J. Great Lakes Res. 22(l):77-82 Internat. Assoc. Great Lakes Res., 1996

New Records of Freshwater (Porifera) for Southern Lake Michigan

Thomas E. Lauer and Anne Spacie Department of Forestry and Natural Resources Purdue University West Lafayette, Indiana 47907

Abstract. Four species of freshwater were identified in southern Lake Michigan: Spongilla lacustris (Linneaus), Eunapius fragilis (Leidy), fluvi- atilis (Linneaus), and Ephydatia muelleri (Lieberkuhn). Samples were collected from artificial substrates in Calumet (IL), Hammond (IN), and Michigan City (IN) harbors and represent the first reports of sponges in these waters. Numerical densities for all species combined were as high as 698 sponge colonies/m2 on the iron hull of the permanently moored 110-m-long ship Milwaukee Clipper in Hammond Harbor. These densities were lowest at 1-m depth and greater at 2- and 3-m depths. A positive correlation (r2 = 0.74) was found between the density of sponge colonies and the angle of the hull as it changed from near vertical at the water line to overhanging at greater depth (toward the keel). Ephydatia muelleri was the most common species based on frequency of occurrence and percent surface area covered. Observations of epizoic growth of sponges on live zebra mussels (Dreissena polymorpha Pallas) suggest that an ecological interaction exists between these two groups of organisms on these arti- ficial substrates. INDEX WORDS: Freshwater sponges, Lake Michigan, Zebra mussel, Spongilla, Eu- napius, Ephydatia, Porifera.

Introduction The distribution and of freshwater sponges (Porifera: ) in North America has not received widespread attention, although some notable works exist for regions surrounding the Great Lakes (Potts 1887, Smith 1921, Old 1931, Jewell 1935, Neidhoefer 1940, Eshleman 1950, Ricciardi and Reiswig 1993). In northern Wiscon- sin, ten species of sponges were found in 103 of the 127 lakes and 15 of the 17 streams examined (Jewell 1935). Old (1931) found twelve sponge species among the 84 lakes, 42 streams, and 19 ponds he surveyed in Michigan. Ricciardi and Reiswig (1993) exa- mined over 1,500 specimens from eastern Canada and demonstrated that sponges were more diverse than previously indicated. They found fifteen different species and sugg- ested that additional taxa could be identified in this geographical region with further study. Sponges may be more ubiquitous than the available scientific literature indica- tes and the absence or limited ranges of some species may not be due to their true zoo- geographical distribution, but rather a lack of observation (Frost 1991). We have been unable to find published sponge records for Lake Michigan, despite the vast amount of scientific information that has been gathered on this lake. The recent appearance of the zebra mussel, Dreissena polymorpha (Bivalvia: Drei- ssenidae), in the Great Lakes (Hebert et al. 1989) has focused new attention on the sessile benthic communities. Significant ecological changes in the community struc- ture have been forecast because of this introduction (U.S. Office of Technology Asses- sment 1993), but it is unknown how the zebra mussel has affected many of the benthic populations. Garton et al. (1993) predicted zebra mussels could cause a restructuring of benthic communities and suggested that species requiring hard substrates for attachment, such as freshwater sponges, might colonize zebra mussel shells. Without knowing the historical and current distributions of native freshwater sponges, however, it is not possible to identify changes in community composition over time. This study takes an initial step toward identifying and reporting the species of fresh- water sponges and describing their distribution in southern Lake Michigan. An initial measure of sponge abundance is presented as well as relative densities of the species found at Hammond Harbor. Observations on sponge-mussel interactions are included and suggest that an important ecological interaction exists between these two groups.

Methods The study sites were located in the southern end of Lake Michigan at Calumet, IL (Lat. 41°43'20", Long. 87°37'30"), Hammond, IN (Lat. 41°41'40", Long. 87°30'32"), and Michigan City, IN (Lat. 41°43'40", Long. 86°54'35") harbors. Fifteen sponge sam- ples for identification were collected by divers at Hammond Harbor in July and Octo- ber 1994 from the hull of the permanently moored 110-m-long ship Milwaukee Clipp- er, from the adjacent steel revetment wall, and from other artificial hard surfaces locat- ed in the harbor. Eight samples were collected at Michigan City in July 1995 from the revetment wall, associated stone riprap, and dock posts made of both iron and wood. Only two colonies were found at Calumet Harbor in July 1995, at a location on the un- derside of the stone breakwall. Samples from both were collected. Individual speci- mens were scraped using a knife, or hand picked from the substrate. Sponge identifi- cation was based largely on gemmule spicule morphology, and care was taken to col- lect specimens bearing gemmules if possible. Each of the different sponge morphs ob- served was collected, as sponges show a large variability in growth forms. These in- cluded both a flat encrusting variety, a branched variety showing a growth habit per- pendicular to the attachment point, and a variety that was found growing on (covering) zebra mussel valves. All sponges were preserved in 70% ethyl alcohol and returned to the laboratory for analysis. Laboratory preparation of the megascleres, microscleres, and gemmoscleres was beg- un by boiling small portions of appropriate sponge tissue in nitric acid to separate the siliceous spicules from the remainder of the sponge tissue. After three washings in water and three washings in 100% ethyl alcohol, spicules were drawn up in a pipette and placed on a glass slide. The alcohol was burned off and the spicules mounted in Permount. This technique created a permanent mount and allowed for the microscopic examination of the spicules as described by Pennak (1989). Identification was then made using the taxonomic keys of Pennak (1989), Frost (1991), and Ricciardi and Reiswig (1993). Permanent slides of the specimens have been archived at Purdue University, Department of Forestry and Natural Resources. Field counts of sponge colonies at Hammond Harbor were performed during 21-28 July 1994 using a 0.25 m2 quadrat sampler divided into 0.01 m2 sections. Stations used for this analysis were located along three parallel transects on the starboard side of the Milwaukee Clipper hull, one each at the 1-, 2-, and 3-m depths. A maximum harbor depth in the area of 4 m precluded sampling deeper. Each transect line began at the bow and extended 100 m toward the stern. Eleven individual sample stations were equally spread along each transect, making a total of 33 stations for the three depths. Sponge colony counts were made at each station location by enumerating all sponge colonies larger than 1 mm (minimum limit of detection) within the 0.25 m2 quadrat. A single count was recorded for each sponge colony observed. Identification of the sponge colonies by species during field enumeration was not possible. Several of the largest sponges were measured to obtain the maximum size (longest axis) found in the study area. Comparison of the sponge densities among sample stations was performed using an analysis of covariance model (Montgomery 1991) using SAS programming (Lit-tell et al. 1991). Individual colonies were identified to species and evaluated to determine relative areal coverage. Twenty-three sample areas were photographed using a Nikonos V camera with a 35-mm lens, 3:1 extension tube, and focusing frame. The surface area sampled by each photo was 76.7 cm2. Sample locations used for this analysis included the hull of the Milwaukee Clipper and the associated revetment wall at depths from 0.5 to 3.5 m. Quadrats for analysis were chosen by divers with the condition that the area conta- ined at least 10% coverage by sponges. Although this nonrandom method does not provide an unbiased estimate of total population size, it does provide a measure of relative abundance of the three species found at Hammond. Slide transparencies of the samples were later projected on white paper and the sponge and zebra mussel images traced and calculated. Sponges were identified to species when possible by comparing sponge morphology shown in the slide transparencies to the samples obtained for laboratory analysis of the spicules described above. It was not possible to conclusively identify all sponges or other organisms in the transparencies using this method, parti- cularly the small colonies. In such cases the organisms were labeled as "unidentified." The hull angle of the Milwaukee Clipper was taken at each sample location by divers using a protractor with a weighted string attached. The vertical hull surface near the waterline represented an angle of 180° downward. As depth increased and the boat hull curved under, readings increased, approaching 225° in the most rounded area near the stern.

Results and Discussion Four species of freshwater sponges were identified in the study: Spongilla lacustris (Linneaus), Eunapius fragilis (Leidy), Ephydatia fluviatilis (Linneaus), and Ephydatia muelleri (Lieberkuhn). This record represents the first published account of these species in Indiana and Lake Michigan, although they have been previously reported from streams and smaller lakes in Wisconsin, Michigan, and Illinois (Old 1931, Jewell 1935, Smith 1921). Three species were found at Hammond (S. lacustris, E. fragilis, and E. muelleri), three at Michigan City (S. lacustris, E. fluviatilis, and E. fragilis), and one at Calumet (E. fragilis) harbors. The freshwater sponges found within Hammond Harbor were growing in close proximity, and in some cases touching or overlapping each other. This association among S. lacustris, E. fragilis, and E. muelleri has been commonly reported in North America (Old 1931, Jewell 1935, Ricciardi and Rieswig 1992). Individual colonies ranged from barely discernible to greater than 20 cm in diameter for E. muelleri and E. fragilis. Branching colonies of S. lacustris extended perpendicularly out from their attachment point as much as 25 cm. Sponges in Hammond Harbor were located principally on the revetment wall, the underside of the dock structures, and the hull of the Milwaukee Clipper. A small number of sponges were found on the rocky riprap used in the harbor construction. In most cases, sponges were attached to vertical or overhanging hard surfaces, particu- larly the steel boat hull and revetment. In some cases, sponges were growing on the zebra mussels that were located on these same vertical or near vertical surfaces. This epizoic association was also observed for E. fragilis in the St. Lawrence River and was thought to be a strategy in the competition with other sessile benthic organisms for limited substrate (Ricciardi and Reiswig 1993). Sponges at Michigan City were found on the same types of harbor structures as at Hammond. The sponges at Calumet Har- bor were found on the underside of a concrete slab associated with a retaining wall. Most sponges were growing below the 1 m depth, but in late summer colonies were observed within 0.25 m of the surface. Wave action and ice scour can limit sponge growth in the shallow water of lakes (Bader 1984). The bottom substrate of both Hammond and Michigan City harbors was primarily silt, and only 5. lacustris was found growing sparsely on this material. Both S. lacustris and E. muelleri have been observed growing in soft sediment in other locations (Frost 1991).

Numerical densities of sponge colonies growing on the hull of the Milwaukee Clipper at Hammond varied with depth and hull angle (Table 1). An analysis of covariance model using density of sponge colonies (log transformed) as the response variable, depth as the class variable, and angle of the hull as the covariant was used to examine differences. As the angle of the hull changed from vertical to overhanging as depth increased, the density of the sponges increased (p < 0.0001, r2 = 0.74). In addition, depth had an effect on the density of sponges at a given hull angle (p < 0.0001), with the 1-m densities being less than the 2- or 3-m densities. These results identify two physical factors that may be controlling sponge densities in Hammond Harbor, and provide suggestions for future investigations. The size of the sponge colonies counted in the 0.25 m2 sampling frame varied in size from (the minimally detectable size of) 0.1 cm to > 20 cm along their longest axis. Sponges may increase in size and eventually overlap or grow together forming larger colonies, but this reduces the apparent number of distinct colonies (Sara 1970). Therefore, colony counts can be misleading. A different method, using percent area coverage of the substrate, provides an alternative measure of sponge densities based on a definitive evaluation of size. This latter technique may also be useful in surveying sponge populations (A. Ricciardi, pers. comm., McGill University, 1995). The three sponge species found at Hammond Harbor were evaluated in an area of high sponge densities. The quadrat samples, although biased in their portrayal of sponge coverage in the entire harbor, provided a measure of relative abundance among the three sponge species and the zebra mussel (Table 2). Percent coverage was reported only for the visible organism growing on the substrate, e.g., if a sponge species was found completely overgrowing a zebra mussel, then the sponge coverage was measured, with no account given for the underlying zebra mussel. Ephydatia muelleri was the most abundant sponge found, occurring in 57% of the quadrats with a maxi- mum areal coverage of 94%. All three species of sponge were common, however, and each was found in at least 10 of the 23 samples taken. Figure 1 illustrates the close association between sponges and zebra mussels in an area on the revetment wall.

We have not yet examined direct ecological interactions between the sponges and zeb- ra mussels at these sites. Growth of sponges on shells of live zebra mussels, however, was commonly observed at Hammond and Michigan City harbors. Ricciardi et al. (in press) suggest that sponge overgrowth on zebra mussels may smother mussel siphons, interfere with normal feeding and respiration, and create sponge-enhanced mussel mortality. Both zebra mussels and sponges have a strong preference for solid sub- strates and are sessile benthic filter-feeders (Frost 1991; Stanczykowska and Lewan- dowski 1980, 1993; Marsden et al. 1993; Sprung 1993). The large coverage by spon- ges and zebra mussels in selected areas that we observed suggests both are major com- ponents of this community. Focusing future ecological research on these two groups may provide additional biological information on this interrelationship, and clarify the relative importance of biotic and abiotic factors controlling growth of freshwater spon- ges and zebra mussels.

Acknowledgments

The authors wish to thank Mr. Timothy Early and his staff of the Aquatic Resources Center, Hammond IN for the field support, Mr. Anthony Ricciardi, McGill University, for confirmation of the sponge identifications and Dr. J. Ellen Marsden, Illinois Natu- ral History Survey, and two anonymous reviewers for providing valuable comments on the manuscript. This research was funded in part by the Illinois-Indiana Sea Grant Program, Small Grant Initiative #94-124. This is Journal Paper No. 14793 from the Agricultural Research Programs, Purdue University.