forms (Brooks & Dodson, 1965). In support of these results the sam- pling of unmanipulated small lakes lacking plank- tivorous fish shows zooplankton dominated by large forms, while in similar lakes with plank- tivorous fish the zooplankton consists mainly of small forms (Nilsson & Pejler, 1973). Changes in body size of Bosmina populations recorded from material preserved in lake sediments are attributed to differences in ptanktivore composition and abundance over long periods (Kerfoot, 1981; Salo et al., 1986). In addition, some data show that planktivory maintains different morphs within a zooplankton species. For example Zaret (1969) es- tablished that fish preferred large-eyed morphs of Ceriodaphnia cornuta over small-eyed individuals (but see Confer et al., 1980; Kerfoot, 1980a). Hes- sen (1985) made similar observations, showing with enclosure experiments that the eye diameter of Bos- mina longirostris was significantly smaller in bags to which fish had been added. However, Hessen did not observe changes in body size, length of anten- nae, or length of mucro between experimental and control populations of B. longirostris. Zaret and Kerfoot (t975) suggested that the eye pigment di- ameter of B. longirostris decreases during daytime due to the selective removal of large-eyed individu- als by planktivorous fish.
In other studies we have shown that the rock- pool vertebrate planktivores (smooth newts, Ranta & Nuutinen, 1985a; tench, Ranta & Nuutinen, 1984) readily select large prey' from small and large Daphnia provided together. In contrast, rock-pool insects (A. carinata, C. producta, P griseostriatus) prefer small prey over larger ones (Ranta & Espo, in prep.). These observations are in agreement with the theory of size-selective predation. In the present data, however, the variability in D. longispina body length within pools over the season was as great as that found between predator regimes. Two of the observations of D. longispina body characters sepa- rate along the line of different predator regimes. First, in large rock-pools with vertebrate plankti- vores D. longispina body length needed to produce one egg was smaller than in small rock-pools with- out vertebrate predators. At the same time the num- ber of eggs produced by a standard 1.5-ram female was on average lower in rock-pools with vertebrates than in those rock-pools where invertebrates were the only planktivores. Second, the body shape of D. longispina in rock-pools supporting vertebrate planktivores was different from that in rock-pools with only invertebrates. We found that D. longispi- na body exuberances were longer in large rock- pools with vertebrate planktivores than in the small rock-pools with only invertebrate planktivores. Head length and tail-spine length showed within- season changes consistent with changes earlier reported as cladoceran cyclomorphosis (Kerfoot, 1980b).
Our greatest concern in the present data is, how- ever, the mismatch between the predictions of size- selective predation and D. longispina body size in rock-pools with the two differing planktivore re- gimes. Several possible factors might account for the observed discrepancy between our rock-pool observations and the expectation. First, the intensi- ty of planktivory by the different predators is not known. All the predators (corixids, diving beetles, smooth newts, tench) feed on zooplankton, but they also eat other prey, such as chironomid larvae and larvae of other water insects in the rock-pools. Second, predation pressure may vary according to the time of season and from year to year. Third, in- teractions between herbivorous Daphnia and their phytoplankton food in rock-pools might be an im- portant, but as yet unstudied, factor affecting body size of D. longispina in rock-pools.
The planktivorous predators in rock-pools exert varying pressure on the plankton. In rock-pools of the Tv~irminne area corixid densities frequently reach up to 300-400 ind./m 2 (Pajunen, 1977), which seriously deplete the chironomid larvae stock in the rock-pool sediment (V.I. Pajunen, pers. comm.), making plankton prey the remaining food supply for the corixids. The situation seems to be the same for the diving beetle (P griseostriatus) as well (Pajunen, 1983). In the lssk~ir newt pond (IN) we observed densities of smooth newt tadpoles up to 10 ind./m 2 in late summer. Dolmen and Koks- vik (1983) showed that planktonic crustaceans are the main prey of smooth newt tadpoles. For tench Ranta and Nuutinen (1984) observed that in early summer their stomachs were full of plankton prey. Thus, these predators are certainly important planktivores in rock-pools, but predation pressure upon D. longispina may become temporally relaxed.
Paleolimnological studies have documented the reduction of cladoceran (Bosmina) body sizes over time (Kerfoot, 1981; Salo et al., 1986). These changes are, in the first place, attributed to changes in the planktivore guild. However, increasing lake eutrophication has occurred with the fish introduc- tions. Gliwicz (1977), and Gliwicz and Siedlar (1981) have provided an alternative explanation to size-selective predation for the occurrence of size reduction in plankton of eutrophicated lakes. Ac- cording to them the loss of larger herbivores in lakes undergoing eutrophication is related to the inhibi- tory effects of large-sized ('net') phytoplankton on zooplankton filtering dynamics. At the same time the filter feeding of small herbivores is not affected by the 'net' phytoplankton. Gliwicz (1977) maintains that the large forms are replaced in the plankton be- cause of their lowered fecundity compared with smaller cladocerans.
Whether the explanation for reduced cladoceran sizes is due to differences in planktivory or due to filtration inhibition must be addressed with proper experiments. Before the results of these experi- ments became available, caution should be taken in attributing any observed changes in cladoceran size as solely the result of differences in predation pres- sures. One example suffices to stress our point. An- derson (1980) studied 320 small lakes in the Moun- tain National Parks of western Canada. Of these lakes 177 were classified as lakes where fish were the
principal predators of large zooplankton and inver- tebrates, and in 143 lakes no fish were present (An- derson, 1980: 638). He found that large cladocerans (Daphnia spp.) were almost as common in the fish lakes as in the fishless lakes. Hence, it is perhaps no wonder that D. Iongispina reaches about the same body size in rock-pools with and without vertebrate predators.
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