We examined seasonal reproductive patterns for the terrestrial isopod Hyloniscus riparius in Wisconsin. Samples collected from habitats in three Wisconsin regions were compared with regard to the reproductive status of population members. We also recorded differences in sex ratio and stage of female gravidity in all samples. Males were a minority in all samples. Southern samples showed the greatest reproductive capacity based on the proportion of females that were found to be gravid

Jass, Joan

Klausmeier, Barbara

English

Valparaiso University

The Great Lakes Entomologist Volume 36 Numbers 1 & 2 - Spring/Summer 2003 Numbers 1 & 2 - Spring/Summer 2003 Article 11 April 2003 The Terrestrial Isopod Hyloniscus Riparius (Isopoda: Oniscidea: Trichoniscidae) in Wisconsin Joan Jass Milwaukee Public Museum Barbara Klausmeier Milwaukee Public Museum Follow this and additional works at: https://scholar.valpo.edu/tgle Part of the Entomology Commons Recommended Citation Jass, Joan and Klausmeier, Barbara 2003. "The Terrestrial Isopod Hyloniscus Riparius (Isopoda: Oniscidea: Trichoniscidae) in Wisconsin," The Great Lakes Entomologist, vol 36 (1) Available at: https://scholar.valpo.edu/tgle/vol36/iss1/11 This Peer-Review Article is brought to you for free and open access by the Department of Biology at ValpoScholar. It has been accepted for inclusion in The Great Lakes Entomologist by an authorized administrator of ValpoScholar. For more information, please contact a ValpoScholar staff member at scholar@valpo.edu. 70 THE GREAT LAKES ENTOMOLOGIST Vol. 36, Nos. 1 & 2THE TERRESTRIAL ISOPOD HYLONISCUS RIPARIUS(ISOPODA: ONISCIDEA: TRICHONISCIDAE) IN WISCONSINJoan Jass1 and Barbara Klausmeier1ABSTRACTWe examined seasonal reproductive patterns for the terrestrial isopodHyloniscus riparius in Wisconsin. Samples collected from habitats in three Wis-consin regions were compared with regard to the reproductive status of popula-tion members. We also recorded differences in sex ratio and stage of femalegravidity in all samples. Males were a minority in all samples. Southern samplesshowed the greatest reproductive capacity based on the proportion of femalesthat were found to be gravid.____________________Hyloniscus riparius (Koch 1838), a European species not native to theNearctic, has been reported from Michigan, Newfoundland, New York, NorthCarolina and Pennsylvania (Jass and Klausmeier 2000), as well as Wisconsin.The first report of H. riparius from Wisconsin was from a survey of the terres-trial isopods at the University of Wisconsin-Milwaukee Field Station in OzaukeeCounty (Jass and Klausmeier 1987). In the Field Station Upland Woods, H.riparius was found in association with Trachelipus rathkii (Brandt 1833). Alongthe boardwalk in the adjacent Cedarburg Bog, it was found with T. rathkii andTrichoniscus pusillus Brandt 1833.Our collections of H. riparius from Wisconsin between 1987 and 1998showed that its habitats can be grouped into three categories: shorelines, dampwoodland soils, and quarries. Common to all Wisconsin collection sites for H.riparius is the high moisture content of the ground. Hyloniscus riparius has beenfound in damp sites such as lakeshores and river banks, and in damp micro-habitats such as the water seeps from rocks in a quarry.This isopod quickly desiccates and expires when removed from its habitatand placed in an empty collecting container. Two factors account for this suscep-tibility in H. riparius. One is its exceptionally small size and the other is that itlacks pseudotracheae (“humidity chambers” for oxygen exchange) on its ab-dominal pleopods, which in other terrestrial isopods act to conserve body mois-ture. Thus it is more strictly confined to moist habitats than some other terres-trial isopod species.Few studies exist documenting the seasonal cycles of terrestrial isopodsin North America. Hatchett (1947) studied the life histories of four Michiganspecies under laboratory conditions and also recorded size frequencies, sex ra-tios and gravidity for a population of one of them, Cylisticus convexus (DeGeer1778), in the field from June through August. In Wisconsin, we have previouslyconducted field studies of the seasonal cycles for populations of Porcelliospinicornis Say 1818 (Jass et al. 1991) and T. rathkii (Jass and Klausmeier1996). For terrestrial isopods in temperate climates, the majority of individu-als have been found to live slightly longer than one year (as in McQueen’s 1976combined field and laboratory study of P. spinicornis in Ontario), with youngadults from the prior season mating and giving birth to the next generationduring their second summer. In this study we collected samples of H. riparius inorder to describe its seasonal cycle in Wisconsin.1 Invertebrate Zoology, Milwaukee Public Museum, Milwaukee, WI 53233-14781Jass and Klausmeier: The Terrestrial Isopod Hyloniscus Riparius (Isopoda: OniscPublished by ValpoScholar, 20032003 THE GREAT LAKES ENTOMOLOGIST 71MATERIALS AND METHODSBetween 1987 and 1998 we collected H. riparius from 37 Wisconsin coun-ties (Fig. 1), indicating that the range for this species in the state includesportions each of the three ecological regions of Curtis (1959). We used our gen-eral collection data from Wisconsin to select sites for a more intensive study ofthe life cycle of H. riparius, choosing for further study those locations that hadyielded larger numbers of specimens in the past (Table 1).We gathered 18 samples from 13 June through 7 November 1997, 6 fromeach of the three ecological regions: the northern, the southern, and the interven-ing tension zone as defined by Curtis (1959) (Fig. 1). At least 30 individuals of H.riparius were gathered at each sampling. Samples were collected with aspirators,preserved in the field in 70% ethanol, and taken to the laboratory for sorting undera dissecting microscope. Adult males were identified as individuals having pleo-pods modified for transfer of sperm and females as those having a marsupium.All collections are maintained at the Milwaukee Public Museum.A series of samples taken throughout the growing season can be analyzedto describe the annual reproductive pattern for a particular isopod species. Thisis done by noting the percent of gravid females in the samples and following thetiming of development of embryos from eggs to young. The developmental pro-cess takes place inside the female’s marsupium and can be seen through theoostegites (transparent overlapping body flaps) that form the marsupial cavityon the ventral surface of the thorax. This allows further classification of gravidfemales into two subgroups, those whose broods are still at the egg/embryostage and those whose broods have developed to the manca stage. When theembryo develops to the stage in which it closely resembles a miniature adult, ithas been termed a “manca” (Holdich et al. 1984). Mancas are active inside themarsupium for at least several days before they become independent. Immaturesare whitish immediately after exiting the marsupium and undergo several moltsin the process of gradually maturing to display the external sexual charactersand wine-red coloration of adults (Heeley 1942). Because of the impossibility ofaccurately identifying and sexing such individuals, the results of our field stud-ies refer to adults only.RESULTSIn each of six populations we sampled in 1997, males comprised less than45% of the population (Table 2). The percentage of males from all 18 sampleswas low, averaging 27.2%. Our northern samples had a significantly higherproportion of males (mean = 34.7%) than did southern samples (mean = 24.5%)(χ2 = 10.60, P < 0.001, df = 1) (SAS 1985). The mean for tension zone males was22.4%, intermediate in value but not statistically significantly different frommeans for the other zones.All samples showed a reproductive cycle that displayed a definite patternof differences across the season in Wisconsin. June through August was theperiod in which the highest frequency of gravid females was found in all threeregions of Wisconsin. Reproductive activity was significantly less after Septem-ber. No samples from October had any gravid females.Our 1997 collections suggested that the reproductive period of H. ripariusis shorter and more concentrated north of the tension zone in Wisconsin than itis in, or south of, the tension zone (Table 2). Females with mancas were foundonly in the August sample from one of the northern populations, whereas mancaswere found from June through September in more southerly populations. Be-cause the females in our samples from the Brown County population consistedalmost entirely of non-reproductive individuals, this had a strong effect on theoverall totals for the northern region.2The Great Lakes Entomologist, Vol. 36, No. 1 [2003], Art. 11https://scholar.valpo.edu/tgle/vol36/iss1/1172 THE GREAT LAKES ENTOMOLOGIST Vol. 36, Nos. 1 & 2Table 1. Collection sites for Hyloniscus riparius in 1997. Regions are as defined byCurtis (1959).Collection dates County LocationWisconsin Northern Hardwoods Province13 Jun, 1 Aug, 3 Oct Ozaukee UWM Field Station Upland Woods11 Jul, 5 Sep, 7 Nov Brown Bayshore ParkWisconsin Tension Zone20 Jun, 8 Aug, 3 Oct Fond du Lac Kettle Moraine Noyes Woods18 Jul, 12 Sep, 07 Nov Milwaukee Saint Francis Seminary WoodsWisconsin Southern Prairie-Forest Province27 Jun, 31 Aug, 17 Oct Green New Glarus State Park Woods25 Jul, 12 Sep, 7 Nov Racine Karcher Marsh Wildlife AreaFigure 1. Shading indicates the Wisconsin counties from which Hyloniscus ripariuswas collected 1987 - 1998. The map depicts Curtis’ (1959) northern and southernecological regions and the tension zone between them.3Jass and Klausmeier: The Terrestrial Isopod Hyloniscus Riparius (Isopoda: OniscPublished by ValpoScholar, 20032003 THE GREAT LAKES ENTOMOLOGIST 73Four other terrestrial isopods were associated with H. riparius in our1997 collections: C. convexus, Miktoniscus medcofi (Van Name 1940), P. spinicornisand T. rathkii.DISCUSSIONWe found H. riparius in a fairly wide range of sites, as defined either byfeatures of the landscape (such as lakeshore, river bank, quarry, gravel pit,garden) or dominant vegetation (floodplain/upland forest, deciduous/mixed de-ciduous and coniferous forest, native plants/introduced species). The most im-portant factor shared by all of these sites is high soil moisture. Vandel (1960)called H. riparius “epigean” to define its microhabitat among the loose soilbeneath dead leaves, mosses, and woody debris. Using the Brower and Zar(1977) qualitative categorization of soil moisture content, we consistently foundH. riparius at the moist to wet end of the scale, where the wetness can be felt andsoil adheres to the hand during collecting.Schultz (1965) discussed the North American distribution and generalbiology of H. riparius, and described a North Carolina population with datacollected in June and September. In fall he found a large number of very smallindividuals, too immature for the development of external sexual traits. Schultz(1965) identified these as the members of the next generation, which would notmature until the following season, a size class distinctly separate from even thesmallest members (body lengths of about 2 mm) of the current generation.While we did not count members of this very small size class, we did note a largenumber of tiny whitish immature isopods in our September sampling. Amongthe matures he measured, Schultz (1965) found that the smaller individualswere males and that females outnumbered males in all collections, the male-female ratio being 1:2.Males were also a minority in all of our Wisconsin samples, averaging27.2% of all the isopods collected. Our two northern populations had a higherTable 2. Collections of Hyloniscus riparius adults in 1997, with percent of the totalthat were male, percent of females that were gravid, and percent of gravid femalesthat were carrying mancas rather than embryos.JUN JUL AUG SEP OCT NOVNorthern WisconsinTotal number of individuals 56 70 37 68 64 81% Male 26.8 32.9 29.7 38.2 43.8 37.0% Females gravid 43.9 0 34.6 2.4 0 0% Gravid females with mancas 0 0 11.1 0 0 0Tension ZoneTotal number of individuals 59 32 50 123 104 73% Male 27.1 15.6 18.0 28.5 29.8 15.1% Females gravid 27.9 48.1 43.9 10.2 0 0% Gravid females with mancas 0 7.7 5.6 11.1 0 0Southern WisconsinTotal number of individuals 48 52 67 84 45 55% Male 22.9 21.2 28.4 22.6 35.6 16.4% Females gravid 40.5 9.8 35.4 9.2 0 6.5% Gravid females with mancas 26.7 25.0 0 50.0 0 04The Great Lakes Entomologist, Vol. 36, No. 1 [2003], Art. 11https://scholar.valpo.edu/tgle/vol36/iss1/1174 THE GREAT LAKES ENTOMOLOGIST Vol. 36, Nos. 1 & 2proportion of males than the more southerly samples. Our studies of the terres-trial isopod T. rathkii (Jass and Klausmeier 1996) showed a similar statisti-cally significant difference between the proportions of males in northern andsouthern Wisconsin, with females dominating southern populations to a greaterdegree than northern ones. In that species we were able to correlate the differentsex ratios with a significantly decreased reproductive capacity in the north interms of gravidity. Interestingly, the greatest percentage of H. riparius malesoccurred in October in both north and south samples.An important difference between T. rathkii and a less surface-active spe-cies like H. riparius is the extension of the breeding season of the latter intoautumn. For terrestrial isopods in temperate climates, Dangerfield and Telford(1990) hypothesized that the less surface-active the species, the weaker wouldbe the tie between the timing of its reproductive cycle and environmental cuessuch as day length. The occurrence of a strong August-September increase inreproductive activity in H. riparius was described as typical by Schultz (1965)and was seen in the preliminary data reported for this species by Jass andKlausmeier (1996). In contrast, for T. rathkii we found that southern popula-tions had their reproductive peak in June (76.9% of females gravid) and north-ern populations in July (91.9% of females gravid), followed by drastic reductionsin the proportion of females gravid in both the north and south in August (<5%)and none after that. For H. riparius we found high percentages of females withembryos in all three August 1997 samples (Table 2).There was a rather abrupt end to the H. riparius reproductive season afterthe release of the last maturing mancas in September. In only one sample, fromthe southernmost region, was there any reproductive activity at all later in theseason (6.5% females gravid in November).Sutton (1972) showed that increasing cold triggered a change in isopodbehavior. He found that freezing temperatures in the field caused an increase interrestrial isopod activity and resulted in migration away from the surfacelayers. Perhaps the only individuals that will survive to begin the reproductivecycle again the following spring are those that have descended far enough intothe lower levels of the soil to find sufficient protection to withstand the extremetemperatures of winter. Our focus has been on the reproductive portion of the lifehistory of H. riparius, and we recognize that many other aspects, such as overwintering, remain to be elucidated by further research.ACKNOWLEDGMENTSWe gratefully acknowledge editorial suggestions received from J. Reinartzof the University of Wisconsin-Milwaukee Field Station and assistance in manu-script preparation from S. Otto of the Milwaukee Public Museum.LITERATURE CITEDBrower, J. E. and J.H. Zar. 1977. Field and Laboratory Methods for General Ecology.Wm. C. Brown Company Publishers, Dubuque, IA.Curtis, J. T. 1959. The Vegetation of Wisconsin, An Ordination of Plant Communities.University of Wisconsin Press, Madison, WI.Dangerfield, J. M. and S.R. Telford. 1990. Breeding phenology, variation in reproduc-tive effort and offspring size in a tropical population of the woodlouse Porcellionidespruinosus. Oecologia 82: 251-258.Hatchett, S.P. 1947. Biology of the Isopoda of Michigan. Ecol. Monogr. 17: 48-79.Heeley, W. 1942. Observations on the life-histories of some terrestrial isopods. Proc.Zool. Soc. London Series B 111: 79-149.5Jass and Klausmeier: The Terrestrial Isopod Hyloniscus Riparius (Isopoda: OniscPublished by ValpoScholar, 20032003 THE GREAT LAKES ENTOMOLOGIST 75Holdich, D. M., R. J. Lincoln and J. P. Ellis. 1984. The biology of terrestrial isopods:terminology and classification. Symposia Zool. Soc. London 53: 1-6.Jass, J. and B. Klausmeier. 1987. Terrestrial isopods at the UWM Field Station. FieldSta. Bull. 20(2): 17-21.Jass, J. and B. Klausmeier. 1996. Comparison of Wisconsin terrestrial isopods andtheir life cycle traits. Field Sta. Bull. 29(2): 14-26.Jass, J. and B. Klausmeier. 2000. Endemics and immigrants: North American terres-trial isopods (Isopoda, Oniscidea) north of Mexico. Crustaceana 73: 771-799.Jass, J., K. Zandi and B. Klausmeier. 1991. Life cycle information from a Wisconsinpopulation of Porcellio spinicornis, the dark-headed isopod. Field Sta. Bull. 24(2):21-26.McQueen, D. J. 1976. Porcellio spinicornis Say (Isopoda) demography. II. A compari-son between field and laboratory data. Can. J. Zool. 54: 825-842.SAS Institute, Inc. 1985. SAS User’s Guide: Statistics, Version 5 Edition, Cary, NC.Schultz, G. A. 1965. The distribution and general biology of Hyloniscus riparius (Koch)(Isopoda, Oniscidea) in North America. Crustaceana 8: 131-140.Sutton, S. 1972. Woodlice. Pergamon Press, Oxford, England.Vandel, A. 1960. Isopodes Terrestres (Premiere Partie). Faune de France 64: 1-416.6The Great Lakes Entomologist, Vol. 36, No. 1 [2003], Art. 11https://scholar.valpo.edu/tgle/vol36/iss1/11

The Terrestrial Isopod \u3ci\u3eHyloniscus Riparius\u3c/i\u3e (Isopoda: Oniscidea: Trichoniscidae) in Wisconsin

ValpoScholar

https://scholar.valpo.edu/cgi/viewcontent.cgi?article=2080&amp;context=tgle

The Terrestrial Isopod \u3ci\u3eHyloniscus Riparius\u3c/i\u3e (Isopoda: Oniscidea: Trichoniscidae) in Wisconsin

Abstract

Similar works

Full text

Available Versions

Valparaiso University

ValpoScholar