Marine Biology 78, 59-67 (1983) Marine

...... Biology Springer-Verlag1983

Selection of unicellular algae by the littoral amphipods Gammarus oceanicus and laeviusculus (Crustacea)*

C. Hudon **

D6partement de Biologie, Universit6 Laval; Ste-Foy, Qu6bec G1K 71)4, Canada

Abstract with various groups such as meiofauna (Hicks, 1977; Lee etal., 1977; Berghie and Bergmans, 1981), gastropods The effect of microalgal strength of adhesion to surfaces (Calow, 1973; Nicotri, 1977; Kesler, 1981; Sumner and was examined with regard to their susceptibility to grazing McIntire, 1982) and amphipods (Meadows, 1964; Grez 6, by Garnmarus oceanicus Segerstr~le and Calliopius laevius- 1968a, b; Ravanko, 1969; Hargrave, 1970; Moore, 1977b; culus (Kroyer). Observations of the feeding behaviour and Zimmerman et al., 1979). The effects of grazing on benthic two feeding experiments were carried out under labora- unicellular algae are numerous, and in this study I intend tory conditions. Naturally attached periphyton (strongly to examine some which involve omnivorous amphipods. attached cells), homogenized periphyton (loosely attached With respect to amphipods, the constraints imposed by cells), filtered phytoplankton (unattached cells) and bare body size and feeding apparatus play a large role in the surfaces (controls) were randomly located in a grid and mechanism of food selection. Feeding mechanisms of offered for grazing to a fixed number of amphipods of amphipods have been studied for many soft-bottom each species separately. The number of individuals visiting species (Enequist, 1949), but have seldom been used to each type of food presented in the grid was recorded for interpret results of feeding experiments (Meadows, 1964; 24-h periods. The feeding habit of each species, their effect Nicolaisen and Kanneworff, 1969). Many omnivorous on food distribution and their efficiency at collecting small amphipods are known to assimilate the microscopic flora particles were also recorded. G. oceanicus has a low effi- (bacteria, fungi, diatoms) more efficiently than the organic ciency at collecting particles and does not select a par- substratum on which it grows (detritus, macroalgae, vascu- ticular type of food, owing to its feeding habit of indis- lar plants) (Kostalos and Seymour, 1976; Harrison, 1977; criminately resuspending loosely attached particles. Morrison and White, 1980; Zimmerman et al., 1980). This C. laeviusculus is a highly efficient and selective grazer, physiological selection could at least compensate for the preferring homogenized periphyton and phytoplankton to low capture efficiency (Dagg, 1974) related to the un- naturally attached periphyton. For epibenthic diatoms, specialized morphology of many littoral amphipods strong adhesion to surfaces is advantageous to avoid (Barnard, 1969). Behavioural adaptations can also be used grazers. to improve food capture (Zimmerman et al., 1979). Since handling time of unicellular benthic algae is negligible to most grazers, a specialized diet should not be expected (Hughes, 1980). Instead, food density and ease in Introduction detaching cells from the surface should be determining factors in food selection. However, factors other than Prey choice among littoral herbivores feeding on benthic active selection by the grazers can artificially create a unicellular algae has attracted little attention considering "selective" diet. For unicellular benthic algae, preferential the important position these algae are likely to occupy in ingestion of certain species can result from differential the food chain. Feeding experiments have been carried out availability of the cells, due to vertical stratification of the community or differential adhesion of cells to the surface (Moore, 1977a; Kesler, 198l; Hudon and Bourget, 1981). * Contribution to the program GIROQ (Groupe Interuniversi- Indeed, a survey of the literature indicates that a large taire de Recherches Oc6anographiques du Qu6bec) ** Present address: Department of Biology, University of Water- variety of grazers apparently prefer aborescent (Licmo- loo; Waterloo, Ontario N2L 3G1, Canada phora sp., Cyrnbella ventricosa, Rhoicosphenia sp., Gorn- 60 C. Hudon: Selection of microalgae by littoral amphipods

Table 1. Summary of the results on selective grazing of diatoms available in the literature

Type of Grazer Types of diatoms selected Types of diatoms avoided Reference experiment

Counts of slides Ciliates Navicula spp., Nitzschia spp., Achnanthes minutissima, Brook, 1952 Cymbella ventricosa Amphora ovalis, Cocconeis placentula Observation in vivo, Protozoa Rhoicosphenia sp. - Jansson, 1967 examination Proales rheinhardti Licmophorasp. and small - Jansson, 1967 of feces Chironomids diatoms Correlation Mayflies Achnanthes spp. (minutissima, Cocconeisplacentula Douglas, 1958 algal versus A gapetus linearis, microcephala), Eunotia pectinalis var. minor grazers density fuseipes and others Gomphonemaspp. Synedra spp. Amphipods Diatoma spp., Melosira spp. Cocconeisspp., Amphora spp. Moore, 1975 A sellus aquaticus Gammarus pulex Lake enclosures Gastropod Synedra sp., small diatoms Cocconeisplacentula Kesler, 1981 Amnicola limosa Pillars on a Littorinid gastro- FragilariastriatuIa, Achnanthes brevipes, Nicotri, 1977 tidal mud flat pods and limpets Melosiranummuloides, A. parvula, Navicula M. moniliformis praetexta Gut contents, Gastropod Gomphonema sp. Amphora, Navicula, Calow, 1973 laboratory experi- Ancyclusfluviatilis Achnanthes spp. ments Laboratory Gastropod Nitzschia linearis Navicula minima, Sumner and streams Juga plicifera Nitzschia amphibia McIntire, 1982 Laboratory Physa heterostropha Cocconeis sp. Roop, cited by streams Patrick, 1970 Gut content, Lampreys Nitzschia acicularis, A chnanthes minutissima, Moore and laboratory Petromyzon marinus N. subtilis, Coceoneis placentula, Beamish, 1973 experiments Lampetra lamotei Navieula gregania, Amphora ovalis N. eryptocephala Lake enclosures Tadpoles Filamentous algae Blue-green algae Dickman, 1968 Rana aurora phonema spp.) and filamentous (Diatoma spp., Melosira experiment) or a 10-d (2nd experimen0 immersion at 1-m spp., Fragilaria striatura) colonial diatoms over small, depth under an offshore buoy. The natural assemblage in- prostrate forms such as Amphora and Cocconeis (Table 1). cluded adherent and stratified cells according to the Thus, the present study was carried out to compare two growth form of the diatom species (see Hudon and species of omnivorous littoral amphipods, Gammarus Bourget, 1981, 1983). Briefly, the assemblage comprised (Lagunogammarus) oceanicus Segerstrgde and CaIliopius adnate cells of Cocconeis spp. and Amphora spp., erect laeviusculus (Kroyer) with respect to their feeding mech- fan-shaped Synedra tabulata, filamentous Melosira num- anisms and selectivity for unicellular algae of different muloides and Biddulphia aurita as well as highly motile adhesive strength. Nitzsehia spp. and Navicula sp. (2) Periphyton growing beside the plastic panel was scraped off, homogenized and filtered using white fiberglass filters (Whatman GF-C) to provide the same cell composition without strong cell Material and methods adhesion and community stratification. (3) Phytoplankton contained in 1 1 of Mitis water was filtered to provide an Two experiments were carried out in August 1981 at the assemblage of loose cells usually unavailable to the am- GIROQ (Universit6 Laval) field station located at Pointe phipods. (4) Bare filters and (5) bare plastic units were Miffs, on the south shore of the lower St. Lawrence used as controls. The ranking of decreasing strength of cell Estuary (Qu6bec, Canada) to determine the preference of adhesion is (1) natural periphyton (strongly attached cells), the amphipods for food of different densities and/or dif- (2) homogenized periphyton (loosely attached cells) and ferent strength of attachment to the surfaces. (3) phytoplankton (unattached cells). Since periphytic Food was obtained and offered to the amphipods in diatoms often secrete mucus appendages, it was supposed the following way: (1) Black laminated plastic panels that even after homogenization, the resuspended peri- (Conoflex LOF-Plastic, Illinois, USA) were coated with a phyton would be more adherent to the filter than phyto- natural epibenthie diatom community after a 20-d (lst planktonic cells. C. Hudon: Selection ofmicroalgae by littoral amphipods 61

The filters and the plastic panels supporting the dif- the experiments. Feeding activity with only one type of ferent types of food were cut into square (2 x 2 cm) units. food offered on one 4 cm 2 unit was also measured with This size was large enough to allow at least two individuals one G. oceanicus or two C. laeviusculus, all factors (volume to feed without interference on the units. The individual of water, number of food units, number of individuals) square units were fixed to a wooden frame following a being reduced by a factor of 25. latin-square design (5 • 5 grid) (Fig. 1). Five replicate traits of each of the five treatments (types of food+controls) were fixed to each frame, arranged so that each treatment Data collection and analysis occurred once in each row and once in each column. This design is similar to that used by Russ (1980) for macro- Observations were made for 15 rain every three hours (lst epifauna. In addition to the units used in the grids, two exp.) or every hour (2nd exp.), for a period of 24 h. During replicate square units of each type of food were kept each observation period, the number of amphipods unused for comparison with grazed units. In the first ex- present on each unit of the grid was recorded for the four periment, the effect of location within the grid was tested dishes. In the second experiment, a further distinction was by using two different grid designs (Fig. 1). In the second made between the individuals spending less than 15 s on a experiment, four identical (Type II, Fig. 1) grids were used. given unit and those remaining longer than 15 s on a The two species of amphipods, Gammarus (Laguno- feeding unit. A summary of the experimental conditions is gammarus) oceanicus and were given in Table 2. selected because they are vagile, free-swimming species, At the end of the 24-h period of observations, the the most likely to have exerted grazing pressure on other amphipods were fixed in 4% formalin and the units were experimental devices suspended offshore from the study air-dried. Units utilized in the experiments were compared area (Hudon, 1983). Prior to each experiment, individuals with unused intact units, using both light and scanning were collected from the littoral area, sorted and kept in electron microscopy (SEM). Estimates of cell density were filtered seawater for 24 h. Their feces were later examined made for phytoplankton-covered units using replicate to identify food items under natural conditions. The grids SEM observations on randomly selected fields. Cell den- were prepared within four hours of the beginning of each sities could not be compared for natural and resuspended experiment to insure food freshness. Each grid was put periphyton due to unquantifiable cell breakage and into a dish filled with 1 1 of filtered seawater (26%0 S) and detritus accumulation on the latter type. kept at 15 ~ throughout the experiment. Water oxygena- For each grid, the short, long and total (short+ long) tion was maintained by gentle air bubbling. Four dishes, number of visits on each food unit were pooled for the 24- representing two replicates of the two amphipod species h period and were analysed independently for the two were used for each experiment. Twenty-five G. oceanicus experiments. In the first experiment, a higher weight of or fifty C. laeviusculus were added to each dish and main- periphyton was used on the filter, as well as on the tained under normal daylight conditions for 24 h. Enough naturally colonized plastic (20-d immersion ) to test the free space was available in the dishes to allow the am- preference between attached and loosely attached assem- phipods to rest elsewhere than on the grid or to swim blages of similar composition. The observations obtained freely. The amphipods always remained active throughout from this preliminary experiment were also used to verify the validity of the experimental set-up. Data normality and heteroscedasticity were verified 1 2 3 4 5 1 2 3 4 5 using Kolmogorov-Smirnov's (Lilliefors, 1962) and Bart-

A lett's (Sokal and Rohlf, 1981) tests, respectively. For the second experiment, a two-way nested ANOVA involving B replicate grid and type of food (fixed factors) was per- formed on the number of visits, with the distinction be- C tween short (< 15 s) and long (> 15 s) visits. An identical D analysis was carried out with the degree of cell attachment (strongly attached cells, loosely attached cells, unattached E cells) as source of variation. The influence of position was analysed separately, using a two-way ANOVA with rows 1 cm I I and columns as fixed factors. Fig. 1. Two grid designs supporting five rows of five types of food units located in different positions. The five types of food are in- dicated as follows: Results Dark plastic colonized with natural periphyton; OllllllIIJ Whitefilter with homogenized periphyton; Feeding observations ~!.:.:.:.:~ Whitefilter with phytoplankton; Gammarus oceanicus is a marine coastal species, occa- k J Intact white filter; sionally observed at depths greater than 25 m, but reaching / Non-colonized dark plastic its highest abundance in the intertidal zone (Steen, 1951). 62 C. Hudon: Selection of microalgae by littoral amphipods

Table 2. Summary of the experimental conditions for the two experiments, n: number of individuals per dish, i: mean weight of material on each 2 • 2 cm square food unit; (SE): standard error on the mean; (F): material resuspended on filter; (P): material attached on plas- tic; Idem: same treatment as above;---: not applicable

Species Grid Type of food Cell adhesion Initial weight Observations design of food (g) Frequency Type

(SE) Gammarus I Phytoplankton (F) Unattached 0.93 (0.13) Every three Number of oeeanicus Homogenized periphyton (F) Loosely attached 5.30 (2.19) hours individuals per n=25 Natural periphyton (P) Attached 4.77 (2.6t) food unit Bare filter Bare plastic II Idem Idem Idem Idem Idem Calliopius I Idem Idem Idem Idem Idem laeviusculus II Idem Idem Idem Idem Idem n=50 Gammarus I Phytoplankton (F) Unattached 1.65 (0.18) Every hour Number of oceanicus Homogenized periphyton (F) Loosely attached 1.81 (0.11) individuals per n =25 Natural periphyton (P) Attached 1.31 (0.35) food unit Bare filter Bare plastic I Idem Idem Idem Idem Idem Calliopius I Idem Idem Idem Idem Idem laeviusculus I Idem Idem Idem Idem Idem n=50

Although the biology of the species is well known (Steele Calliopius laeviusculus is also a very common littoral and Steele, 1972), little information is available on its amphipod, the bioloy of which was described by Steele feeding ecology. Observations prior to and during our ex- and Steele (1973). Our observations on its feeding be- periments showed that individuals of this species generally haviour are in good agreement with those reported by remain lying on their side on the substratum and perform Enequist (1949) for other genera of . Indi- strong body flexions, which cause particles to be resus- viduals generally stand upright on the substrate, clinging pended in the surrounding water. The rapid beating of the pleopods induces a vortical current bringing the particles towards the mouth, where periodic cleaning and tearing movements take place (Fig. 2). A slower current on the dorsal side of the individual also brings particles towards A. the mouth-parts. Although these currents are mainly Gammarus respiratory, they may also help in catching particulate oceamcus food. Under natural conditions, active grasping of prey with the gnathopods and subsequent biting with the m~)uth parts was commonly observed. Under laboratory condi- 0.5 cm tions, this behaviour was observed when a piece of dead was directly presented to Gammarus oceanicus. Microscopic observation of the feces of individuals freshly collected at sea showed animal remains, macroalgal frag- ments and detritus, together with occasional benthic ,iusculus diatoms. These observations agree with previous work, in- dicating that amphipods of this are omnivores (Segerstrtde, 1959; Bousfield, 1973; Zimmerman etaL, 1979), feeding on detritus (Grez6, 1968b; Harrison, 1977; Morrison and White, 1980), dead fish (SegerstrNe, 1959) and ephemeral algae (Steele and Steele, 1975; personal Fig. 2. Feeding behaviour of the experimental organisms (modi- observation). They were also observed to feed on fucoids fied from Bousfield, 1973). Arrows indicate the direction and rela- (Gibb, I957), but apparently prefer fragile filamentous tive speed of currents. (A) Gammarus oeeanicus. Top-view of the algae (Ravanko, 1969; A. Cardinal, personal communica- general posture on the substratum. (B) Calliopius laeviusculus. tion). Side-view of the general posture on the substratum C. Hudon: Selection of microalgae by littoral amphipods 63

firmly to the surface with peraepods 3 to 7. The abdomen both grid designs. Callioflius laeviusculus showed a very is curved ventrally so that the beating of the pleopods strong preference for resuspended pefiphyton while Gam- necessary for respiration also generates a current resus- marus oceanus did not show any coherent pattern (Fig. 3A). pending and carrying the particles towards the mouth When the two species of amphipods are compared parts (Fig. 2). All cephalic appendages seem to form a regarding their effects on homogenized periphyton (Fig. funnel, directing the particles towards the maxillipeds and 4A, B), it is apparent that Gammarus oceanicus resuspends distal articles of the gnathopods, among which the particle particles over a wide area and produces a uniformly sifting most likely occurs. Direct chewing on a piece of perturbed surface. On the other hand, Calfiopius laeviuscu- loose algal mat was also occasionally observed. lus has a more discrete impact on the surface, some areas The diet of Calliopius laeviusculus was described by being completely cleared of cells, while others remained Dagg (1976) as consisting of settled planktonic algae, untouched. For C. laeviusculus, a slight preference for detritus or live zooplankton. The gut content of the grazing near the edge of food-units could be observed. amphipods in our experiments were always of a greenish These results show that the two species of grazers had colour when collected, indicating a more herbivorous diet. consistently different feeding behaviours. Gammarus Microscopic examination of the feces of these individuals oceanieus foraged at random, which could reflect the showed fine detritus and benthic diatoms to be the major omnivorous and mainly macroscopic composition of its food components. diet under natural conditions. The diffuse relationship between the actual location of the amphipod and the food source could also have been due to the massive resuspen- First series of experiments sion of loose particles and the large volume of water moved by this species. Calliopius laeviusculus appeared to The first series of experiments was carried out to test the be a more selective and efficient collector of small par- effects of different grid designs and cell attachment on ticles. As expected, loosely attached (resuspended) peri- food selection by the two species of amphipods. For both phyton was strongly preferred to an equivalent amount of species, the number of visits on each unit was similar on tightly attached natural periphyton. C. laeviusculus also

SPECIES Gammarus oceanicus Calliopius laeviusculus

GRID DESIGN I I I[ A. .o_,~;g g

First 80' ~ 2L~ ~ 2 c~ o., ,o. experiment 60" 13- "~ 40" m "5 20-

I I I I

~o -~ 80- > 60- O ..c 40- r B. "6 20-

Second experiment

ffl "~ 80- > 60- e-- _o 40- 20-

Fig. 3. Gammarus oeeamcus (left) and Calliopius laeviusculus (right). Frequency histograms of percent of number of visits for each type of food for the two grid designs presented to the two species of amphipods. (A) First experiment, percent of total visits. (B) Second exper- iment, (1) percent of short visits, (2) percent of long visits 64 C. Hudon: Selection of microalgae by littoral amphipods

ignored unattached phytoplanktonic cells, probably owing to the much smaller (five-fold) weight of phytoplankton offered on the filters (Table 2). Following these pre- liminary results, if the strength of cell attachment is a determining factor in amphipod food selection, then the selection pattern of C. laeviusculus for resuspended peri- phyton should be divided between both phytoplankton and resuspended periphyton when they are presented in comparable amounts.

Second series of experiments

A second series of experiments was designed to determine if cells would be selected according to their strength of attachment to the surface, regardless of species composi- tion. For this, equivalent weights of phytoplankton (non- attached cells), resuspended periphyton (loosely attached cells) and natural periphyton (tightly attached cells) were placed in the grid and offered to the grazers as in the previous experiment. ANOVA was first carried out to detect the main sources of variation among the different factors (species, replicate grids, type of food and cell attachment). The results of ANOVA were then examined in the light of the individuals' gut contents and effect on phytoplanktonic cell density. For Gammarus oceanicus, selection patterns of food units between replicate grids were significantly different from one another for long, short and total number of visits (Table 4). This result confirmed the inconsistent pattern of visits on food units (Fig. 3 B), similar to that of the first series of experiments. Further interpretation of other sources of variation was therefore unnecessary since any trend may have been due to chance. For Calliopius laeviusculus, no significant differences were present be- tween replicate dishes, nor between rows and columns on the grids. Thus, the influence of unit position could be ignored. Cell attachment did not appear to have a sig- nificant effect on food selection by this species, perhaps because of the low number of degrees of freedom avail- able (df=2). However, the number of long visits (> 15 s) differed significantly according to the type of food. This result may be interpreted as actual feeding taking place Fig. 4. (A) Gammarus oceanicus. First experiment. Light micro- graph of dense homogenized periphyton resuspended on filter af- during long visits, while short visits could merely reflect ter undistinctive grazing. Note that the entire surface is disturbed. movements of . The comparison of the number of Scale bar indicates 0.2 cm. (B) CalIiopius laeviusculus. First exper- short and long visits also showed the occurrence of a sig- iment. Light micrograph of dense homogenized periphyton re- nificantly (paired Student's t-test, P<0.001) larger num- suspended on filter after grazing on well defined areas. Note that some areas were untouched (unt), while others were completely ber of long visits for both species of amphipods. cleared of cells, leaving the filter exposed (ill). Same scale as A detailed ranking of food types according to the mean above. (C) Calliopius laeviusculus. Second experiment. Scanning frequency of long visits by Calliopius laeviusculus (Table 5) electron micrograph of a control white filter after 24 h of grazing. shows that the homogenized periphyton was preferred to Note the contamination by detritus (de) adhering to the filter all other types of food. Phytoplanktonic cells and bare fibers (arrows). Scale bar indicates 100/~m. (D) Ungrazed phy- toplankton resuspended on filter. Second experiment. Note the filters were next on the list of preference. The number of large size variation of phytoplanktonic cells, from very small long visits spent on phytoplankton-coated units was con- Thalassiosira sp. (circle) to chains of Chaetoceros spp. (ch). Same siderably higher than in the first experiment, in which this scale as above. (E) Ungrazed periphyton resuspended on filter. type of food was totally neglected (Fig. 3A, B). The Second experiment. Broken chains of Biddulphia sp. (hi), pennate diatoms (pe) and detritus (de) can be seen adhering to filter fibers identical preference for phytoplankton coated units and (arrows). Same scale as above bare filters could be due to the alteration of the filter C. Hudon: Selection ofmicroalgae by littoral amphipods 65

Table 3. Gammarus oceanieus (left) and Calliopius laeviusculus' (right). Second series of experiments. Summary of the analyses of variance carried out on short (less than 15 s), long (more than 15 s), and total number of visits. Significance of the F ratio: n.s., non significant difference; *, P<0.05; **, P< 0.01 ; ***, P< 0.001

Source of Degrees of Gammarus oceanicus Calliopius laeviusculus variation freedom of the F ratio Short Long Total Short Long Total

Replicate dish 1, 5 ** *** *** n.s. n.s. n.s. Rows• 16, 16 n.s. n.s. n.s. n.s. n.s. n.s. Cell adhesion 2, 3 n.s. n.s. n.s. n.s. n.s. n.s. Type of food 4, 5 n.s. * ** n.s. * n.s.

Table 4. Calliopius laeviusculus. A-posteriori test of significant difference of number of long visit on each type of food (Table 2). The disruption of the underline indicates the occurrence of a significant difference between two adjacent pairs

Types of food: Bare Natural Bare Phyto- Homogenized plastic periphyton filter plankton periphyton

Mean number of visits 1.80 2.60 3.02 4.40 5.32 (adjusted) Significant differences Values of the F ratio 1.80 2.60 4.10 4.40 4.90

Table 5. Gammarus oceanicus (top row) and Calliopius laeviusculus (bottom row). Grazing efficiency on small (8 #m) Thalassiosira offered alone or among other types of food. Initial cell density equaled 5 500.0 (578.3) cells mm-2; ~: mean cell density (cells mm-2); (SE): standard error on the mean; n: number of individuals; % decrease: percentage of the initial cells lost after 24 h of grazing

Alone Among other food units (within a grid)

Final ~ (SE) % Final ~ (SE) % density decrease density decrease

Gammarus 4 083.3 (320.0) 26 4 069.2 (256.7) 26 oceanicus n = 1 n = 25 Calliopius 1 410.8 (198.3) 74 2 500.0 (487.5) 54 laeviuscu/us n = 2 n = 50

surface during the experiment, namely the decrease of the grids, the gut and feces of both amphipod species con- phytoplanktonic cell densities on one hand, and the pro- tained detritus and diatoms. When the amphipods were gressive contamination of the initially bare filters by loose left for 24 h with a single food unit coated with phyto- material (feces, detritus, resuspended cells) (Fig. 4C) on planktonic cells, their gut contained large amounts of a the other hand. Finally, the least visited units were the small (8 ~m) species of Thalassiosira. Both species are thus ones supporting tightly attached natural periphyton, and able to collect and ingest very small particles, despite their the control bare plastic. In summary, although the resus- completely different food collecting habits and preference. pended periphyton remained the most frequently selected To compare the relative efficiency of cell collection of type of food, a displacement of the preference was ob- the two species, the change of density of phytoplanktonic served towards units covered with other types of loose and cells was examined after 24 h of grazing. Phytoplankton- unattached material (phytoplankton) when offered in coated food units were chosen for this comparison because comparable amounts. This result also suggests that food this assemblage was largely characterized by one small quality could play a role in food selection for C. laevius- (8/~m) easily recognizable species of Thalassiosira and CldU& bore small amounts of detritus (Fig. 4D). Results are To ascertain that feeding had taken place, the gut con- presented (Table 5) for phytoplankton food units offered tents of individuals from both species was examined after alone or among other types of food, within a grid. Gam- the second series of experiments. After 24 hours spent on marus oceanicus removed a low (26%) proportion of small 66 C. Hudon: Selection of microalgae by littoral amphipods cells whether or not phytoplankton food units laid among For the epibenthic diatom community, grazing pres- other food types. For Calliopius laeviusculus, cell numbers sure can have a determinant effect on species composition were reduced by more than half in the grid experiment, and diversity (Sumner and McIntire, 1982), biomass indicating a higher grazing efficiency than G. oceanicus. (Castenholz, 1961; Calow, 1973)and vertical development The proportion of cells removed by C. laeviusculus rose to (Nicotri, 1977; Kesler, 1981). In turn, I have shown that 74% when no alternate food was present. Unfortunately, the strength of attachment of diatoms to the surfaces similar comparisons could not be made for the resus- directly affects their susceptibility to be grazed. As a con- pended periphyton, due to the larger and unquantifiable sequence, loosely attached and upright cells are com- amount of detritus present on resuspended periphyton- paratively more exposed to predation than highly ad- coated units (Fig. 4 E). hesive, adnate cells. Thus, for adnate forms, such as Cocconeis spp. and Amphora spp., lower susceptibility (Moore, 1975; Kesler, 1981) to grazers could be traded in Discussion against the drawbacks of being more readily smothered and shaded out by upright diatom species and detritus The two species of littoral amphipods investigated in this (Hudon and Bourget, 1981). study were shown to have consistently different feeding Acknowledgements. The author thanks Dr. E. Bourget for habits, producing differential exploitation of epibenthic logistic support (NSERC grant A0511) during the experi- diatoms as food resource. ments and helpful comments on the manuscript. The Gammarus oceanicus is an occasional diatom grazer, advice of Dr. P. Brunel on early drafts of the manuscript most probably ingesting them when they grow upon and of two anonymous referees on the ecology of am- macroscopic food items such as vascular plants, macro- phipods were very much appreciated. Scanning electron algae or detritus. Although this species is not well adapted microscopy was carried out at the Department of Civil for efficient collection of small particles, it can resuspend Engineering of Laval University with the help of Mr. J.-P. and ingest loosely attached cells because of the relatively Tremblay. R. Cardinal and H. Rainer provided valuable large volume of water displaced accessorily by respiratory technical support. During this study, the author was sup- currents. In my experiments, this feeding habit homoge- ported by scholarships from the Natural Sciences and nized food material and produced the inconsistent local- Engineering Research Council of Canada and the Quebec ization of these amphipods on the grids. Accordingly, no Ministry of Education. preference for any given food unit could be inferred from the position of the individuals, although they obviously could feed on the material in suspension in the water (e.g. Literature cited loose or unattached cells and detritus). Barnard, J. L.: The families and genera of marine gammaridean In comparison, Calliopius laeviusculus is more efficient amphipods. Bull. US natl. Mus. 271, 1-535 (1969) feeder on small bottom particles. In this respect, body Berghie, W. V. and M. Bergmans: Differential food preferences in posture on the surface helps to produce feeding currents three co-occurring species of Tisbe (Copepoda, Harpacticoida). similar to those observed for tube-dwelling amphipods Mar. Ecol. Prog. Ser. 4, 213-219 (1981) (Enequist, 1949). This feature represents a behavioural Bousfield, E. L.: Shallow water gammaridean of New England, 312 pp. Ithaca: Cornell University Press 1973 adaptation for high efficiency of food collection while still Brook, A. J.: Some observations on the feeding of protozoa on permitting free swimming and pelagic life-habit. As ex- freshwater algae. Hydrobiol. 4, 281-293 (1952) pected, a strong preference for loose and unattached cells Calow, P.: The food ofAncyclusfluviatilis (Miall.), a littoral, stone- was observed, with resuspended periphyton, unattached dwelling herbivore. Oecologia 13, 113-133 (1973) Castenholz, R. W.: The effect of grazing on marine littoral diatom phytoplanktonic cells and loose detritus ranking ahead of populations. Ecology 42, 783-794 (1961) natural, tightly attached periphytic cells. The preference Dagg, M. 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