Trophic spectrum of corymorphid hydroid polyps in the Bahía Blanca estuary (Southwestern Atlantic Ocean)

DUTTO, M. SOFÍA; NAHUELHUAL, EUGENIA; CARCEDO, M. CECILIA; GARCIA, MAXIMILIANO D.; CONTE, ALBERTO F.; GENZANO, GABRIEL N.; HOFFMEYER, MÓNICA S.

Camboriú

Congreso; XVII Congreso Latinoamericano de Ciencias del Mar; 2017

INTRODUCION Hydroids play important roles in bentho-pelagiccoupling processes in shallow marine ecosystems since their great abundances and the high predation impact on a wide food spectrum, from detritus and benthic microplankton to egg and fish larvae (GILI and HUGHES 1995; OREJAS et al. 2013). The ecological significance of these benthic stages was particularly proved for colonial hydroid polyps (COMA et al. 1995; OREJAS et al. 2000; OREJAS et al. 2001; GENZANO 2005;GILI et al. 2008; OREJAS et al. 2013). Comparatively, trophic knowledge for solitary polyps is scarce and practically no data exists on the natural feeding of Corymorphiidae polyps. The record in nature of these kind of polyps comprises a rare event, sometimes fortuitous (e.g. WATSON 2008; VERVOORT 2009; GENZANO et al. 2009). Numerous polyps of Corymorpha januarii, however, have been observed in muddy-bottom channels of the Bahía Blanca Estuary in Argentina during late spring by local fisherman for more than 20 years. Our work aimed at describing a population of the conspicuous and solitary C. januarii polyp providing one of the first approaches to its feeding ecology.To this end, our objectives were to describe the zonation and the main features of the population and its individuals, and to analyze the diet and the trophic preference of C. januarii polyps.METHODS: Polyps density was estimated at random using quadrants of 1 m2 (n= 7). The accompanying fauna was taken by sediment samples (n= 4) with plastic cores. Core samples were softly sieved through mesh bags (1 mm aperture size) to separate the fauna from the sediment and retained organisms were fixed in 4% Formaldehyde-seawater solution. Additional samples were also collected with the same core for sedimentological analysis (n= 4). Polyps were collected by handfor biometric and gastric analyses (n= 55) and fixed in 4% formaldehyde-seawater solution. The biometrical variables: size (length and width in cm), length (cm) and number of aboral tentacles, and length (cm) of oral ones of each polyp were recorded. Wet and dry weight (g) were obtained by using an analytical balance and oven at 60ºC for 24h. Gastrovascular content of each polyp was then carefully removed and, following Coma et al. (1995), the type of trophic item(zooplankton and/or particulate organic matter -POM-), the number, and the size of each item were recorded. Planktonic potential preys were sampled by tows using nets of 250 µm and 500 µm mesh and 30 and 70 cm of diameter, respectively. An HydroBios® flowmeter was used to estimate the filtered volume of seawater. These samples were preserved and each potential prey was counted (ind m-3) and identified to the lowest possible taxonomic level. Ivlev´s (1961) indexof electivity [E= (r-p)/(r+p), where r= frequency of a given prey item in the diet; p= its frequency in the natural prey population] was used to estimate prey selectivity.RESULTS AND DISCUSSION: The substrate of the channel in which polyps were rooted was composed by 11.8% of sand and 88.2% of mud (79.6% silt and 8.6 clay) and a high content of organic matter (4.97 ± 0.1%). The octocoral Stylatula sp. and diverse polychaetes dominated the accompanying fauna. The C. januarii population observed along an area of 250 m long and 3 m wide was composed by an estimated number of 31,500 polyps (mean of 42 ± 5 polyps m-2). In accordance with observations by Da Silveira & Migotto (1992), most of the polyps were lying on the mud, completely exposed to the sun and air during almost the whole ebb tide. Some polyps were observed in an intertidal zone in contact with the water and a few were accidentally captured from the bottom of the channel. Polyps were visible from the motorboat, although quite smaller than those found in Brazil(DA SILVEIRA and MIGOTTO 1992), and their apical part was yellow to purple. Corymorpha januarii polyps´ total length (hydrocaulus + hydrant) varied from 2.34 to 9.75 cm and the width of the hydrant ranged from 0.40 to 1.10 cm, the mean length and number of aboral tentacles were 2.13 cm and 32 respectively, whereas the mean length (cm) of oral tentacles was 0.35 cm. The 91% of the population showed a polyp size of 5 cm or less. The mean wet weight and mean dryweight of polyps were of 13.67 ± 0.44 g and 0.03 ± 0.02 g, respectively; being water ~78% of the body mass. Potential preys were represented by 34 taxa. The dominant taxa was the mysid Arthromysis magellanica (1,387.63 ind m-3, 47% of total taxa), followed by the copepod Acartia tonsa (630.57 ind m-3, 21%). Cladocerans contributed importantly (14%) to the zooplankton community, particularly Bosmina longirostris which accounted for 11% of total taxa (335.15 ind m3). Polyps are mainly known as carnivorous organisms but their diets may be diverse (GILI et al. 1997, OREJAS et al. 2013). Fifteen types of trophic items were found in the gastrovascular contents of the polyps analyzed and the 88% of them contained at least one trophic item in their gastrovascular cavities. The most frequent trophic items consumed were POM (65.45% of total polyps), the copepods harpacticoids, being Microarthridion aff. littorale the most consumed(56.36%), the copepod Acartia tonsa (43.64%) and the mysid Neomysis americana (40%). All these items were positively selected along with cypris of Balanus glandula and eggs of invertebrates and fish (E index). Logically, for those items which were found in the gastrovascular cavities but did not in the environment, extreme negative selectivity values (-1) were recorded.Fish larvae and the mysid A. magellanica were the only trophic items which having been recorded in the environment, were avoided by the polyps. Benthic suspension feeders in shallow marine ecosystems are responsible for a large proportion of the energy flow from the plankton to the benthos. Besides plankton, other types of trophic items, such as bacteria, fecal pellets and vegetal and animal derived-detritus may be expected to be exploited by polyps (GILI and HUGHES 1995). This may be particularly true in highly turbid and eutrophic ecosystems with intense resuspension processes which show great abundance of POM available as a potential food resource for filter feeders (GILI and COMA 1998).CONCLUSION: In the Bahía Blanca Estuary (Argentina), Corymorpha januarii polyps seem to be omnivorous, feeding on particulate organic matter and zooplanktonic organisms, mainly copepods. Benthic suspension feeders in shallow ecosystems are responsible for large proportion of the energy flow. Due to their considerable size and their year after-year record in such a turbid and naturally eutrophic environment as the Bahía Blanca Estuary, Corymorpha januarii polyps may have a significant predation impact thus playing an important role in the estuarine benthic-pelagic coupling