Food choices and values for a benthic herbivore, Idotea wosnesenskii Shelby Burgess 1 , Morgan E. Eisenlord 2 , Aaron W. E. Galloway 2,3 , Megan N. Dethier 2; 1 University of Michigan, 2 University of Washington (UW) Friday Harbor Laboratories, 3 UW School of Aquatic and Fishery Sciences Background: Spatial subsidies of aged detrital seaweed from habitats of high primary production may provide a significant source of energy to adjacent food webs 3,4 (Figure 1). As aging occurs, algae are thought to increase in food value due to bacterial colonization 2,3,4 . Aged kelp may be more palatable due to increased nitrogen content, breakdown of secondary metabolites (e.g., phlorotannins), and decreased thallus thickness due to natural degradation 2,3,4 . Previous studies have shown preference for aged detrital material 5 . Nereocystis luetkeana and Agarum fimbriatum are common kelp (Phaeophyceae, Laminariales) species of the Pacific Northwest. Nereocystis has lower phlorotannin content than Agarum and studies have shown that it is often preferred by herbivores 2,3,6 . In this study we investigate preferences and food values of differing macroalgal diets to a benthic herbivore, Idotea wosnesenskii. We asked two primary questions: 1) Are there differences between algal species? and 2) is there a difference between fresh and aged algae? Methods: Adult Preference Trial • We collected Idotea, Agarum and Nereocystis from San Juan Island, WA. Samples were taken from blades cut 20cm from the stipe. ‘Aged’ diets were aged in a dark flow-through seawater tank for 14-17 d. • Two males and two females were distributed to each of 20 containers. Idotea were fed for 24-120 hours or till there was noticeable biomass change of the samples. • Treatments in included: Fresh Agarum versus fresh Nereocystis; fresh Agarum versus aged Agarum; fresh Nereocystis versus aged Nereocystis; aged Agarum versus aged Nereocystis. • We used 10 control containers to measure and correct for biomass loss due to natural degradation. T represents the feeding treatment, C represents the Control (averaged over 5 replicates), subscript i represents initial mass, and subscript f represents final mass = T i *(C f /C i )-T f Juvenile Growth Trial: • Juvenile Idotea (mean length=3.07±0.11 mm) were removed from the brood pouch of brooding females, distributed to 3 replicate feeding containers per treatment, and raised for 10 weeks in filtered seawater in a temperature controlled room. • Diets were provided ad libitum and water replaced every 2-3 days. Growth rates were determined by measuring from the tip of the head to the tip of the uropod. Two experiments were conducted at two different temperatures. 1) Temp=8°C, n=41 Idotea per replicate. 3 treatments: Ulva sp., fresh Nereocystis luetkeana, and aged Nereocystis. Kelp blades were aged in the dark for 14-21 days. 2) Temp=14.5°C, n=105 Idotea per replicate. 4 treatments: Ulva spp., fresh Nereocystis, Fucus gardneri, and Mazzaella splendens. Figure 2. There was more Nereocystis consumed per day than Agarum across all treatments regardless of the pairing. Figure 4. Consumption of fresh Agarum versus Nereocystis by Idotea corrected for biomass loss of controls. There was significantly more Nereocystis consumed (t-test, p<0.01). Figure 3. Consumption of aged versus fresh Nereocystis by Idotea corrected for the biomass change of the controls. More fresh biomass was consumed than aged (t-test, p=0.01), but there was significant biomass loss of aged algae in the controls (t-test, p=0.01). Results: Adult Preference Trial: • Nereocystis consumption (fresh or aged) was always higher than Agarum of any age (Figure 2 and 4) • There was significantly more fresh Nereocystis consumed than aged Nereocystis (Figure 3) Juvenile Growth Trial: • Significant difference in growth rates were found on different diets. Highest growth rates were on Ulva spp., fresh Nereocystis, and aged Nereocystis. • Animals grew slower on diets with chemical (Fucus gardneri) and structural (Mazzaella splendens) anti- herbivore defenses. • Temperature did not significantly affect growth rates (Figure 5) Conclusions: • Our results indicate that the importance of algae in providing useful subsidies to benthic grazers may depend strongly on species 1 • There was a significantly higher growth rate for juvenile Idotea and significant preference by adult Idotea for fresh Nereocystis, this suggests that aged algae do not increase in nutritional value 1,6 • We would expect Idotea to consume the most abundant kelp species, but this was not shown in either the preference treatments or the juvenile growth trials • Agarum may play a smaller role as a subsidy than its biomass in the ecosystem would suggest 6 • Kelp species high in polyphenolic content or of low nutrient content may not only not be of low preference, such as Agarum, but also may negatively impact growth, such as is seen in Fucus 2,3,4 Abstract: Spatial subsidies of aged detrital seaweed from habitats of high primary production may provide a significant source of energy to adjacent food webs. Previous studies indicate nearshore consumers use aged algal material as a food source. As aging occurs, algae are thought to increase in food value due to bacterial colonization. To test this, two experiments were conducted. The first examined preference of aged versus fresh thalli of two different kelp species, Nereocystis luetkeana and Agarum fimbriatum, in laboratory feeding experiments. Adults of Idotea wosnesenskii, an intertidal isopod common to the Pacific Northwest, were given four treatments of aged and fresh kelp of both species. Significantly more Nereocystis was consumed than Agarum, but contrary to expectations, there were no significant differences in consumption of fresh versus aged tissue for either species. The second experiment was a 10 week long feeding trial with newly hatched Idotea to determine growth rates on five different diets: aged Nereocystis, fresh Nereocystis, and fresh Ulva sp., Fucus gardneri, and Mazzaella splendens. Diets of algae with anti-herbivore defenses, one chemical (Fucus) and one mechanical (Mazzaella), resulted in significantly lower growth rates than algae without these defenses. There was also a significant difference in growth rates between aged and fresh Nereocystis. Our results suggest the species of algae may be more important in providing useful subsidies to benthic grazers than the degree of aging. The effects of aging on the nutritional value of algal blades needs further investigation. Funding: NSF REU-Blinks-Beacon to S.B.; Mary Gates Foundation Funding to MEE; NSF OACIS-GK12 Teaching Fellowship (DGE-0742559) to AWEG and NSF (BIO-OCE 0925718) support to MND and AWEG Acknowledgments: Dr. David Duggins, Dr. Sophie George, Alex Lowe, Spencer Roberts, Wendel Raymond, and UW’s Friday Harbor Labs References: 1 Bell, T.M. and E.E. Sotka. 2012. Local adaptation in adult feeding preference and juvenile performance in the generalist herbivore Idotea balthica. Oecologia. 170:383-393 2 Chenelot, H. and B. Konar. 2007. Lacuna vincta (Mollusca, Neotaenioglossa) herbivory on juvenile and adult Nereocystis luetkeana (Heterokontophyta, Laminariales). Hydrobiologia 583:107-118. 3 Duggins, D. O. and J. E. Eckman. 1997. Is kelp detritus a good food for suspension feeders? Effects of kelp species, age and secondary metabolites. Marine Biology 128(3):489-495. 4 Mann, K. H. 1988. Production and use of detritus in various freshwater, estuarine, and coastal marine environments. Limnology and Oceanography 33 (4):910-930. 5 Pennings, S. C., T. H. Carefoot, M. Zimmer, J. P. Danko, and A. Ziegler. 2000. Feeding preferences of supralittoral isopods and amphipods. Canadian Journal of Zoology-Revue Canadienne De Zoologie 78:1918-1929. 6 Sosik, E.A. 2012. Trophic effects of decomposition on detrital foodwebs: an isotopic perspective. Dissertation. University of Washington. In review. 75 85 90 77 209 177 80 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Ulva spp. Nereocystis fresh Nereocystis aged Fucus Mazzaella Mean Growth (mm) Diet 8 C 14.5 C 8°C 14.5°C Ulva sp. Nereocystis Fresh Nereocystis aged Nereocystis aged Nereocystis Aged Fucus Mazzaella Figure 1. Spatial subsidies in coastal ecosystems Figure 5. Mean growth rates for experiments 1 and 2. ANOVAs were run at each temperature separately; all treatments in each experiment were significantly different from each other at p < 0.02. Error bars report standard error; n for each treatment reported at top of column. ©2004 FHL Marine Invertebrate Zoology Course ©Dean Janiak 20 20 10 10 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Agarum Nereocystis Algal Mass Consumed (g) Treatment Control 20 20 20 20 20 20 20 20 -5 0 5 10 15 20 25 30 35 Agarum Agarum Agarum Nereocystis Nereocystis Nereocystis Agarum Nereocystis Algal mass consumed (g) 20 20 10 10 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Algal Mass Consumed (g) Treatment Control Fresh Aged Fresh Fresh Fresh Aged Aged Aged Treatment 1 Treatment 2 Treatment 3 Treatment 4 Fresh Fresh Nereocystis Nereocystis Fresh Aged View publication stats View publication stats