Behavioural and Metabolic Adaptations of Marine Isopods to the Rafting Life Style
Total Page：16
File Type：pdf, Size：1020Kb
Marine Biology (2006) 149: 821–828 DOI 10.1007/s00227-006-0257-9 RESEARCH ARTICLE Lars Gutow Æ Julia Strahl Æ Christian Wiencke Heinz-Dieter Franke Æ Reinhard Saborowski Behavioural and metabolic adaptations of marine isopods to the rafting life style Received: 20 July 2005 / Accepted: 16 January 2006 / Published online: 7 February 2006 Ó Springer-Verlag 2006 Abstract Rafting on floating objects is a common dis- Introduction persal mechanism for many marine invertebrates. In order to identify adaptations to the rafting life style, we Seven isopod species of the genus Idotea form resident compared behavioural and metabolic characteristics of populations around the island of Helgoland (German two isopods, the obligate rafter Idotea metallica and the Bight, North Sea; Franke et al. 1999). They are dis- facultative rafter Idotea baltica. In laboratory experi- tributed benthically along the coast from the supratidal ments, I. metallica showed low locomotive activity and a down to subtidal habitats (Naylor 1972). The distribu- tight association to the substratum. Idotea baltica,in tion of Idotea baltica and Idotea emarginata is primarily contrast, was more active with more frequent excursions the result of interspecific competition and subsequent in the surrounding water column. Oxygen consumption habitat segregation (Franke and Janke 1998). Although rates were similar in both species. Idotea metallica fed on they prefer benthic habitats, all species also raft on zooplankton making this species widely independent of floating objects at the sea surface; most of them, how- autochthonous food resources of the raft. Feeding rates ever, only sporadically. Only I. baltica is abundant on and digestive enzyme activities were low in I. metallica. floating macroalgae in the German Bight, especially on Reduced egestion rates may indicate slow gut passage brown algae of the order Fucales, with average numbers and, thus, efficient resorption of nutrients. Efficient food of about 90 adult animals per kg seaweed (Franke et al. utilization and the ability to accumulate high amounts of 1999). Since 1994 another species, I. metallica, has storage products, i.e. lipids, indicate a possible adapta- occurred regularly in small numbers on floating objects tion of I. metallica to low food availability or starvation. in the North Sea. Gutow and Franke (2001) showed that The feeding behaviour of I. baltica, in contrast, was I. metallica is only a summer resident. Populations go more herbivorous and appeared wasteful and inefficient. extinct in winter because temperatures are too low for Low lipid contents in I. baltica also indicate poor storage reproduction. Nevertheless, due to repeated annual reserves. Thus, I. baltica requires a permanent access to re-introduction, I. metallica has become a regular food. member of the isopod community of the German Bight. Idotea metallica lives exclusively on often abiotic floating objects (Moreira 1972; Tully and O´Ceı´digh Communicated by O. Kinne, Oldendorf/Luhe 1986) on which they can be transported passively over long distances by surface currents (van der Baan and & L. Gutow ( ) Æ J. Strahl Æ C. Wiencke Holthuis 1969). Benthic populations of I. metallica are Alfred Wegener Institute for Polar and Marine Research, P.O. Box 12 01 61, 27515 Bremerhaven, Germany not known even though the species is reported from E-mail: [email protected] coastal waters in many parts of the world (Miller 1968; Tel.: +49-471-48311708 Hartmann 1976; Sano et al. 2003; Abello´et al. 2004), Fax: +49-471-48311724 probably as a consequence of inferiority in interspecific J. Strahl competition with benthic species (Gutow and Franke Department of Marine Zoology, University of Bremen, 2003). Apparently, rafting is a specific and evolution- P.O. Box 33 04 40, 28334 Bremen, Germany arily developed life style of I. metallica. Living close to the surface requires tolerance against changing physical H.-D. Franke Æ R. Saborowski and chemical conditions such as temperature and Alfred Wegener Institute for Polar and Marine Research, Biologische Anstalt Helgoland, P.O. Box 180, salinity or intensive UV-radiation (e.g. Cheng 1975; 27483 Helgoland, Germany Herring 1969). 822 In contrast to I. metallica, the predominantly benthic performed for each species within 1 week. The total I. baltica is found rafting almost exclusively in coastal number of observations was 600 per species. After each waters. Abundances are usually low offshore (Morris experimental period, all individuals were returned to the and Mogelberg 1973) indicating a poor ability for long mass culture. distance rafting. Idotea baltica is almost entirely lacking Low activity: Sitting calmly on the substratum (sed- on abiotic rafts which provide only little plant food iment or algae), grooming antennae and pleopods, (Gutow and Franke 2003). feeding algae, catching and feeding on Artemia nauplii In order to identify specific adaptations to the rafting (catch posture). Social contact: touch without movement life style, we compared in laboratory experiments behavioural and physiological characteristics in terms of Medium activity: Slow movement (striding) on sub- locomotive activity and respiration rates as well as the stratum (sediment or algae), turning/spinning around nutritive demands of the obligate rafter I. metallica and the body. Social contact: touch with movement the facultative rafter I. baltica. We hypothesize that High activity: Swimming horizontally or vertically I. metallica has evolved strategies that allow the species through the water, fast movement (running) on sub- to persist under conditions of limited food availability in stratum (sediment or algae). Social contact: casing, oligotrophic open oceans. scuffle, cannibalism Material and methods Respiration Animals Oxygen consumption rates were determined in a closed system by the Winkler method (Grasshoff 1983). Prior In order to avoid metabolic variation due to the repro- to the experiments, individuals of both species were ductive state, all experiments were carried out with adult kept separately for defecation in glass vials (40 ml) for males rather than females. Isopods of the species 12 h without food. Seawater was filtered (0.2 lm), I. metallica and I. baltica were obtained from mass allowed to adjust overnight to the experimental tem- cultures which were run routinely at the Marine Station perature of 16°C, and then filled into eight incubation Helgoland. Wet weight (WW) of the animals ranged bottles (0.57–0.61 l). Six of these bottles contained from 130 to 230 mg for I. metallica and from 100 to 1 isopod (3·1 I. metallica and 3·1 I. baltica) and a 300 mg for I. baltica. During the experiments, the ani- piece of gauze each. Two bottles served as controls. mals were exposed to a constant temperature of 16°C Animals were incubated for 4–5 h at 16°C before sub- and a light–dark cycle of 16:8 h (06:00–22:00 local time). samples of the water were transferred into Winkler flasks (50–60 ml) and fixed for the quantification of dissolved oxygen. The isopods were carefully blotted on Activity filter paper and weighed (WW). The experiment was repeated four times resulting in a total number of 12 Two aquaria (28 cm high, 35.5 cm wide, 19 cm deep) individuals per species. These experiments were con- were equipped with a 2 cm sediment layer of fine sand ducted in the morning as well as in the afternoon. and were filled with 10 l of seawater. Each aquarium was furnished with two fragments (10 g each) of the brown alga Fucus vesiculosus which served as both a substratum Food ingestion and egestion for the isopods to cling to and as food. Algal fragments were fixed in the aquaria with a thin cotton string to a The animals were acclimated to the experimental con- tripod above the aquaria but were allowed to float on ditions for 1 week. For acclimation, individual isopods the water surface. The distance between both algal pieces were maintained individually in glass vials (40 ml) and was about 20 cm. Two-day-old Artemia nauplii were were fed with pieces of F. vesiculosus and Artemia nau- offered as additional food. plii in excess. The seawater was changed daily. One aquarium was equipped with ten individuals of For experiments, 26 isopods of each species were fed I. metallica and the other with ten I. baltica. After 24 h individually with weighed pieces of F. vesiculosus and of acclimation, the behaviour of the isopods was moni- 200 Artemia nauplii. After 24 h, the isopods were re- tored three times per day at 11:00, 14:00 and 19:30 local moved and WW of the animals was recorded. The time. Randomly selected animals were observed for 10 s. remaining Fucus fragments were weighed and remaining Their locomotive activity was recorded and classified as Artemia nauplii were counted. ‘‘low’’, ‘‘medium’’ and ‘‘high’’ (see below). In total, 40 Faecal pellets of the animals were collected immedi- observations (10 s each) were done per species and ately after the experiments, transferred into reaction monitoring period. Consequently, on average, each cups and centrifuged for 15 min at 13,000 g and 4°C. animal was recorded four times during each monitoring Supernatants were removed and the pellets were weighed period. Five replicates with ten individuals each were and lyophilised. Dry weight (DW) was measured for 823 food items F. vesiculosus and Artemia nauplii as well as Statistics for the faeces. The distribution of the behaviour of the animals on different activity levels was analyzed with a v2-text. A t Biochemical investigations test was applied for comparing two normally distributed data sets (respiration rates, egestion rates and eges- Digestive enzymes and total lipids were measured in tion:ingestion ratio, enzyme activities). If normal distri- aqueous extracts of individual adult males. Animals bution failed, a Mann–Whitney U test was applied were lyophilised and then ground to a homogeneous fine instead (lipids).