Habitat selection by the burrowing brittlestar ophiophragmus filograneus in the banana river lagoon%2c florida
Habitat Selection by the Burrowing Brittlestar Ophiophragmus filograneus in the Banana river Lagoon, Florida. Indira Brown, Department of Biology and Marine Science, Jacksonville University. AbstractBrittlestars are motile Echinoderms inhabiting the oxidized subsurface layer of soft-bottomsubstrates in every marine environment. Burrowing brittlestars typically have a patchydistribution that may be influenced by sediment organic content or presence of sub-aquaticvegetation. Few studies have been undertaken to determine specific substrate preferences.Ophiuroids from the Banana River Lagoon near Merritt Island, FL were collected to determinetheir substrate preferences based on four sediment grain sizes. In controlled laboratory tests ofgrain size preference, brittlestars preferred to burrow in the very fine and fine grainedsediments (N=48). Subsequent experiments testing preference between these two grain sizesrevealed a preference for the very fine sediments. Further experiments of grain size withvegetation and artificial vegetation were performed. Results indicated Ophiophragmusfilograneus’ patchy distribution in the natural environment may correlate with very finesediment, seagrass and structural distribution.KEYWORDS: Ophiophragmus filograneus, brittlestars, sediment preference.IntroductionBenthic invertebrates play a vital role in the physical, chemical and biological structure of theirsedimentary environment. They are known to recycle nutrients and detoxify pollutants throughburrowing, feeding, and respiring (Peterson and Quammen 1982; Fukuyama and Oliver 1985;Kvitek et al. 1992; Micheli 1997). The mobility of sediment-associated cadmium (Cd), for
example, may be increased during resuspension of sediments by a shift from reducing tooxidizing conditions and by altered pH (Khalid et al., 1981; Förstner, 1987; Peterson et al.,1996). Ophiuroids are one of the numerous benthic invertebrates that contribute to theoxidization state of the subsurface sediment layer in the marine environment. They are also animportant component of the marine trophic system in shallow waters (Turner and Meyer 1985). Ophiophragmus filograneus belongs to the Family Amphiuridae, which is also known asthe burrowing brittlestars. O. filograneus burrow in the sediment 5-10 cm deep. They are oneof the smallest known brittlestars with disk diameters of approximately 1 cm, and arms up to10 cm in length. O. filograneus are common in the marine littoral regions from the southern tipof Florida to Pensacola Bay on the North West coast and to Cape Canaveral in the North East.They have also been found in Cedar Key, Florida. O. filograneus are unique because they occurin estuaries at reduced salinities and not in the open sea. In the past, distribution ofOphiophragmus was assumed to be limited because it was known to adapt to environmentswith reduced salinities. However, recent studies indicate O. filograneus functions better athigher salinities and therefore its restriction to estuaries is probably due to other factors suchas food resources, sediment type, competition, or predation or some other factor (Talbot andLawrence 2006). Within estuaries Ophiophragmus filograneus still shows patchy distribution. The factorsthat define their irregular distribution are unclear. Some possible factors influencing theirdistribution are sediment size and the presence of sea-grass, or other submerged aquaticvegetation. The burrowing brittlestar Micropholis gracillima preferred to burrow in fine grainedsediment with some organic content (Zimmerman 1987). The goal of this study is to determine
the substrate preference of Ophiophragmus filograneus with regards to sediment grain size,seagrass and artificial seagrass substrate. The proposed hypothesis is that Ophiophragmusfilograneus would prefer fine and very fine.Methodology: The collection site for this experiment was in the Banana River, near Melbourne, FL(28o12’ N, 80o37’ W) (Figure 1). The salinity of the water was recorded at 24 ppt. Sediment wascollected by the shovelful in shallow water (no more than 3 feet in depth) and then sievedthrough 0.15 cm sieve in order to obtained the specimens burrowed within the sediment. Thespecimens were placed in a bucket with portable air pumps filled with natural seawater fromthe site. The specimens were transported back to Jacksonville University’s wet lab. Thespecimens were placed in two ten-gallon aquaria with a 2 inch deep layer of natural sedimentfrom the collection site and natural seawater at 24 ppt. The brittlestars remained in the holdingtanks for 24 hours to acclimate to their new environment before testing began. The salinity wasadjusted with distilled water as necessary to maintain a constant 24 ppt. Specimens were fedevery other day with finely powered fish food flakes. The first experiment was designed to determine brittlestar preference when presentedwith four sediments. The sediment sizes were very fine (50/140), fine (45/60), medium (30/65),and coarse (20/30). Sediment was purchased locally from (Standard Sand & Company Services,Jacksonville, FL) and was clean of debris and organic material. Finger bowls (8 inches diameter)were used as individual testing arenas. Natural sea water was placed into each of the bowls to adepth of 2 inches. Each bowl was divided into four pie-shaped sections with plastic dividers toprevent mixing of the different sand sizes. Sand was placed into each section so that all four
sizes of sand were represented in each bowl. The size of sand in each section was indicatedalong the outer perimeter of the finger bowls (Figure 2). For example, the coarse treatment wasplaced between the very fine and fine at the southern perimeter of the bowl, while the mediumtreatment was placed between the very fine and fine at the northern perimeter of the bowl.The sediments were allowed to settle for 24 hours before placing brittlestars in each bowl. An individual O.filograneus was placed at the center of its assigned bowl at 3:00p.m.After placing the brittlestars in each of the 12 bowls, the blows were covered with a box toprevent light penetration that might influence the specimens’ substrate preference. After 24hours the boxes were removed and the location of the burrowed brittlestar was recorded as itssediment preference (Figure 3). The specimens were then removed and placed into a separateholding tank from the brittlestars that had not yet been tested. The 12 finger bowls werecleaned in order to remove all scent of previous specimens tested. The finger bowls were thenreset with new sediments and allowed to settle for 24 hours before testing resumed. Each setof 12 finger bowls tested is referred to as a block. The experiment was repeated 4 times for atotal N of 48. The data were analyzed with a Freidman’s Rank Sum test (Table 1). A second set of experiments was done to determine the preference of brittlestarsbetween the two sediment sizes most often chosen in the first experiment, which were thevery fine and the fine (Figure. 4). The same exact method from the previous study was used,except in this case the arenas were divided into 2 sections instead of 4. The data was alsoanalyzed with a Freidman’s Rank Sum test (Table 2). The third set of experiments was performed to determine brittlestars preferencebetween the very fine sediment and seagrass (Holodule wrightii). The same exact method from
the second set of experiment was used, except the brittlestars remained in the holding tanksfor 48 hours to acclimate to their new environment before testing (Figure 4). In addition, theseagrass rhizomes were submerged within the very fine sediment as its’ blades were above thesediment submerge in water. Due to the irregularity of placing the sediment at the edge of thearenas, the experiment was performed again. During the second set of testing, the seagrasswere placed in the center on its side of the arenas. The data were analyzed with a Freidman’sRank Sum test (Table 3). The fourth and final set of experiments was set up exactly as the pervious experiment inorder to determine the preference of brittlestars between the very fine sediment sizes andartificial seagrass (Figure 5). The artificial seagrass was constructed using Diamond coffeestirrers and Berwick curling ribbons. The ribbon, which was seven inches in length, was foldedin half then tied around a coffee stirrer in an overhand bend knot. The same exact method fromthe previous study was used. The data was also analyzed with a Freidman’s Rank Sum test(Table 4).Results In the first set of experiments, the brittlestars within each block appeared to haveselected the very fine and fine sediments most often, with very few brittlestars selecting themedium or coarse sediment. Block 1 is significantly different from blocks 2, 3, and 4, (Figure 6).However, despite this anomaly, the overall trend was that the majority of animals preferred thevery fine and fine sediment size, and rarely preferred the medium and coarse sediment size(Figure 7). In this case, 44% of the brittlestars preferred the very fine sediment, 40% preferred
the fine sediment, 6% preferred the medium sediment and 10% preferred the coarse sediment(Figure 8). In the second set of experiments, the brittlestars’ preference between the fine and veryfine sediments was tested. Once again block 1 showed a different trend from blocks 2, 3 and 4.For example, in block 1, seven brittlestars preferred the fine sediment and five brittlestarspreferred the very fine sediment. However, in all of the other blocks the very fine sediment wasmost often selected (Figures 9 & 10). In total, 69% of the brittlestars preferred the very finesediment and 31% preferred the fine sediment (Figure 11). In the third set of experiments, the brittlestars’ preference between the seagrass andthe very fine substrate was tested. The brittlestars within each block appeared to havepreferred the seagrass in the very fine sediments most often. Very few brittlestars selected theboarder or the region between the seagrass and very fine sediment. For example in block 1, sixbrittlestars preferred the seagrass bed, four preferred the very fine sediment and 2 preferredthe border. Block 3 was slightly different since no brittlestars preferred the boarder (Figures 12& 13). In total, in all the blocks the seagrass substrate was often selected. Therefore, 60.42% ofthe brittlestars preferred the seagrass substrate, 29.17% preferred the very fine sediment and10.42% preferred the border (Figure 14). In the final experiment, brittlestars’ preference between the artificial seagrass and veryfine sediments was tested. Once again the brittlestars within each block appeared to haveselected the artificial seagrass and very fine sediments most often, with very few brittlestarsselecting the border or the region between the seagrass and very fine sediment. There was nodifference between the trends in each block. For example in block 1, seven brittlestars
preferred the fine sediment, four brittlestars preferred the very fine sediment and onebrittlestar preferred the border. In all of the blocks the artificial seagrass substrate was mostoften selected (Figure 15 & 16). In total, 60.42% of the brittlestars preferred the artificialseagrass substrate, 31.25% preferred the very fine sediment and 8.33% preferred the border(Figure 17). Figure 1. The collection site of O.filograneus in the Banana River is indicated by the red Oval.
Figure 2. Photograph of experimental set Figure 3. Brittlestar burrowed in one bowl Of testing arenas from the first set of experiments. Location of the brittlestar is very obvious.Figure 4. Photograph of experiments 2 through 4 testing arenas
Figure 5. Photograph of testing arina with artificial seagrass.Table1. Statistical analysis of data from the first set of experiments. The numbers in the tablereflect the rank of choice within each block. A choice of very fine sediment was assigned thevalue of 1, fine sediment was assigned the value of 2, medium sediment the value of 3, andcoarse sediment the value of 4.Bowl Number Block 1 Block 2 Block 3 Block 4 1 3.5 3.5 8.5 3.5 2 8.5 8.5 3.5 11.5 3 3.5 8.5 8.5 8.5 4 9 3.5 3.5 3.5 5 11.5 9 8.5 3.5 6 3.5 8.5 8.5 8.5 7 3.5 8.5 3.5 8.5 8 11.5 8.5 9 8.5 9 3.5 8.5 8.5 3.5 10 11.5 3.5 3.5 3.5 11 3.5 3.5 3.5 8.5 12 8.5 3.5 11.5 8.5 Rank 81.5 77.5 80.5 80 Rank2 6642.25 6006.25 6480.25 6400 ∑ Rank2 553.5208 ∑ Ranks2/12 46.12674 H -0.51466 t 11 11 10 11 T 1320 1320 990 1320 sum of T 4950 adjusted H -3.50669
Table 2. Statistical analysis of data from the second set of experiments. The numbers in thetable reflect the rank of choice across the entire experiment. A choice of very fine sedimentwas assigned the value of 1, and the fine sediment was assigned the value of. Bowl Number Rank 1 Rank 2 Rank 3 Rank 4 1 41 17 41 17 2 41 17 17 17 3 17 17 17 17 4 41 17 41 17 5 17 17 17 41 6 41 17 17 17 7 17 17 17 41 8 41 17 17 41 9 17 17 17 41 10 41 17 17 17 11 17 17 41 17 12 41 41 17 17 ∑ Ranks 372 228 276 300 ∑Ranks^2 138384 51984 76176 90000 ∑ Ranks^2/12 11532 4332 6348 7500 H 150.8418 t 33 15 T 35904 3360
sum of T 39264 adjusted H 233.9318Table 3. Statistical analysis of data from the third set of experiments. The numbers in the tablereflect the rank of choice across the entire experiment. A choice of very fine sediment wasassigned the value of 1, seagrass was assigned the value of 2, and the broader between veryfine and seagrass was assigned the value 3. Bowl Number Rank 1 Rank 2 Rank 3 Rank 4 1 7.5 29 29 7.5 2 7.5 29 7.5 7.5 3 29 7.5 29 7.5 4 29 29 7.5 46 5 46 46 7.5 29 6 29 29 29 29 7 7.5 29 29 7.5 8 29 7.5 29 29 9 46 29 29 46 10 29 29 7.5 29 11 29 29 29 29 12 7.5 29 29 29 ∑ Ranks 296 322 262 296 ∑Ranks^2 87616 103684 68644 87616 ∑ Ranks^2/12 7301.333 8640.333 5720.333 7301.333 H 147.0221 t 14 29 5 T 2730 24360 120
sum of T 27210 adjusted H 195.0274Table4 . Statistical analysis of data from the third set of experiments. The numbers in the tablereflect the rank of choice across the entire experiment. A choice of very fine sediment wasassigned the value of 1, artificial seagrass was assigned the value of 2, and the broader betweenvery fine and artificial seagrass was assigned the value 3. Bowl Number Rank 1 Rank 2 Rank 3 Rank 4 1 46.5 7 7 7 2 7 30 30 30 3 7 30 30 30 4 30 30 30 30 5 30 7 30 7 6 7 30 46.5 30 7 30 7 7 46.5 8 30 46.5 30 30 9 30 7 30 30 10 7 7 30 7 11 30 30 7 30 12 30 30 30 30 ∑ Ranks 284.5 261.5 307.5 307.5 ∑Ranks^2 80940.25 68382.25 94556.25 94556.25 ∑ Ranks^2/12 6745.021 5698.521 7879.688 7879.688 H 143.1424 t 15 29 4 T 3360 24360 60 Sum of T 27780 adjusted H 191.1886
7 6 Number of Brittlestars 5 4 VF F 3 M C 2 1 0 Block1 Block2 Block3 Block 4Figure 6. Results of sediment preference among four sediment grain sizes with each blockshown separately. VF = very fine, F=fine, M= medium, C= coarse.
7 6 Number of Brittlestars 5 Block1 4 Block2 3 Block3 Block 4 2 1 0 VF F M CFigure 7. Results of sediment preference among four sediment grain sizes grouped by grainsizes. VF = very fine, F=fine, M= medium, C= coarse. 50.00% 45.00% 40.00% 35.00% Percentage 30.00% 25.00% 20.00% 15.00% 10.00% 5.00% 0.00% VF V M CFigure 8. Results of sediment preference among four sediment grain sizes withall blocks combined. VF= very fine, F= fine, M= medium, C= coarse.
12 Number of Brittlestars 10 8 VF 6 F 4 2 0 Block1 Block2 Block3 Block 4Figure 9. Brittlestar sediment preference between very fine (VF) and fine (F) sediment size 12 10 8 Block 1 Number of Briittlesars Block 2 6 Block 3 Block 4 4 2 0 VF FFigure 10. Brittlestar sediment preference by sediment grain size—second set of experiments.Blocks are indicated by the different colors.
80.00% 70.00% 60.00% Percentage 50.00% 40.00% 30.00% 20.00% 10.00% 0.00% VF VFigure 11. Brittlestar sediment preference by sediment grain size with blocks combined. VF=very fine, F= fine. 12 10 Number of Brittlestars 8 VF 6 SG B 4 2 0 Block 1 Block 2 Block 3 Block 4Figure 12. Brittlestar preference between seagrass (SG) and very fine (VF) sediment size.
12 10 Number of Brittlestars 8 Block 1 6 Block 2 Block 3 4 Block 4 2 0 VF SG BFigure 13. Brittlestar preference between seagrass (SG) and very fine (VF)-third set ofexperiments. Blocks are indicated by the different colors. 70.00% 60.00% 50.00% Percentage 40.00% 30.00% 20.00% 10.00% 0.00% SG VF BFigure 14. Brittlestar substrate preference between seagrass and very fine grain size with blockscombined. VF= very fine, F= fine.
12 10 Number of Brittlestar 8 VF 6 A B 4 2 0 Block 1 Block 2 Block 3 Block 4Figure 15. Brittlestar preference between artificial seagrass (AG) and very fine (VF) sedimentsize. 12 10 Number of Brittlestars 8 Block 1 6 Block 2 Block 3 4 Block 4 2 0 VF A BFigure16. Brittlestar preference between artificial seagrass (SG) and very fine (VF)-third set ofexperiments. Blocks are indicated by the different colors.
70.00% 60.00% 50.00% Percentage 40.00% 30.00% 20.00% 10.00% 0.00% A VF BFigure 17. Brittlestar substrate preference between artificial seagrass and very fine grain sizewith blocks combined. AF= very fine, VF= very fine.DiscussionAlthough block 1 was statistically different from the other blocks in both experiments, theresults from the experiment indicated that Ophiophragmus filograneus preferred smaller grainsizes, and specifically the very fine grain size in the absence of aquatic vegetation. The reasonfor the difference in brittlestar behavior between block 1 and the subsequent blocks may havebeen a result of handling time during experimental set-up. The process of transferring thebrittlestars from the holding tank to the bowls took longer for block 1 than for any of thesubsequent blocks. This trend was also true for the second set of experiments.In addition, it was taken under consideration that brittlestars tend to be associated withseagrass, (Halodule wrightii). This raises the questions “Do they prefer soft sediment orseagrass substrate? Or do seagrass prefer the soft sediment? Due to the above mention quires,
further research in the laboratory and in the field was undertaken. The results fromexperiments three and four indicated that Ophiophragmus filograneus preferred structuresubstrate, especially in the presence of aquatic vegetation. The reason the third experimentwas performed a second time was due to the position of the seagrass within the bowl and thedifference in brittlestar behavior in block 1. In addition, the variation in block 1 may have beena result of not having sufficient acclimation time. The process of transferring the brittlestarsfrom the holding tank to the bowls took longer for block 1 than the other blocks duringexperimental set-up. The brittlestars were given an additional 24 hours to acclimate insubsequent experiments. As a result there were no statistical differences between the blocks inthe third and fourth experiment. Despite the anomaly of block 1, the overall trend was for brittlestars to prefer thesmaller grain sizes. In addition, the overall trend suggests that brittlestars prefer seagrasssubstrate in the presence of aquatic vegetation. Just as important, brittlestars prefer structuralsubstrate in the absence of aquatic vegetation. This suggests that the hypothesis aboutbrittlestars sediment preference, seagrass and structural substrate were correct.Ophiophragmus filograneus’ patchy distribution in the natural environment may correlate withsediment distribution, seagrass distribution and structural distribution. This remains to beverified in the field. However, It must be taken into consideration that the different sedimentswere clean and major organic compounds were absent. Zimmerman et al 1987 showed thatorganic content of the sediment is an important factor for other burrowing brittlestars.
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