New Records of Freshwater Sponges (Porifera) for Southern Lake Michigan

J. Great Lakes Res. 22(l):77-82 Internat. Assoc. Great Lakes Res., 1996 New Records of Freshwater Sponges (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 sponge were identified in southern Lake Michigan: Spongilla lacustris (Linneaus), Eunapius fragilis (Leidy), Ephydatia fluviatilis (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 artificial substrates. INDEX WORDS: Freshwater sponges, Lake Michigan, Zebra mussel, Spongilla, Eu- napius, Ephydatia, Porifera. Introduction The distribution and taxonomy of freshwater sponges (Porifera: Spongillidae) 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) examined over 1,500 specimens from eastern Canada and demonstrated that sponges were more diverse than previously indicated. They found fifteen different species and suggested 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 freshwater 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 samples 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 located 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 specimens were scraped using a knife, or hand picked from the substrate. Sponge identification 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 observed was collected, as sponges show a large variability in growth forms. These included 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).

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