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7-21-1988

Foraging Patterns of Two Syngnathid Fishes: Importance of Harpacticoid

Kevin Tipton University of South Florida

Susan S. Bell University of South Florida, [email protected]

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Scholar Commons Citation Tipton, Kevin and Bell, Susan S., "Foraging Patterns of Two Syngnathid Fishes: Importance of Harpacticoid Copepods" (1988). Integrative Biology Faculty and Staff Publications. 13. https://scholarcommons.usf.edu/bin_facpub/13

This Article is brought to you for free and open access by the Integrative Biology at Scholar Commons. It has been accepted for inclusion in Integrative Biology Faculty and Staff Publications by an authorized administrator of Scholar Commons. For more information, please contact [email protected]. p MARINE ECOLOGY PROGRESS SERIES Vol. 47: 31-43, 1988 - Published July 21 Mar. Ecol. Prog. Ser.

Foraging patterns of two syngnathid fishes: importance of harpacticoid copepods

Kevin Tipton, Susan S. Bell

Department of Biology, University of South Florida, Tampa, Florida 33620, USA

ABSTRACT: The diets of juvenile (

INTRODUCTION Alheit & Scheibel 1982, Zander 1982). While such studies combine to illustrate trophic links between har- Fish predation on macrofaunal invertebrates in both pacticoids and fishes, few studies are from seagrass temperate and tropical seagrass beds is well studied beds. Information on fish feeding on harpacticoids in (Reid 1954, Carr & Adams 1973, Young et al. 1976, seagrass beds has been gleaned mostly from subtropi- Brook 1977, Nelson 1979, Stoner 1979, 1980, 1982, cal areas, specifically Florida (Table 1). Two limitations Stoner & Livingston 1980, Ryer 1981, Livingston 1982, of previous studies are obvious. First, few data have 1984, Ryer & Boehlert 1983). However. a gap exists in been recorded on prey (meiofauna) availability (see information on predation on meiofauna by fishes within Petraitis 1979 for discussion), an important aspect of such sites. Meiofauna are abundant in seagrass beds feeding studies. Second, no investigation on fish feed- (Bell et al. 1984), with as many as 3000 harpacticoid ing on meiofauna in seagrass beds has identified prey copepods reported from a single blade of Thalassia to species level, an analytical problem with studies of testudinum Banks ex Konig (M.O. Hall pers, comm.). fish feeding on invertebrates in general (see Stoner Given the great abundance of copepods on blades, in 1979). Species level information, however, is vital for combination with that of typical sediment dwelling detailed understanding of benthic food selection by forms, meiofauna may serve as important trophic links fishes. in seagrass beds. Members of the family (pipefishes and Some investigators have reported harpacticoid seahorses) are consistently abundant members of sea- copepods to be unimportant to higher trophic levels grass fish communities (Reid 1954, Carr & Adams 1973, (Marshal1 1970, McIntyre & Murison 1973, Heip & Smol Adams 1976, Kulczycki et al. 1981, Ryer 1981, Sogard 1976), but these studies are from unvegetated areas. 1982, Huh 1984, Livingston 1984, Targett 1984, How- More recent work from shallow water has shown that ard & Koehn 1985). Little is known about the ecology of harpacticoids are a prevalent dietary item for some syngnathids (Howard & Koehn 1985), with most infor- fishes, especially juveniles and smaller-sized species mation being anecdotal. Aspects of seahorse life history (e.g. Feller & Kaczynski 1975, Bodiou & Villiers 1979, have been noted (Strawn 1958), but the feeding ecol-

O Inter-Research/Printed in F. R. Germany Mar. Ecol. Prog. Ser. 47. 31-43, 1988

Table 1 Overview of fish feedlng studies report~ngpredation on harpacticoid copepods in seagrass beds

Location Fish species/size Fish Expression of im- Conclusion Source collection portance values metbods

North Lagodon rhomboides Bag seine % Body weight Harpact~coideaten by Adams 1976 Carolina Orthopristis chrysoptera of fish small fishes USA Australia Sillaginodes puncta tus Seine Percent volume Harpacticoids important Robertson 1977 25-320 mm (%V) to 0+ age class

Florida Anchoa mitchelli 10-69 mm; Otter trawl O/O Dry weight Harpactlcoids important Sheridan & USA Leiostomus xanth urus (% DW) to juvenlle L xanthurus Llvingston 1979 10-1 09 mm; Micropogonius undulatus 10-1 19 mm; size classes vary according to species Florida Lagodon rhon~boides Trawl Harpacticoids important Stoner 1979, USA 11-121 mm, 5 mm size to smaller size classes 1980 classes Florida Callionymuspauciradiatus Pushnet % Frequency, Harpacticoids most im- Sogard 1982. USA 5-38 mm "/o Number, portant prey for all size 1984 % DW, classes; positive selec- Chesson's elect~vity tion for harpacticoids Florida 14 species for quantitative. Otter trawl '10 DW, Small or young stages of Livingston USA 12 species for qualitative cluster analysis species eat harpacticoids 1982, 1984 analysis; size classes vary according to species Florida Diplodus holbrooki Trawl Harpacticoids important Stoner & USA Lagodon rhom boides to smaller size classes of Livingston 1984 11-121 + mm, 5 mm size both species classes North Lagodon rhomboides Otter trawl Ivlev's electivity M. menidia avoid Euter- Fulton 1985 Carolina Leiostomus xanthurus pina. Harpacticoids im- USA ~Venidlamenidia portant to juveniles of Orthopristis chrysop tera 3 species Australia Mitotichthvs semistriatus Beam trawl "10 V Harpacticoids important Howard & phillipi to U. carinorostris Koehn 1985 Urocampus carinorostris; size classes vary according to species

ogy of these fish is virtually unreported. Livingston MATERIALS AND METHODS (1984) and preliminary investigations in a Tampa Bay seagrass bed (Tipton 1987) have shown that the gulf Study site. Fishes and epifaunal , repre- pipefish Syngnathus scovelli Evermann & Kendall and senting 'available prey', were collected from a subtidal the dwarf seahorse Hippocampus zosterae Jordan & site at the south end of the Sunshine Skyway bridge Gilbert feed during the daytime and are consumers of causeway near the mouth of Tampa Bay, Florida (27" large numbers of harpacticoids. The purpose of this 35' N, 82" 36'W). Seagrass beds, composed primarily of study was to investigate the feeding ecology of 2 Thalassia testudinum, dominated the site (approxl- species of syngnathids in a subtropical Thalassia tes- mately 10 X 105 m2), but patches of Halodule wrightii tudinum bed over a 6 mo period with special reference (Ascherson) were common in nearshore areas. Drift to meiobenthic copepod prey. Specifically we examine algae, mainly Acanthophora sp., Gracilaria sp. and ontogenetic patterns of feeding of prey for major taxa Hypnea sp., were prominent over the study period and meiobenthic copepod species and prey selectivity throughout the entire seagrass bed. Sediments were by syngnathids durlng times of peak fish abundances. muddy-sand and water depths during this study Tipton & Bell: Foragilng of 2 syngnathid fishes 3 3

ranged from ca 0.25 to 1.0 m. Salinity ranged from 31 to by number tends to overemphasize the contribution of 34 Ym and temperature was 24 to 32 "C over the study small dietary items, the percent composition by dry period. weight biomass (% DW) of prey was determined to Field sampling. Collections of fishes were made du- provide a complete description of the diet (Hyslop ring daylight hours (10:OO to 18:00 h) twice monthly 1980). To summarize the data the F, % N and O/O DW from Apnl to October 1984. Fishes were collected with were used to calculate an index of relative importance a 92.0 cm w~de,1 mm mesh pushnet from randomly (IRI)(Pinkas et al. 1971). The IRI was calculated as selected 100 m2 plots within the Thalassia testudinum follows: bed. While the pushnet may underestimate the abun- IRI = ('G N +"'o DW) (% F). (1) dance of large and mobile fishes, it is an efficient method Dry weight biomass of prey was estimated from meas- of collecting small and cryptic species, such as gobies, urements on field-collected organisms. Freshly col- blennies, pipefishes and seahorses, which live in sea- lected organisms from the study site were returned to grass beds (Strawn 1954). Five 20 m long pushnet runs the laboratory for biomass analyses. Freshly killed indi- were made through each plot. Two plots were sampled viduals were used because of possible error introduced on dates when few fish (i.e. less than 20 Syngnathus into dry weight estimates by preservation or freezing of scovelli < 90 mm) were captured in the first plot. Fishes samples (Williams & Robins 1982). Organisms were were preserved immediately in 10 O/O buffered forma- anaesthetized with an isotonic solution of MgCl,, lin-seawater. Fishes were not anaesthetized and no assigned to a morphotype, measured and assigned regurgitation was observed. length classes. Each organism was rinsed in deionized Laboratory procedures. All fishes caught in pushnet water and batches of 5 to 20 individuals of each size samples were enumerated and identified to species. class were placed on preweighed aluminum pans. The The standard length of each Syngnathus scovelli was pans and their contents were weighed after drying in measured to the nearest millimeter and individuals an oven for 24 h at 60 "C. Cyclopoid copepods, harpac- <90 mm (that size found to have eaten large numbers ticoid copepods and ostracods were weighed on a Cahn of harpacticoid copepods in preliminary investigations) 29 electrobalance (f O.l,ug). Larger taxa were were saved for gut analysis. Hippocampus zosterae weighed on a Mettler balance (f 0.01 mg). These were measured to the nearest millimeter from the top of weights were then used to calculate length-weight the head crest to the tip of the tail (Strawn 1958) and all regressions for each morphotype and provide estimates individuals were retained for gut analysis. of prey biomass. If the length-weight regression for a Syngnathids have a tubelike gut with no differentia- morphotype was non-significant (p> 0.05) the mean tion between the stomach and intestines, so the entire weight of the morphotype was used for biomass gut of both Syngnathus scovelli and Hippocampus 20s- estimates. Regression equations derived by Williams & terae was examined for investigation of feeding habits. Robins (1982) and Sogard (1982) were used for calanoid The gut was removed from each fish and placed into a copepods and amphipods, respectively. solution of 70 % isopropanol and Rose-Bengal stain. Estimates of prey 'availability', were made by sam- Following staining, guts were examined under a dis- pling blades of Thalassia testudinum from within the secting microscope. Prey were identified, enumerated 100 m2 plots where fish were collected on the 6 dates and measured to the nearest 0.04 mm with an ocular when both species of syngnathids were most abundant micrometer and dissecting microscope. Most prey were (see below). Blades were haphazardly selected and identified at least to the ordinal level and harpacticoid sampled prior to the fish collections. A plastic tube with copepods were identified to the species level. A prey a 0.063 mm mesh on one end was filled with filtered category of unidentifiable copepods was made up to seawater, placed over a seagrass blade and corked. include those copepods (cyclopoid and harpacticoid) The grass blade and all epifaunal organisms trapped in which could not be identified due to digestion. Each the tube were washed into a jar and preserved with major prey taxon was assigned a n~orphotypeand each 10 % buffered formalin-seawater with Rose-Bengal species of harpacticoid copepod found was assigned to stain. The samples were returned to the laboratory and one of 6 harpacticoids morphotypes based upon body crustaceans from 4 blades from each sampling date shape. Guts of all S. scovelli and H. zosterae up to a were identified and enumerated. Harpacticoid maximum of 20 of each species, including all size copepods were identified to species and all other classes, from each sampling date were examined. A organisms were identified to lowest possible taxon. total of 205 juvenile S. scovelli and 87 H. zosterae guts Selectivity was determined using fauna1 abundances were included in this study. from seagrass samples, and gut contents of the syn- Dietary analysis. The frequency of occurrence (% F) gnathids. Vanderploeg & Scavia's relativized electivity and percent composition by number (% N) were deter- index (E') (Vanderploey & Scavia 1979) was calculated mined for each prey taxon. Since percent composition as: 34 Mar. Ecol. Prog. Ser. 47: 31-43, 1988

the available prey and the gut contents. Most crusta- cean eggs found in the guts were larger than copepods (shrimp or amphipod eggs?) and were not included in where Ri= proportion of food item in the diet; P, = the calculations of E' because no practical way to proportion of food item in the environment; and N = quantitatively sample their 'availability' exists. number of kinds of food items. E' values range between + 1 and -1; a range of + 0.3 to -0.3 was arbitrarily chosen to indicate 'ran- RESULTS dom' selection. Positive values above + 0.3 indicate selection of prey items and negative values below -0.3 Dietary analysis of Syngnathus scovelli indicate avoidance. A necessary assumption, based on laboratory observations, of the above approach is that Harpacticoid, cyclopoid and calanoid copepods con- fauna on seagrass blades represented 'available' prey. sistently had the highest percent frequencies of occur- Syngnathus scovelli, observed in aquaria, actively rence (% F) in guts of all size classes of Syngnathus searched for prey by swlmming among and striking at scovelli (Table 2). Amphipods, shrimp, ostracods and prey on seagrass blades or in the water column with crustacean eggs showed increased percent frequencies fairly equal frequency. Hippocampus zosterae, how- of occurrence with increasing length of S. scovelli. ever, attached themselves to seagrass blades with their Significant differences in both O/O N and '10 DW of the prehensile tails and attacked blades more frequently harpacticoid, cyclopoid and unidentified copepod prey than the water column (Tipton unpubl.). items were detected among the 4 size classes of Syn- Unlike other selectivity indices that vary with the gna thus scovelli (Fig. 1 ; Kruskal-Wallis test, p < 0.05). relative abundance of other food items in the sample Inspection of the data shows a strong pattern of higher (Chesson 1978, Vanderploeg & Scavia 1979, Lechowitz percentages of both number and dry weight of these 1982), E' is unaffected by the relative abundance of prey in the smaller size classes of fish (Fig. 1).The % N prey types and is probably the best overall selectivity and % DW of crustacean eggs and shrimp were also index (Lechowitz 1982). In this study E' was calculated significantly different among the 4 size classes of S. by combining data from 3 sampling dates in spring and scovelli but, in contrast, more of these prey were con- 3 sampling dates in summer. Three dates were lumped sumed by the larger size classes (Fig. 1). No other so that all available prey were represented by a suffi- differences In either % N or % DW were recorded for cient number and no E' values were undefined when remaining prey among the size classes of S. scovelli. prey in the environment were equal to zero (P, = 0). Harpacticoid copepods had the highest index of rela- These dates were chosen because they contained the tive importance (IRI)value of any prey for all sizes of highest number of both Syngnathus scovelli and Syngnathus scovelli (Table 3). A value of 10 427 (max- Hippocampus zosterae. E' values were calculated for imum IRI = 20 000) was obtained for harpacticoids in 4 size classes of S. scovelli and 3 size classes of the 30-49 mm size class of S. scovell~. Harpacticoids H. zosterae for both spring and summer. Prey selection were the most important prey for the 2 smallest size was calculated from numerical percentages. Organ- classes while crustacean eggs had the highest IRIS for isms such as isopods, insects, and mites were not the 50-69 and 70-89 mm size classes. Calanoid included in analysis as they represented < 1 % of both copepods and caridean shrimp also had high IRISfor

Table 2. Syngnathus scovelli.Percent frequency of occurrence (% F) of major prey taxa for 4 size classes of fish Number of guts examined is given in parentheses for each size class. Total = X F for all size classes combined

Prey taxon Size class in mm < 30 30-49 50-69 70-89 (n= 36) (n= 46) (n= 58) (n = 39) Total

Amphipods 0 0 39.1 62.1 92 3 50.3 Calanoid copepods 72 2 76.1 89.7 94 9 83.8 Cyclopoid copepods 52.8 65.2 46.6 46.2 52.5 Harpacticoid copepods 72.2 93.5 87.9 87.2 86.0 Crustacean eggs 2.8 21.7 67.2 64.1 49.2 Ostracods 0.0 4.4 10.3 23.1 9.5 Caridean shrimp 2.8 8.7 29.3 56.4 24.6 Unidentified copepods 13.9 39.1 50.0 35.9 36.9 - Tipton & Bell: Foraging of 2 syngnathid fishes 3 ,

Fig. 1. Syngnathus scovel0. Per- Ioi- centage number and percentage 50 6qmm n = 58 dry weight biomass for prey items in guts of 4 size classes of fish over all dates combined. Number of guts examined [n) is given for each size class. AM: 7, 50- amphipods; CA: calanoid cope- - pods; CY: cyclopoid copepods; CE: crustacean eggs; OS: ostra- cods; SH: caridean shrimp; UC: unidentified copepods; HC: har- pacticoid copepods 5 CM CA CY CE OS SH UC HC

the largest size class and cyclopoid copepods had the ticoids with high importance values were Harpacticus 2nd highest IRI for the <30 mm size class. Other prey sp. 2, Ectinosomatid sp. B and unidentified copepo- were not important components of S, scovelli diets. dites. No strong seasonal differences in dietary composi- tion of Syngnathus scovelli over the dates was apparent with crustacean eggs and harpacticoids constituting Dietary analysis of Hippocampus zosterae the majority of the diet on most sampling dates (data available in Tipton 1987). Additionally the IRI values Every Hippocampus zosterae examined contained for major prey of S. scovelli did not show any differ- harpacticoid copepods (Table 4). Calanoid copepods ences among seasons mirroring the trends for abun- and unidentified copepods also had high O/O F values. dance and biomass of food items (Tipton 1987). Amphipods increased in frequency of occurrence from The IRI of Harpacticus sp. 1, the dominant species of the smallest size class (13.3 %) to the largest size class harpacticoid copepod found in guts, closely followed (44.7 %). Other prey did not have high % F values in H. that of total harpacticoids for all 4 size classes of Syn- zosterae, except for cyclopoid copepods (44.4 %) for gnathus scovelli (Fig. 2 and 3). Other species of harpac- the 20-30 mm size class.

Table 3. Syngnathus scovelli. Index of Relative Importance (IN) of major prey taxa for 4 size classes of fish. Number of guts examined is given in parentheses for each size class. (Maximum IRI = 2.0 X 104).Mean = IRI over all sizes classes combined

Prey taxon Size class in mm < 30 3049 50-69 70-89 (n = 36) (n = 46) (n = 58) (n = 39) Mean

Amphipods Calanoid copepods Cyclopoid copepods Harpacticoid copepods Crustacean eggs Ostracods Candean shrimp Unidentified copepods

Tipton & Bell: Foraging of 2 syngnathid fishes 37

Table 4. Hippocampus zosterae. Percent frequency of occur- Table 5. Hippocampus zosterae. Index of Relative Importance rence (%F) of major prey taxa for 3 size classes of fish. (IRI)for prey items in 3 size classes of fish. Number of guts Number of guts examined is glven in parentheses for each slze examined is given in parentheses for each size class (maximum class. Total = % F for all size classes combined IRI = 2.0 X 10-4). Mean = IRI for all size classes combined

Prey taxon Size class in mm Total Prey taxon Size class in mm Mean c20 20-30 >30 c20 20-30 >30 (n=15) (n=36) (n=36) (n=15) (n=36) (n=36)

Amphipods 13.3 41.7 44.7 36.8 Amphipods 25 190 199 165 Calanoid copepods 66.7 94.4 97.2 90.8 Calanoid copepods 786 1246 2196 1560 Cyclopoid copepods 33.3 44.4 25.0 34.5 Cyclopoid copepods 221 112 13 90 Harpacticoid copepods 100.0 100.0 100.0 100.0 Harpacticoid copepods 15348 15934 15432 15622 Ostracods 0.0 27.8 27.8 23.0 Ostracods 0 26 23 20 Candean shrimp 0.0 5.6 0.0 2.3 Candean shrimp 0 3 0 1 Unidentified copepods 73.3 88.9 80.6 82.8 Unidentified copepods 1919 1664 1389 1594

The diet of Hippocampus zosterae in terms of % N slightly with increasing fish size but were generally and % DW was dominated by harpacticoid copepods small. Cyclopoid copepod IRI values were also small (Fig. 4) near or exceeding 80 % of the diet of all size ranging from 221 for the <20 mm size clases to 13 for classes. Only unidentified copepods and calanoid cope- the > 30 mm size class. Because harpacticoids domi- pods made up > 10 % of the diet of any H. zosterae size nated % F, % N and % DW over all dates, no strong class. seasonal pattern in feeding by H. zosterae was appar- Harpacticoid copepods had IRI values greater than ent. Thus by all measurements, harpacticoids were the 15 000 for all size classes of Hippocarnpus zosterae overwhelmingly important prey item of H. zosterae. (Table 5). Calanoid copepods and unidentified Some seasonal variation in IRI values for total har- copepods were the only other prey with IRI values pacticoids and for selected species of harpacticoids of exceeding 1000. Amphipod IRl values increased all 3 size classes of Hippocampus zosterae was re-

Fig. 4. Hippocampus zosterae. Percentage number and per- centage dry weight biomass for prey items in guts of 3 size classes of fish over all dates combined. Number of guts examined (n) is given for each size class. AM: amphlpods; CA: calanoid copepods; CY: cyclopoid copepods; OS: ostracods; SH: shrimp; UC: unidentified copepods; HC: harpacticoid copepods 38 Mar. Ecol. Prog. Ser. 47: 31-43. 1988

Fig 5. Hippocampus zosterae. Temporal variation of Index of Relative Importance (IRI) values for total harpacticoid copepods and dominant harpacticoid copepod species for 3 size classes. Numbers of guts exanlined are indicated above data points. Dates same as Fig. 2. Maximum IRI = 2.0 X 104. TH: total harpacticoids; HI: Harpacticus sp. 1; H2: Harpacticus sp. 2; PB: Paradactylopodia brevicornis; DT: Dactylopodia tisboides

corded (Fig. 5). Harpacticus sp. 1 was the dominant Available prey harpacticoid species in guts but 3 other species also were important to H. zosterae on selected dates. Para- Harpacticoid copepods were abundant on seagrass dactylopodia brevicornis had IRI values higher than blades from the study site from April to October 1984 Harpacticus sp. 1 in May for the 20-30 mm size class and were the dominant prey taxon on all dates sampled and > 30 mm size class. Harpacticus sp. 2 and Dacty- with a maximum abundance of 7.05 per cm2 of blade lopodia tisboides also were important prey species, but surface area in April (Tables 6 and 7). Other major prey their IRI values never exceeded those of Harpactjcus taxa were less abundant. The harpacticoid Paradacty- sp. 1. lopodia brevicornis was abundant in the spring as was

Table 6.Mean number (k SE) of dominant epifaunal crustaceans per cm2 on Thalassia testudinum blades, spring 1984. Only taxa found in guts of S~~ngnathusscovelli or Hippocampus zosterae are included. (N=4 blades per date) Information on other copepod species is available in Tipton (1987)

27 Apr 11 May 29 May Total spring

Amphipods 0 47 (0.27) 0.08 (0.03) Caridean shrimp 0.01 (0.01) - Ostracods 0.37 (0 08) 1.17 (0.40) Calanoid copepods -

Taxon not present Tipton & Bell: Foraging of 2 syngnathid fishes 39

rable 7. Mean number (L SE) of dominant epifaunal crustaceans per cm2 of Thalassia testudinum blades, summer 1984. Only taxa found in guts of Syngnathus scovelli or Hippocampus zosterae are included. (N=4 blades per date). Information on other copepod species is available in Tipton (1987)

Taxon 24 Aug 7 Sep 21 Sep Total summer

Amphipods 0.02 (0.18) a 0.07 (0.01) 0 28 (0.01) Caridean shrimp - - 0.01 (0.01) <0.01 Ostracods 0.23 (0.01) 0.20 (0.13) 0.19 (0.07) 0.20 (0.03) Calanoid copepods 0.04 (0.03) 0.31 (0.55) 0.05 (0.04) 0.13 (0.56) Cyclopoid copepods 0.06 (0.03) 0.12 (0.05) 0.70 (0.25) 0.29 (0.12) Total harpacticoid copepods 2.39 (0.37) 4.18 (0.36) 1.68 (0.71) 2.75 (0.41) Unidentified copepodites 0.96 (0.14) 1.76 (0 36) 0.40 (0.17) 1.04 (0.21) Harpacticus sp. 1 0.01 (0.01) 0.05 (0.02) 0 02 (0.01) Harpacticus sp. 2 0.60 (0.17) 1.53 (0.19) 0.34 (0.15) 0.82 (0.18) Dactylopodia tisboides 0.04 (0.02) 0.48 (0.15) 0.31 (0.15) 0.28 (0.08) Paradactylopodia brevicornis 0.54 (0.15) 0.24 (0.10) 0.28 (0.17) 0.35 (0.08)

a Taxon not present

Harpacticus sp. 2 in late summer. Harpacticus sp. 1 had cyclopoid copepods by > 30 mm S. scovelli in summer very low abundances from blades on all 6 sampling (Table 8). Hippocampus zosterae < 20 mm selected dates. Unidentified copepodites were found in high cyclopoids in summer and >30 mm H. zosterae densities in both spring and summer with 1.76 per cm2 selected calanoids in spring (Table 8). Harpacticus of blade surface in early September. sp. 1 was the only harpacticoid copepod prey species which was selected by S. scovelli or H. zosterae, being chosen disproportionately to its field abundance by all Selectivity of prey by Syngnathus scovelli and size classes in spring and summer (Tables 8 and 9). Hippocampus zosterae

Vanderploeg & Scavia's Electivity Index (E') for DISCUSSION Syngnathus scovelll and Hippocamplis zosterae indi- cated that most major prey taxa were avoided (77 % of Harpacticoid copepods comprised the largest per- cases) by all size classes of both S. scovelli (Table 8) centage of the diet of Hippocampus zosterae and the and H. zosterae (Table 9). Only 14 % of cases were small size classes of Syngnathus scovelli in the Tampa assigned 'random' feeding. Few prey taxa were Bay, Florida, seagrass bed. S, scovelll (size class selected. Calanoid copepods were selected by the unidentified) from a Texas seagrass bed also had a 50-69 mm and 70-89 mm S, scovelli in spring and large proportion of copepods in gut samples (Huh &

Table 8. Vanderploeg & Scavia's Electivity Index (E') for major prey taxa and dominant harpacticoid copepod species in guts of Syngnathus scovelli in spring and late summer 1984. E' ranges from + 1 (selectivity) to - 1 (avoidance)

Prey organism Size class in mm < 30 3049 50-69 70-89 Spring Summer Spring Summer Spring Summer Spring Summer

Amphipods -1.00 -1.00 -0.90 +0.15 -0.69 -0.48 -0.24 -0.41 Calanoid copepods -1.00 -0.32 +0.18 -0.64 +0.67 0.00 +0.78 +0.09 Cyclopoid copepods -0.34 +0.71 -0.75 -0.96 -0.87 -0.98 -0.07 -0.97 Ostracods -1.00 -1.00 -1.00 -1.00 -0.99 -1.00 -0.99 -0.99 Caridean shrimp - 1.00 -0.08 -0.73 -0.63 +0.02 -0.84 -0.26 -0.37 Total harpacticoid copepods -0.02 -0.94 -0.72 -0.73 -0.88 -0.65 -0.95 -0.76 Unidentified copepodites -0.03 -0.99 -0.96 -0.99 -0.96 -0.99 -0.97 -0.99 Harpacticus sp. 1 +0.43 +OS5 +0.85 +0.80 +0.85 +0.83 +0.44 +0.77 Harpacticus sp. 2 -1.00 -1.00 -0.99 -0.97 -0.96 -0 97 -1.00 -0.98 Dactylopodia tisboides -1.00 -1.00 -1.00 -1.00 -1.00 -0.99 -0.99 -1.00 Paradactylopodia brevicol-nis - 1.00 - 1.00 -0.99 -1.00 -0.96 -0.99 -0.95 -0.99 40 Mar. Ecol. Prog. Ser. 47: 31-43, 1988

Table 9. Vanderploeg & Scavia's Electivity Index (E') for major prey taxa and dominant harpacticoid species from guts of Hippocampus zosterae In the spring and late summer 1984. E' ranges from + 1 (selectivity) to - 1 (avoidance)

Prey organism Slze class in mm < 20 20-29 > 30 Spring Summer Spring Summer Spring Summer

Amphipods Calanoid copepods Cyclopoid copepods Ostracods Caridean shrimp Total harpaclcoid copepods Unidentified copepodites Harpacticus sp. 1 Harpacticus sp. 2 Dactylopodia hsboides Paradactylopodia brevicornis

Kitting 1985). Given that the small size classes of sea- contents of H. zosterae from Biscayne Bay, Florida, grass resident fishes such as the pinfish Lagodon rhom- passed through a 1 mm mesh sieve, perhaps indicating boides (Stoner 1979, Livingston 1980), spot Leiostomus that small prey like harpacticoid and cyclopoid xanthurus (Sheridan & Livingston 1979), mojarra copepods were eaten. Eucinostomus gula, and silver perch Bairdiella chrys- Information on prey selectivity of syngnathids is ura (Livingston 1982) utilize harpacticoid copepods as limited. Ryer (1981) found that Syngnathus fuscus prey, the dietary patterns observed for syngnathids in (< 100 mm) selected calanoid copepods in June and our study site are not unique. Moreover, because November and various species of amphipods and patterns of abundances for S. scovelli and H. zosterae isopods in other months. The larger size classes in Tampa Bay, Florida (Tipton 1987), as well as dietary (50-69 mm, 70-89 mm) of Syngnathus scovelli in this characteristics reported here, in general accord with study also selected calanoid copepods in the spring. reports from other geographical areas, a common feed- Total harpacticoid copepods which had the highest IRI ing pattern emerges for these fishes in seagrass beds. values were not selected by S. scovelli, although the The diet of Syngnathus scovelli changed ontogeneti- dominant harpacticoid species, Harpacticus sp. 1, in cally from harpacticoid copepods to larger prey as fish the gut was selectively chosen. Prey selectivity of size increased (Table 2; Fig. 2). The ontogenetic switch Hippocampus zosterae was similar to that of S. scovelli, from small prey to larger prey was also shown for S. (Tables 8 and 9) but some differences in diet content scovellj from Appalachicola Bay, Florida (Livingston are notable, probably attributable to differences in food 1982, 1984), but harpacticoids were not as prevalent in capturing behavior and the syngnathids' size. H. zos- the diet as in the present study. Syngnathus fuscus terae, being smaller and less motile than S, scovelli, from North Carolina (Adams 1976) and Chesapeake may not be able to capture the larger and more mobile Bay (Ryer 1981) also switched from smaller to larger prey, such as amphipods and shrimp, since small fishes prey as adult fish size increased but no information is are particularly ineffective at capturing large, motile available to determine if a strong dependence on har- prey (Eggers 1977). These bigger prey would be more pacticoids by juveniles of S. fuscus existed. susceptible to predation by S. scovelli, especially the In contrast to Syngnathus scovelli, little ontogenetic large sizes. A similar explanation was offered by How- change was apparent in Hippocampus zosterae diets ard & Koehn (1985) for Australian syngnathids. They with a strong dominance of harpacticoid copepods over found Urocampus carinorostris, the smallest of 4 pipe- all size classes (Fig. 5) and dates (Tipton 1987). This is fish species examined, to have had the highest percent- the first study to report harpacticoids as important prey age of harpacticoids in its diet. U. carinorostris has a of H. zosterae. Reid (1954) found shrimp and microcrus- prehensile tail and uses a 'sit and wait' type feeding taceans to be the major prey of seahorses near Cedar behavior, similar to that of H. zosterae, which Howard Key, Florida. Shrimp were found in only 2 guts of H. & Koehn (1985) maintain may restrict ability to capture zosterae in the present study, but Reid (1954) lumped mobile prey such as amphipods. Behavioral differences all seahorse species together for diet descriptions and between syngnathids may thus be important in this was probably including shrimp eaten by larger sea- study. horse species. Brook (1977) mentioned that the gut In general, selectivity values need to be interpreted Tipton & Bell: Foraging of 2 syngnathid fishes 4

cautiously. What is deemed available prey by a species in all the samplings. Sibert (1979) found a researcher may not be available to a predator due to slmilar situation for harpacticoids in the Nanaimo estu- behavior of the predator, behavior of the prey, area ary, British Columbia. Harpacticus uniremis was the sampled or the sampling method employed (Eggers dominant prey species of the chum salmon Oncorhyn- 1977, Petraitis 1979). Determining prey availability in cus keta (Healey 1979), but was rare in sediment core estuaries may be especially difficult for mobile organ- samples. Because very few studies have looked at isms since prey may reside in more than one mi- copepod prey at the species level it 1s ~mpossibleto crohabitat (Mine110 & Zimmerman 1984).The sampling know if this is a general phenomenon. method chosen for this study was designed to effi- Attempts have been made to determine whether fish ciently capture meiobenthos on blades but underesti- predation controls harpacticoid copepod populations mated planktonic prey such as cyclopoid and calanoid with contrasting views offered in other studies. Many copepods. The larger, more mobile crustaceans, such authors maintain that fish predation exerts little reg- as shrimp and amphipods, probably avoided the sam- ulatory control on harpacticoid populations (Bregnballe pling tube. These 2 sources of possible sampling bias 1961, Sibert et al. 1977, Alheit & Scheibel 1982, Sogard would tend to cause the E' values of these more mobile 1984, Hicks 1985). Feller & Kaczynski (1975), however, organisms to be overestimated and the selectivity of observed a decrease in densitites of benthic harpac- other organisms, such as harpacticoids, to be under- ticoids by 3 orders of magnitude from April to May in valued. As such, the evaluation of strong selectivity for Puget Sound coinciding with the influx of juvenlle any harpacticoid is probably conservative. Moreover, a salmon and suggested that the salmon predation was consistent pattern of relative abundances for major responsible for the decrease. Sibert (1979) and Gee et taxa and harpacticoid copepod species on blades from al. (1985) provided evidence for fish predation having a various dates was observed (Tables 6 and 7); therefore controlling influence on a single harpacticoid species the lumping of 3 dates for electivity calculations was but not the total abundance of copepods. In the Tampa justified (see 'Materials and Methods'). Bay seagrass bed, a high level of predation combined The harpacticoid copepod Harpacticus sp. 1 was with very low abundances of Harpacticus sp. 1 on the always selected by all sizes of both Syngnathus scovelli seagrass blades could suggest that the population of and Hippocampus zosterae. Although very little is this copepod species is being controlled by syngnathid known about the life history of Harpacticus sp. 1, many predation, although further experimentation is factors may contribute to its attractiveness as prey for necessary to test this hypothesis. S. scovelliand H. zosterae.First, the behavior of Harpac- It is clear that harpacticoid copepods are entwined in ticus sp. 1 may be important as motion has been iden- the trophic network of the seagrass bed examined here. tified as the best single predictor of predator risk for While the position of syngnathids in the food web is not planktonic prey of brown trout (Ware 1973). Second, firmly established because little is known about preda- the high selectivity values of Harpacticus sp. l tion on these fishes (Strawn 1958, Lascara 1981, How- reflected extremely low abundance on seagrass blades ard & Koehn 1985), their continued appearance as (available prey). Two possible explanations can be common or dominant fishes in seagrass beds suggests offered. First, the microhabitat of Harpacticus sp. 1 that they may provide a significant energy source. within the grassbed is unknown and the sampling Accordingly, the large numbers of harpacticoids eaten regime chosen for this study may not have accurately by these syngnathids provides evidence that the fish- predicted available prey (see above). An attempt was copepod link is integral to seagrass ecosystem function. made to more widely assess the microhabitat utilization of Harpacticus sp. 1 by sampling the water column Acknowledgements. We thank Drs B. C. Cowell and H. R. around and above the seagrass blades, sediment near Mushinsky and anonymous reviewers for their critical reviews the blades, and drift algae. Ve~yfew Harpacticus sp. 1 and/or discussions concerning this project. The following per- were found in any of these sampling efforts suggesting sons provided much needed assistance with field collections and computer analyses: M. Mann, M. Vitale, D. Miller,and W. that the low numbers in the field may not be a sampling Bros. This work was completed in partial fulfillment of the artifact. Alternatively, predation by Syngnathus sco- requirements for a Master of Science degree in Zoology (K.T.) velli and Hippocampus zosterae could be responsible at the University of South Florida. for the low numbers of Harpacticus sp. 1 collected in samples from the grassbed. Guts with over 100 Harpac- ticus sp. 1 each were not uncommon for either fish LITERATURE CITED species and as many as 200 individuals per gut were Adams, S. M. (1976). Feeding ecology of eelgrass fish com- found in H. zosterae guts. Predation at this level could munities. Trans. Am. Fish. Soc. 105: 514-519 keep the population ofHarpacticussp. 1 low and alter- Alheit, J., Scheibel, W (1982). Benthic harpacticoids as a food natively might explain the low abundances of this source for fish. Mar Biol. 70: 141-147 42 Mar Ecol. Prog. Ser. 47: 31-43, 1988

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This article was submitted to the editor; it was accepted for printing on May 9, 1988