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Larval Polychaetes Are Strongly Associated with Marine Snow

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Larval Polychaetes Are Strongly Associated with Marine Snow MARINE ECOLOGY PROGRESS SERIES Vol. 154: 211-221, 1997 Published July 31 Mar Ecol Prog Ser Larval polychaetes are strongly associated with marine snow Alan L. shanksl1*,Kimberly A. del carmen2 'Oregon Institute of Marine Biology. University of Oregon, PO Box 5389. Charleston, Oregon 97420, USA 'pacific Biomedical Research Center, Kewalo Marine Laboratory, University of Hawaii. 41 Ahui St., Honolulu. Hawaii 96813, USA ABSTRACT. The assoc~atlonof larval polychaetes with manne snow was investigated (l)w~th SCUBA to sample marine snow In the f~eldand (2) in laboratory experiments. The field sampling took place in the Atlantic Ocean off Charleston, South Carolina, USA 13 sample dates) and in the Pacific Ocean at 2 locations around the San Juan Islands, Washington, US:\ (7 sample dates). On all of the sample dates marine snow was present and abundant (rmge 1 to 63 agg. I-'). Larval polychaetes were signif~cantly concentrated on aggregates on 7 of the 10 sample dates. Larval polychaetes from 12 families \yere pre- sent in the plankton samples of which 10 families were found associated with aggregates. On average. 16% (SD = 22%) of all larval polychaetes were on aggregates. Precompetent and competent larval polychaetes made up 84 and 16% respectively of the larval polychaetes in the plankton. On average 20% of all precompetent polychaete larvae were on aggregates. In contrast, competent polychaete lar- vae were strongly associated with aggregates with 80":) of the competent larval polychaetes on aggre- gates. Laboratory experiments using 4 polychaete species and laboratory-made marine snow also found that larval polychaetes were concentrated on aggregates In d vel tical flume, observations were made on the behavior of larval polychaetes following contact with aggregates. Upon contacting manne snow, larval polychaetes crawled into or over the surface of aggregates for several minutes before swimming away. Thesc field and laboratory observations suggest that a precompetent polychaete larva m~ghtspend about 5 h d.! vlsiting aggregates and that during a day ~t may vis~tabout 90 aggregates. C0mpeten.t larval polychaetes may spend >l9 h d.' on aggregates. Polychaete larvae visiting or resid- ing on aggregates may be feedlng on the aggregate microbial community. Further, the vertical flux and, perhaps, benthic deposition of larvae residing in aggregates is due to the sinking of the marine snow. Clearly, marine snow is an important component of the environment for many types of poly- chaete ldrvae and adaptations to life In aggregates nld): have played a role in the evolution of poly- chaete larval traits. KEY WORDS: Marine snow Polychaetes Larvae Aggregates . Meroplankton Competent . Pre- competent. Settlement INTRODUCTION sampling bottles which 'gra'b' a volume of water, or with plankton nets which filter a larger volume of In a liter of fresh- or seawater the planktonic organ- water and concentrate the trapped organisms into a isms are generally assumed to be randomly distributed collection bucket. Consequently, any fine scale biolog- in space; that is, we assume that biological or physical ical or physical structure in the environment is structure in these environments do not impose fine destroyed during sampling; the sampling technique scale order on the distribution of organisms. This tradi- dictates the world view of the biologist. tional view of the planktonic world is perhaps an arti- Shortly after people began to enter the pelagic realm fact of the sampling techniques researchers have in submersibles (Tsulita 1952, Suzuki & Kato 1953) or historically used. Plankton are collected with water as divers (Hamner et al. 1975) it became apparent that plankton are not necessarily randomly distributed in the water column. For example, using SCUBA, Omori & Hamner (1982) observed dense aggregations of a O Inter-Research 1997 Resale of full artlcle not permitted 212 Mar Ecol Prog Ser variety of zooplanktors, aggregations which were 100 studies. This allowed for a more detailed analysis of the to 1000 times more concentrated than the average pop- taxonomic composition of the larvae in aggregates and ulation density as indicated by a plankton tow. Further, their relative sta.ge of development. in all marine and freshwater environments in which researchers have looked, they have found macroscopic aggregations of detritus and microbes (e.g. marine METHODS snow, lake snow or aggregates), which provide a struc- tured benthic-like micro-habitat in the water column Field sampling. Field sampling of marine snow took (Alldredge & Silver 1988, Kiarboe et al. 1990, Grossart place in the waters of the Atlantic and the Pacific & Simon 1993, Kiarboe 1996). The concentrations of Oceans. Samples were collected on 3 dates between detritus and microbes within aggregates are generally August and November of 1991 in the Atlantic Ocean much higher than concentrations in the surrounding approximately 3 km offshore of Charleston, South Car- aggregate-free water (Alldredge & Silver 1988). olina, USA (31°43'N, 78O43'W). In the Pacific Ocean Marine snow is not only ubiquitous in the ocean and samples were collected on 5 dates between July and has been observed in at least 1 lake, it is often quite August 1992 in Parks Bay, Shaw Island (48"33'N, abundant. In estuarine waters concentrations of 122" 59' W), and on 2 dates in August 1992 in West marine snow can be >l00 1-' (Wells & Shanks 1987), Sound, Orcas Island (48" 37' N, 122'58' W). Shaw and whi1.e in coastal waters concentrations are usually Orcas Islands are located in the San Juan Islands, around 1 to 10 I-' (Alldredge & Silver 1988). In Lake Washing ion, USA. Constance (Germany) aggregate concentrations were From 4 to 6 replicate marine snow samples contain- generally on the order of 10 I-' (Grossart & Simon ing 30 to 200 aggregates >l mm in longest dimension 1993). In the ocean, many benthic invertebrates pro- were collected from between 3 and 5 m depth by duce larvae which spend days tn month< in the p!ank- SCUSA divcrs using the teclinique ul' Shanks & ton. Given the abundance of marine snow it seems Edmondson (1990). Aggregates were drawn into a 1 m1 inevitable that the larvae of benthic invertebrates will sampling syringe and transferred via a connected contact aggregates. What happens when a larva con- T-valve into a 50 m1 reservoir syringe. With this sam- tacts an aggregate? Do larvae swim away following pling technique 100 aggregates can be concentrated in contact or do they remain with the aggregate exploit- <25 m1 of water. In the collection procedure, the sam- ing it as a food source or habitat? pling syringe was mounted on a frame with an under- Shanks & Edmondson (1990) made the first observa- water flashlight to aid visualization of the aggregates. tions on the association of meroplankton with aggre- The beam from the flashlight illuminated the water in gates. They made observations on 4 dates at Cape front of the intake for the sample syringe. Due to the Lookout Bight, North Carolina, USA. They found that light reflected off of the aggregates, even the clear 9 and 74 % respectively of the precompetent and com- mucus matrix of the marine snow was visible. The petent larval polychaetes were associated with aggre- diver swam to the up-current end of their tether and gates. On average 80% of the vertical flux of compe- sampled as the current carried them to the down- tent larvae through the water column was due to stream end of their tether. Sampling was haphazard; larvae associated with aggregates. In a later study, the diver sampled aggregates in sequence as they Bochdansky & Herndl (1992) found larval polychaetes passed through the beam of the flashlight. associated with aggregates sampled in the northern On 20 August 1992 in West Sound it appeared to the Adriatic Sea. Their observations further suggested that divers that there were 2 size classes of aggregates. Small larval polychaetes may have been feeding on the detri- aggregates tended to be between 1 and 5 cm in longest tus and microbes in aggregates. Most recently, Green dimension and large aggregates were > 10 cm in longest & Dagg (1997) found larval polychaetes to be associ- dunension (see Table 2). Divers collected separate repli- ated with marine snow collected in the Gulf of Mexico. cate samples of 'small' (100 aggregates sample-') and In this study we further investigate the association of 'large' (30 aggregates sample-') aggregates. larval polychaetes with marine snow. The previous Plankton samples (i.e. marine snow and the water studies took place in 3 locations and one might assume surrounding marine snow) were collected in the that the association of larval polychaetes with aggre- Atlantic Ocean sam.ples using a submersible bilge gates is a phenomena peculiar to these locations. By pump. Water (37.5 1) was pumped from the depth at sampling additional locations and in a new ocean, we which the marine snow was sampled and filtered greatly expanded the conditions under which one can through a 53 pm mesh plankton net. In the Pac~fic expect to find polychaete larvae associated with Ocean plankton samples were collected with a 25 cm marine snow. The number of aggregates collected per diameter 53 pm mesh plankton net. The net was towed sample was larger in this study than in the previous vertically from 6 m to the surface (approximately 300 1 Shanks & del Carmen Lar\id1 polychaetes on marlne snow 213 filtered). All samples were stained with Rose Bengal Table 1 Estimated mlnlmum numbel- of setigel-S present at and preserved in 3 % Formalin. competency In larvae from 10 of the 12 polychaete families The size and concentration of marine snow was sampled dur~ngth~s study determined using the photographic technique devel- oped by Honjo et al. (1984).About 30 photographs of a Setigers at Source competency 13.2 X 9.2 X 5 cm volume of water (0.607 1) were taken at depths between 3 and 5 m.
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