Hermit Crabs and Their Symbionts: Reactions to Artiﬁcially Induced Anoxia on a Sublittoral Sediment Bottom

Journal of Experimental Marine Biology and Ecology 411 (2012) 23–33

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Journal of Experimental Marine Biology and Ecology

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Hermit crabs and their symbionts: Reactions to artiﬁcially induced anoxia on a sublittoral sediment bottom

Katrin Pretterebner a, Bettina Riedel a, Martin Zuschin b, Michael Stachowitsch a,⁎ a University of Vienna, Department of Marine Biology, Althanstrasse 14, 1090 Vienna, Austria b University of Vienna, Department of Paleontology, Althanstrasse 14, 1090 Vienna, Austria article info abstract

Article history: Hermit crabs play an important role in the Northern Adriatic Sea due to their abundance, wide range of sym- Received 20 June 2011 bionts, and function in structuring the benthic community. Small-scale (0.25 m2) hypoxia and anoxia were Received in revised form 25 October 2011 experimentally generated on a sublittoral soft bottom in 24 m depth in the Gulf of Trieste. This approach suc- Accepted 28 October 2011 cessfully simulates the seasonal low dissolved oxygen (DO) events here and enabled studying the behaviour Available online 24 November 2011 and mortality of the hermit crab Paguristes eremita. The crabs exhibited a sequence of predictable stress responses and ultimately mortality, which was correlated with ﬁve oxygen thresholds. Among the crustaceans, Keywords: Adriatic Sea which are a sensitive group to oxygen depletion, P. eremita is relatively tolerant. Initially, at mild hypoxia (2.0 −1 Dead zone to 1.0 ml l DO), hermit crabs showed avoidance by moving onto better oxygenated, elevated substrata. Eutrophication This was accompanied by a series of responses including decreased locomotory activity, increased body Hydrogen sulphide movements and extension from the shell. During a moribund phase at severe hypoxia (0.5 to 0.01 ml l− 1 Hypoxia DO), crabs were mostly immobile in overturned shells and body movements decreased. Anoxia triggered emergence from the shell, with a brief locomotion spurt of shell-less crabs. The activity pattern of normally day-active crabs was altered during hypoxia and anoxia. Atypical interspeciﬁc interactions occurred: the crab Pisidia longimana increasingly aggregated on hermit crab shells, and a hermit crab used the emerged infaunal sea urchin Schizaster canaliferus as an elevated substrate. Response patterns varied somewhat according to shell size or symbiont type (the sponge Suberites domuncula). Mortality occurred after extended

anoxia (~1.5 d) and increased hydrogen sulphide levels (H2S ~128 μmol). The relative tolerance of crabs and certain symbionts (e.g. the sea anemone Calliactis parasitica) – as potential survivors and recolonizers of affected areas – may inﬂuence and promote community recovery after oxygen crises. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Hermit crabs are therefore ecosystem engineers: by using gastropod shells they affect the abundance and distribution of other inverte- Hermit crabs inhabit a wide range of environments, from polar to brates (Gutierrez et al., 2003; Jones et al., 1994; 1997; Williams and tropical seas and from the supratidal to deep ocean canyons. They McDermott, 2004). play important roles as predators, scavengers, detritivores and even In the Northern Adriatic Sea, which is characterised by high- filter-feeders (Schembri, 1982), and their manifold symbioses can en- biomass macroepibenthic assemblages (Fedra et al., 1976; Zuschin rich the biodiversity of their habitats today and probably in the past and Stachowitsch, 2009), hermit crabs exhibit a high density (1.9 (Reiss et al., 2003). Hermit crab-occupied shells are important islands individuals m− 2). Paguristes eremita (Linnaeus, 1767) (formerly of hard structures for the attachment of epifauna in soft-bottom Paguristes oculatus) is the dominant hermit crab species and harbours benthic communities (Brooks and Mariscal, 1986). There, empty more than 110 symbionts (in the broad sense of De Bary, 1879, cited shells are likely to be buried in the substrate unless they are used in Stachowitsch, 1980). The large number, size and range of species by hermit crabs as protection (Creed, 2000; Stachowitsch, 1977). (Williams and McDermott, 2004) make hermit crab associations a stable yet mobile microbiocoenosis (Stachowitsch, 1977). The encrusting and endolithic species influence the time that a shell, as the key resource for hermit crabs, can function as a housing. This led to a new classification of symbionts based on their function in ⁎ Corresponding author at: Department of Marine Biology, Faculty of Life Sciences, two simultaneous, opposing processes: a constructive process that University of Vienna, Althanstrasse 14, 1090 Vienna, Austria. Tel.: +43 1427757103. strengthens and extends the shell's lifespan, and a destructive process E-mail addresses: [email protected] (K. Pretterebner), [email protected] (B. Riedel), [email protected] (M. Zuschin), [email protected] that weakens the shell (Stachowitsch, 1980). Finally, this association (M. Stachowitsch). plays an important role in structuring the overall community: when

0022-0981/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jembe.2011.10.027 24 K. Pretterebner et al. / Journal of Experimental Marine Biology and Ecology 411 (2012) 23–33 deteriorated shells are abandoned, many symbionts survive and grow dominant taxa (O. quinquemaculata, the sponges Reniera spp., and to establish the multi-species clumps that characterise this soft the ascidians Microcosmus spp.), this benthic community was bottom. named the Ophiothrix–Reniera–Microcosmus community (Fedra et Coastal shallow seas face the greatest anthropogenic threats due al., 1976). The sediment surface between the patchy distribution of to the impacts of accelerated human activities (Jenkins, 2003). Eutro- multi-species clumps is characterised by a low epifaunal density phication, coupled with water column stratification, is one of the (Zuschin et al., 1999) and is dominated by predators and deposit gravest threats. This is manifested in low dissolved oxygen levels feeders, including the hermit crab P. eremita. (Diaz, 2001). Hypoxia (DO concentrationsb2mll− 1) and anoxia have spread exponentially since the 1960s and today are a key stress- 2.2. Experimental design and sampling or in shallow marine ecosystems (Diaz and Rosenberg, 2008). Benthic animals respond to hypoxia and anoxia with a range of stress behav- The experimental anoxia generating unit (EAGU) is an underwater iours, physiological adaptations (Diaz and Rosenberg, 1995; Gray et device designed to artificially create hypoxia and anoxia on a small al., 2002; Wu, 2002) and mass mortalities (in the Adriatic; scale (0.25 m2) on the seafloor. It enables the documentation of the Stachowitsch, 1984). The resulting dead zones (Diaz and Rosenberg, behaviour of macrobenthic organisms to oxygen depletion and in-

2008) have impacts on all scales from the structure and function of creasing H2S. benthic communities to impaired ecosystem services (Sala and The EAGU consists of a cubic aluminium frame or an interchange- Knowlton, 2006). The Northern Adriatic Sea is one of nearly 400 able plexiglass chamber, both measuring 50×50×50 cm (for details recognised dead zones worldwide. As a shallow, semi-enclosed see Stachowitsch et al., 2007). A separate instrument lid is placed water body with abundant nutrient discharges, mainly from the Po on top of one of these two different bases. This lid houses a time- River, coupled with meteorological and climatic conditions, it exhibits lapse camera, two flashes and a data logger with sensor array (Uni- most attributes associated with the development of low oxygen sense®) to record DO, H2S and temperature. Oxygen was measured events (Justić et al., 1993). This makes it a model for ecosystems 2 and 20 cm above the sediment to capture potential stratification, periodically affected by oxygen deficiency (Crema et al., 1991). and the H2S sensor was located 2 cm above the sediment. Photos Due to the difficulty of predicting oxygen depletion events and were taken automatically every 6 min, and sensor data were logged their often rapid onset and course, an experimental anoxia generating every minute. unit (EAGU) was developed to simulate anoxia on a small scale and to The EAGU was deployed in both configurations. First, in the “open” document the full range of behavioural responses, inter- and intra- configuration, the aluminium frame was positioned above an aggre- specific interactions, and mortalities in situ (Stachowitsch et al., gation of benthic organisms for 24 h to document behaviour under 2007). First results on selected species and species groups have normoxic conditions. Then, in the “closed” configuration, the frame already been published for the Adriatic (Riedel et al., 2008a; 2008b). was exchanged with the plexiglass chamber and deployed above In general, crustaceans are sensitive to anoxia (Vaquer-Sunyer and the same assemblage. The chamber allows no water exchange with Duarte, 2008). Our first results, however, indicate that various crusta- the water column. Anoxia was induced within 1–2 d due to natural cean species show different tolerances. Thus, the bioherm-inhabiting respiration rates of the enclosed organisms. This “closed” configura- crab Pisidia longimana was among the most sensitive species, whereas tion was maintained for another 1–2 d to detect the reactions of Pilumnus spinifer and Ebalia tuberosa (the latter buries itself in the more tolerant species. The starting point and duration of all phases sediment during the day) were more tolerant. The mutualistic pea of the deployments are not uniform due to the inherent nature of crab Nepinnotheres pinnotheres (living inside ascidians) survived pro- field experiments (e.g. weather, diving schedules). Remaining organ- longed anoxia as well as higher H2S(Haselmair et al., 2010). Hermit isms (living and dead) were then collected for preservation in a 4% crabs were not evaluated in that study, but our initial observations formalin:seawater solution, and species and biomass were deter- indicated that they were among the more tolerant crustaceans. This mined. The fieldwork was conducted in September 2005 and from would have implications for survival of short-term hypoxia and the July to October 2006. subsequent recolonization of affected areas. Considering the important roles that hermit crabs play in this community, little is known 2.3. Data analysis about their responses to hypoxia (Côté et al., 1998; Riedel et al., 2008b; Shives and Dunbar, 2010; Stachowitsch, 1984). The present Overall, 6705 images were evaluated, yielding a documentation study is designed to fully document their behaviour during oxygen time of 670.5 h (45.5 h open, 625 h closed configuration). The behav- crises and to correlate behaviour and mortality with specific oxygen iour of P. eremita was analysed image by image and recorded in cate- thresholds. gories (Table 1). In the open configuration (normoxia), all visible individuals within the camera's field (inside and outside of the 2. Materials and methods aluminium frame) in two deployments (9, 11) were evaluated, yielding 25 and 26 hermit crabs, i.e. a total of 51 individuals. In the closed 2.1. Study site configuration (rapidly sinking values/hypoxia/anoxia), 25 hermit crabs were evaluated (eight deployments): 9 inhabited the smaller The study site is located 2.3 km off Cap Madona (Piran, Slovenia) A. pespelecani (Linnaeus, 1758), 11 the larger Murex brandaris or in the Gulf of Trieste (45° 32′ 55.68″ N, 13° 33′ 1.89″ E) close to the Hexaplex trunculus, 3 sponge-covered shells (Suberites domuncula, oceanographic buoy of the Marine Biology Station Piran at a depth Olivi 1792) and 2 unidentified shells. If 4 or fewer hermit crabs of 24 m. were present in an experiment, all were evaluated. If more individ- The sublittoral bottom consists of poorly sorted silty sand with uals were present, those 4 individuals visible throughout the experi- high-biomass aggregations of macrobenthic organisms termed ment were selected. Additional criteria for selection included range multi-species clumps (Fedra et al., 1976). These multi-species clumps of sizes, occupied shell species and symbionts. or bioherms are formed by biogenic structures, mainly mollusc shells The hermit crabs themselves are typically only partially visible (i.e. which serve as a basis for sessile species including serpulid tube- antennae, eyestalks). Image evaluation was therefore based on shell worms, ascidians, sponges, anemones and bivalves (Zuschin et al., locomotion and/or position changes (except in shell-less crabs). Dis- 1999). Multi-species clumps provide a substrate for semi-sessile and placement (i.e. major/minor locomotion) in small and large crabs was vagile species such as the brittle star Ophiothrix quinquemaculata. equated to shell displacement. For individuals in large S. domuncula,it Filter- and suspension-feeding species dominate. Based on the 3 was equated to estimated crab length. Behaviours were documented K. Pretterebner et al. / Journal of Experimental Marine Biology and Ecology 411 (2012) 23–33 25

Table 1 regressions were used. H2S concentrations were transformed using Behaviours and reactions of hermit crabs. a log (x+1) transformation. The software package SPSS 17.0 was Category/sub- Criteria used. category Visibility 3. Results Visible Hermit crab, its shell or parts thereof visible. This includes shell largely covered by other organisms (e.g. crab under brittle star 3.1. Sensor data aggregation, i.e. outline recognisable, no exit tracks). Non-visible Neither crab nor its shell or parts thereof visible − 1 (e.g. hidden behind a multi-species clump) During normoxia, DO values varied from 2.6 to 5.6 ml l (2 cm Location above the sediment). The values were typically higher (2.8 to On sediment Crab/shell located on sediment or on bivalve shells on sediment 8.9 ml l− 1) 20 cm above the sediment. In all deployments, the subse- Elevated Crab/shell on multi-species clump (no contact with sediment) fi Locomotion quent closed con guration caused an immediate and constant de- No No displacement crease in DO concentrations. Anoxia was generated within 1 to 3 d. locomotion This depended in part on the initial concentration and on the amount Minor Displacementb1 shell length or b1 body length of biomass enclosed in the respective deployment. It was induced (if crab outside shell) fastest in deployment 11 (17.4 h) and the slowest in deployment 13 Major Displacement≥1 shell length or ≥1 body length (if crab outside shell) (69.8 h). In deployment 2 an intermediate oxygen peak developed. Turn Turning movement without displacement H2S was created in all deployments: values were lower in deploy- Body movement Crab movement without displacement (retraction into or ments with short anoxia (up to 21 μmol l− 1) and highest when stretching out of shell; appendage movements: chelipeds/legs; anoxia lasted 2 d or more (maximum 304 μmol l− 1). The tempera- slight shell movements if crab not visible) Body posture ture remained constant in each deployment (range across deploy- Normal Biologically normal body postures. One or more of the following ments: 18.5 to 21.4 °C) and bottom water salinity was 38‰ (for visible: eyes, cheliped(s), leg(s), antennae. If crab not visible, details and characteristic sensor data see Haselmair et al., 2010). shell aperture slightly elevated (i.e. not flat on sediment surface) Extended Soft (posterior) part of carapace (and occasionally also anterior 3.2. Responses to decreasing dissolved oxygen concentrations part of abdomen) visible Out Crab fully emerged/shell abandoned Decreasing DO values triggered a sequence of predictable stress Shell position responses and atypical interactions in hermit crabs. For significance Upright Aperture facing down Overturned Aperture facing up levels of changes between oxygen categories, see also Suppl. material Interaction Visible interaction of crab, its shell, or a symbiont with another Table 1: all crabs combined, Table 2: small crabs, Table 3: large crabs, organism. Includes: Table 4: crabs in the sponge S. domuncula. • organism on top of shell, • hermit crab on organism that shows reaction, • interaction not directly visible but reconstructed based on 3.3. Location changes track and/or reactions between two images. Excludes organisms used by crab solely as a substrate, e.g. crab climbs up on or over a sponge or ascidian During normoxia, hermit crabs were located mainly on the sediment or partly hidden under multi-species clumps (only 24% of observations on clumps; Fig. 1A). The onset of hypoxia triggered a until mortality or until poor visibility due to decomposition of other highly significant (Pb0.001) avoidance response, i.e. migration onto benthic organisms. Mortality was recorded as occurring 2 h after the clumps. At mild and moderate hypoxia, crabs moved up clumps in last observed locomotion or body movement. “Hours after hypoxia nearly half the observations (47 and 48%, respectively). This value and anoxia” refer to the total time span measured between 2 ml l−1 fell to 39% (Pb0.001) during severe hypoxia and anoxia (Suppl. mate- DO and the evaluated behaviour, whereby the anoxia component is rial Table 1). provided separately (time between 0 ml l−1 DO and the behaviour). The responses differed considerably according to housing type Day and night were defined according to sunrise and sunset of the (large and small shells, S. domuncula). The three categories differed corresponding day of the deployment. Deployment 11 was chosen to in the amount of time spent on the sediment and multi-species graphically depict a representative deployment: it was one of the clumps in each oxygen category. Small crabs (in small shells) were longest experiments in the closed configuration and was also one of mostly on the sediment during normoxia. During mild hypoxia, the the two deployments used to evaluate behaviour in normoxia (open number of observations of crabs located on bioherms increased configuration; Figs. 3 and 4). from 36% in normoxia to 56% (Pb0.001) (Fig. 1B). During moderate hypoxia, slightly less than half the observations (46%) showed crabs on clumps, rising to 52% (Pb0.05) during severe hypoxia and decreas- 2.4. Statistical analysis ing to 39% (Pb0.001) during anoxia (Suppl. material Table 2). Large crabs, in contrast, were mostly on the sediment in all oxygen catego- The behavioural categories were assigned to dissolved oxygen ries (normoxia: 94% of observations; Fig. 1C). Mild hypoxia triggered categories: normoxia (≥2.0 ml l−1), mild hypoxia (2.0–1.0 ml l− 1), movement onto clumps (from 6 to 19%), with a peak during moderate moderate hypoxia (1.0–0.5 ml l− 1), severe hypoxia (b0.5 ml l− 1), hypoxia (30%), followed by a continuous decrease (15 and 9% during and anoxia (0 ml l− 1). severe hypoxia and anoxia, respectively). The changes across all The non-parametric Kruskal–Wallis test was used to determine oxygen categories were highly significant (Suppl. material Table 3). differences in behaviour due to declining oxygen concentrations. During normoxia, most crabs in S. domuncula were on the sediment. The Mann–Whitney U-test was used to compare behaviours between Mild hypoxia caused movement onto clumps from 30 to 97% of obser- different oxygen categories. To detect differences in responses be- vations (Pb0.001; Fig. 1D); this value remained constant during tween day- and night phases, cross tables were computed and the moderate hypoxia and fell during severe hypoxia (93%, Pb0.05) and Pearson Chi-square test was performed. To identify which factor anoxia (85%: Pb0.001, Suppl. material Table 4). (length of the closed configuration, duration of hypoxia and anoxia, The locations at the end of the evaluation also differed; most small development of H2S) affected mortalities and survivorship, linear (9 of 10 individuals) and large crabs (9 of 11) returned to the 26 K. Pretterebner et al. / Journal of Experimental Marine Biology and Ecology 411 (2012) 23–33

A) All individuals B) Small crabs 100 n = 25 n = 9 80

≥2<1<2 <0.5 0 ≥2<1<2 <0.5 0 (6750 1606 1297 5265 6653) (2536 578 457 2070 2354)

C) Large crabs D) Crabs in S. domuncula 100 n = 11 n = 3 80 Mean % of observations (95 Cl)

≥2<1<2 <0.5 0 ≥2<1<2 <0.5 0 DO conc. category (ml l-1) (1716 704 597 2541 2677) (1028 254 186 441 1092)

Fig. 1. Presence of hermit crabs on multi-species clumps (mean percentage) related to five oxygen thresholds: (A) all individuals, (B–D) specific responses. n: number of evaluated crabs in closed configuration. Numbers in parentheses under oxygen thresholds indicate total photographs evaluated in each oxygen category. sediment. Two of 3 individuals in S. domuncula, however, remained was initiated after a mean of 55 h (SD=9.4) of combined hypoxia on multi-species clumps until the end. and anoxia (mean anoxia duration=26.9 h; SD=14.2). Locomotion here ranged from a single location change (minutes) to 13.9 h of movement (mean=3.7 h; SD=4.1). Within this period, the mean 3.4. Decreased locomotion number of location changes was 15 (range: 1 to 40). Overall the highly significantly different locomotory activity be- Most horizontal locomotion was observed during normoxia (23%; tween day and night (normoxia) largely disappeared during mild Fig. 2A). Mild hypoxia caused a decrease to 14% (Pb0.001, Suppl. ma- hypoxia, re-occurred during moderate and severe hypoxia, and disap- terial Table 1) and severe hypoxia a further drop to 4% (Pb0.001) peared again at anoxia (Suppl. material Table 5). (anoxia 5%). Crabs in S. domuncula moved much less than those in shells; the decrease from normoxia (17% of all observations) to severe hypoxia (1%) was constant and steep (Pb0.001, Suppl. material Table 3.6. Changes in body movements 4) and the value remained minimal during anoxia (1%). Initially, during normoxia, locomotion and body movements 3.5. Altered activity pattern accounted for a similar percentage of observations (23 and 26%, respectively; Fig. 2A and B). At mild hypoxia, locomotion decreased Locomotion occurred in episodes, reflecting activity peaks. Initial- (see above) but body movements started to increase highly signifi- ly, during normoxia, more locomotion occurred during the day than cantly (51% of observations), peaking at moderate hypoxia (68%; at night (32 and 14% of observations, respectively). During the day, Fig. 2B), then continuously dropping at severe hypoxia and anoxia slightly more major than minor locomotion occurred (19 and 11%, (42 and 34%, respectively; all changes Pb0.001, Suppl. material respectively); during the night the relationship was more balanced Table 1). Note that, at anoxia locomotion occurred in only 5% of obser- (7 versus 6%). Oxygen depletion dampened this rhythm, i.e. a smaller vations (Fig. 2A). In 20 (out of 25) individuals, body movements daytime activity peak during early anoxia (Fig. 3). All crabs that lasted longer than locomotion (mean=25.2 h; SD=24.1; range: emerged from their shells (one exception), however, moved around 18 min to 81.4 h). The remaining 5 crabs moved around shell-less, shell-less on the sediment or multi-species clumps. This is reflected and locomotion ceased at about the same time as body movements. in the somewhat higher last activity peak (Fig. 3). This last phase In sponge-inhabiting individuals, mild hypoxia triggered a steeper K. Pretterebner et al. / Journal of Experimental Marine Biology and Ecology 411 (2012) 23–33 27

A) Locomotion B) Body movements 30 80

70 25 60 20 50

15 40

30 10 20 5 10

≥2<1<2 <0.5 0 ≥2<1<2 <0.5 0

C) Extension D) Overturned shells 50 30

25 40 Mean % of observations (95 Cl) 20 30 15 20 10

10 5

≥2<1<2 <0.5 0 ≥2<1<2 <0.5 0 DO conc. category (ml l-1) (6750 1606 1297 5265 6653)

Fig. 2. Reactions of all hermit crab individuals (mean percentage) related to five oxygen thresholds: (A–D) selected behaviours. Normoxia n=51; hypoxia/anoxia n=25. Numbers in parentheses under oxygen thresholds indicate total photographs evaluated in each oxygen category (valid for A–D). Note different scales. increase of body movements (70% of observations) and a higher peak rare (7%; Fig. 2C). Eight individuals were extended 19 times briefly value during moderate hypoxia (91%) (Suppl. material Table 4). (maximum 9 images) while examining the sediment or empty, damaged shells. Interactions (both inter- and intraspecific) caused 3 3.7. Body posture: extension from shell individuals to stretch out of their shell towards the opponent (maximum 2 images). With decreasing DO concentrations, extension (all During normoxia, normal body posture was maintained (93% of crabs) increased to 15% at mild and 34% at moderate hypoxia, reach- observations). Extension from the shell and other postures were ing 44% of observations at anoxia (all changes except from moderate to severe hypoxia Pb0.001, Suppl. material Table 1). This pattern was 5 40 more distinct in crabs inhabiting S. domuncula: values were higher during moderate hypoxia (57%) and anoxia (55%), but the major in- Open Closed configuration -1

h crease again occurred at moderate hypoxia. 4 - ) 30 Extension (Suppl. material Fig. 1A) typically occurred in two -1 peaks. The second peak is higher and longer than the ﬁrst. In between, 3 body posture returned to normal. The decrease after the second peak (ml l

-1 reﬂects fully emerged individuals (Suppl. material Fig. 1B) rather than 20 a return to normal posture. 2

DO conc. h 1 3.8. Atypical shell position: overturned Number of observations Initially, most housings were normally positioned (aperture facing 0 0 down: 97, 98 and 98% of observations during normoxia, mild and 1 12 23 1 12 24 36 48 60 72 84 93 moderate hypoxia, respectively). As DO values dropped further, Deployment duration (h) more shells were overturned, with highly signiﬁcant increases at

Fig. 3. Average number of locomotions per hour during deployment 11. n=3 crabs in severe hypoxia (from 2 to 17% of observations) and at anoxia (24%; open, n=4 in closed conﬁguration. White and black bars: day and night. White and Fig. 2D) (Suppl. material Table 1, Fig. 2B). The start and the number black arrowheads: emergence from shell and death, respectively (50% of individuals). of overturned shells differed according to housing: large crabs often 28 K. Pretterebner et al. / Journal of Experimental Marine Biology and Ecology 411 (2012) 23–33 started overturning at severe hypoxia (26% of observations), small 5 40 crabs (17%) and those in S. domuncula (38%) at anoxia. OpenP2 Closed configuration -1 h 4

) P1 30

3.9. Emergence from shell -1

(ml l 3 P. eremita remained inside its shell from normoxia to severe hyp- -1 oxia. Anoxia, however, triggered emergence (13 of 25 individuals; 20 52%). The crabs emerged between 38.9 and 71.3 h of combined hyp- 2 oxia and anoxia (mean=52.8 h; SD=8.5), whereby anoxia lasted P3 from 10.3 to 48.6 h (mean=24.4 h; SD=11.8). At emergence, 77% 10 DO conc. h of the crabs were positioned on the sediment and 62% of shells 1 were still upright. Once emerged, the crabs moved around. One Number of observations large individual (in M. brandaris) returned to its shell. Two images 0 0 (12 min) before it emerged for the first time (after 34 h of anoxia), 1 12 23 1 12 24 36 48 60 72 84 93 nearly the whole abdomen was visible. The crab remained emerged Deployment duration (h) for 24 min, moved around on top of its upright shell, turned around, and partially inserted its abdomen back into the aperture. It remained Fig. 4. Average number of interactions per hour during deployment 11. n=3 crabs in open, n=4 in closed configuration. White and black bars: day and night. White and inside for 12 min before finally emerging again and climbing on an black arrowheads: emergence from shell and death (50% of individuals). P1–3: interac- exposed infaunal sea urchin Schizaster canaliferus. tion peaks. Emergence was clearly a precursor to mortality: 8 of 13 crabs (62%) that abandoned their shell died. Three of the remaining emerged crabs survived due to relatively short anoxia in the deploy- directly, one its symbiotic sea anemone Calliactis parasitica (Couch, ments. Two other individuals disappeared from view and could not 1842). In the former, the crab initially used the sea urchin as an ele- be further evaluated. vated substratum for 5.3 h (Suppl. material Fig. 2C). It then emerged from its shell and climbed on top of the sea urchin, where it remained for 1.8 h. This was followed by a moribund, immobile state on the 3.10. Inter- and intraspecific interactions sediment with sporadic body movements (mortality 15.9 h after onset of the interaction). Short inter- and intraspecific interactions were difficult to observe In response to decreasing DO concentrations, the crabs P. longimana because photographs were taken in 6-min intervals. Nonetheless, often positioned themselves on hermit crab-occupied shells (maximum during normoxia, such interactions took place in 50% of observations, duration 23.2 h). No such interaction ever occurred during normoxia. largely due to individual, long-lasting contacts. During mild hypoxia During mild and moderate hypoxia, 11 and 15 interactions with both inter- and intraspecific interactions highly significantly in- individuals or aggregations of up to 9 individuals, respectively, were creased (57%) and remained stable during moderate and severe hyp- documented. During severe hypoxia and anoxia, the number of interac- oxia (57 and 56%, respectively) until anoxia caused a major drop to tions increased further to 43. 25% (Pb0.001, Suppl. material Table 1). Ophiura spp., a species normally partially buried in the sediment, Interaction patterns differed according to housing category. In also used shells to serve as elevated sites. During severe hypoxia small crabs a marked increase occurred at mild hypoxia (from 53 to 67% of observations) followed by a subsequent decrease until anoxia (19%; all changes Pb0.001, Suppl. material Table 2). In large crabs the Table 2 normoxic value was retained (43, 45 and 47% during normoxia, mild Interspecific interactions between hermit crabs and benthic invertebrates. and moderate hypoxia, respectively) until severe hypoxia triggered an increase to 63% and anoxia a drop to 29% (the latter two changes Class/Taxa Pb0.001, Suppl. material Table 3). In sponge-inhabiting crabs, inter- Anthozoa actions accounted for 61 and 63% of observations during normoxia Cereus pedunculatus (Pennant, 1777) and mild hypoxia, respectively. A highly significant increase occurred Epizoanthus arenaceus (Delle Chiaje, 1823) b Gastropoda at moderate hypoxia (92%), dropping at anoxia to 29% (P 0.001, Hexaplex trunculus (Linnaeus, 1758) Suppl. material Table 4). Murex brandaris (Linnaeus, 1758) During normoxia and mild hypoxia, more interactions occurred at Diodora sp. Bivalvia night. This changed at moderate hypoxia (more interactions during Corbula gibba (Olivi, 1792) the day). During severe hypoxia more interactions were again Polychaeta recorded at night, but this pattern reversed again during anoxia. The Protula tubularia (Montagu, 1803) day/night differences were highly significant during normoxia, Infaunal polychaete individuals moderate hypoxia and anoxia, and significant during mild and severe Crustacea Pisidia longimana (Risso, 1816) hypoxia (Suppl. material Table 5). These day/night differences are Pilumnus spinifer (Milne-Edwards, 1834) illustrated here using deployment 11: two major peaks (peak 1: Nepinnotheres pinnotheres (Linnaeus, 1758) normoxia/open configuration, peak 2: late hypoxia to early anoxia/ Macropodia sp. closed configuration) describe interactions that occurred at night Alpheus glaber (Olivi, 1792) (Fig. 4). The final, small peak (peak 3) represents a lengthier contact Holothuroidea Ocnus planci (Brandt, 1835) in which the crab crawled onto the sea urchin S. canaliferus. Echinoidea Psammechinus microtuberculatus (Blainville, 1825) 3.11. Interspecific interactions Schizaster canaliferus (Lamarck, 1816) Ophiuroidea Ophiothrix quinquemaculata (Delle Chiaje, 1828) Interspecific interactions with numerous organisms were docu- Ophiura spp. mented (Table 2). The interactions with the infaunal S. canaliferus, Ascidiacea Microcosmus sulcatus (Coquebert, 1797) for example, occurred during anoxia, one involving a hermit crab K. Pretterebner et al. / Journal of Experimental Marine Biology and Ecology 411 (2012) 23–33 29 and anoxia, these brittle stars were commonly observed on occupied migrated horizontally from deeper to shallower, better oxygenated shells. sites (Bell and Eggleston, 2005). Large numbers of the West Coast Most interspecific interactions (241) occurred with the brittle star rock lobster Jasus lalandii migrate shoreward during hypoxic condi- O. quinquemaculata. Initially, during normoxia, most individuals tions at high tides in the greater Elands Bay region (West coast, showed escape reactions when approached by a hermit crab: they South Africa), leading to mass strandings (Cockcroft, 2001). Hypoxic interrupted their filter-feeding posture and changed location. Slight bottom water affects the vertical distribution of fish and crustaceans contact between arm tips and crabs was recorded several times. At (mantis shrimp Squilla empusa and blue crab C. sapidus): migrations mild hypoxia, escape reactions ceased entirely although the number in the water column have been documented to avoid unfavourable and duration of interactions increased. During severe hypoxia, mori- conditions (Hazen et al., 2009; Pihl et al., 1991). bund brittle stars even clung to hermit crab shells: some were posi- Hermit crabs responded in a manner similar to a wide range of tioned on top of the shells and completely covered them (maximum bioherm-associated crustaceans from the Northern Adriatic Sea, duration: 21.7 h). Interactions ceased during anoxia because the which first emerged from their hiding places and then escaped to brittle stars had died. more elevated substrates (Haselmair et al., 2010). In the Northern Interactions with H. trunculus occurred during normoxia, ceased Adriatic Sea, sublittoral sediment bottoms are flat and extensive, so during mild hypoxia, re-occurred at severe hypoxia, and were the that hypoxia can affect several hundred to thousands of km2 highest during anoxia. (Stachowitsch, 1984). This makes it unlikely that benthic invertebrates can escape horizontally. Multi-species clumps represent the 3.12. Mortality only elevated structures, and oxygen concentrations were generally higher 20 cm above than directly at the bottom. The normally declin- Overall, 36% of the hermit crabs (n=9) died, all of them during ing oxygen gradients within the benthic boundary layer and increas- anoxia. These 9 individuals were observed in 3 different deployments ing concentrations into the water column (Diaz and Rosenberg, 1995; in which anoxia occurred faster and lasted longer (including one with Jørgensen, 1980) are therefore retained and strengthened during an intermediate oxygen peak). Hermit crabs died between 52.2 and oxygen crises. Migration onto multi-species clumps may therefore 73.5 h after combined hypoxia and anoxia (mean=60.9 h; SD=7.8), provide a refuge for hermit crabs from short-term hypoxia. whereby anoxia ranged from 18.4 to 62.1 h (mean=37.7 h; SD=16.6). Mild hypoxia induced such initial avoidance behaviour in hermit

At anoxia, H2S began to develop. Mortality occurred at H2Svaluesranging crabs. The same threshold was reported in most bioherm-associated from 116.5 to 248 μmol l−1 (mean=128.1 μmol l−1; SD=9.8). Mortali- crustaceans, apart from the more tolerant pea crab N. pinnotheres (a ty was highly significantly affected by H2S concentration and significantly symbiont in bivalves and sea squirts) and the nut crab E. tuberosa affected by the duration of anoxia (Pb0.001 and b0.05, respectively; (Haselmair et al., 2010). The blue crab Callinectes similis actively Suppl. material Table 6). At death, 8 of the 9 crabs (89%) were positioned detects and avoids DO below 2.3 ppm (Das and Stickle, 1994). The on the sediment and were outside their shell; 5 shells were overturned penaeid shrimp Metapenaeus ensis moves away from oxygen-depleted (Suppl. material Fig. 2D). The first shell-less individuals died 2.3 h and water (0.5 mg l−1 DO), seeking normoxic conditions (3.0 mg l−1 DO; the last 18.4 h after emergence (mean=8.4 h; SD=5.1). Wu et al., 2002). The white shrimp Penaeus setiferus and the brown shrimp Penaeus aztecus avoid hypoxic conditions below 1.5 and 3.13. Survival 2.0 ppm DO, respectively (Renaud, 1986). Physiological mechanisms to actively detect dropping oxygen concentrations (and thus impending Overall, 13 individuals (52%) survived 30.2 to 78.0 h of hypoxia hypoxia) (Breitburg, 1992) and to orientate towards more favourable and anoxia (mean=58.8 h; SD=14.15) (5 different deployments). conditions (Bell et al., 2003) are essential to successfully avoid hypoxia. Survivors were typically present only in deployments with shorter For example, the deep-water hermit crab Parapagurus pilosimanus anoxia ranging from 8.5 to 25.1 h (mean=20.8 h; SD=4.9) and adjusts its orientation and movement according to oxygen gradients − 1 lower final H2S concentrations (0 to 126.1 μmol l ; mean=42.2 - and currents (Rowe and Menzies, 1968). μmol l− 1; SD=48.2). Most of the survivors were on the sediment in The density of P. eremita was high during a recent transect survey upright shells (9 of 13: 69%), 4 were on multi-species clumps (1 inside on this Northern Adriatic sediment bottom (2.4 individuals m− 2), − 2 its shell). Ten crabs survived the relatively low H2S concentrations at slightly higher than in an earlier sampling (1.9 m ; Stachowitsch, the end of the respective deployments (maximum 21.0 μmol l−1), 1977). P. eremita is a highly mobile crustacean with an average although 3 of the 13 individuals experienced and survived much higher speed of 1.8 m h− 1 (Stachowitsch, 1979). Based on earlier time- values (121.1 μmol l−1). lapse films, these crabs were estimated to move 21.6 m per day but remained within a defined radius (based on the high tagging reloca- 4. Discussion tion rate). Locomotion is non-directed, apparently determined by multi-species clumps on the sediment and by intraspecific encoun- 4.1. Responses to decreasing dissolved oxygen concentrations ters. Hermit crabs responded to 2.0 ml l− 1 DO by moving less, and at 0.5 ml l− 1 DO locomotion ceased. This pattern was only briefly This study provides a detailed account of the full range of hermit interrupted by a locomotion spurt when shells were abandoned. crab behaviours during declining DO- and rising H2S concentrations Juvenile Norway lobster Nephrops norvegicus also alter their behav- and correlates the specific responses with oxygen thresholds. The re- iour pattern: the first response to 30% oxygen saturation is prolonged actions encompass a succession of stress behaviours and atypical inactivity (decreased walking and digging), with escape bursts of interactions, and many crabs died (Fig. 5). swimming (Eriksson and Baden, 1997). This may correspond with In situ observations and earlier time-lapse films confirm that P. the initial less-active behaviour of hermit crabs. eremita prefers the sediment surface between multi-species clumps Most crustaceans try to escape by first increasing their locomotion, (Stachowitsch, 1979). As oxygen levels fall, however, the crabs tend later becoming immobile at species-specific thresholds. Such reduced to migrate onto the higher multi-species clumps/bioherms, although locomotion may save energy needed for respiration (Johansson, 1997; most ultimately return to the sediment. Such avoidance is common Mistri, 2004). Other crustaceans in the Northern Adriatic Sea increase in crustaceans (Diaz and Rosenberg, 1995; Pihl et al., 1991; Renaud, their locomotion threefold with declining oxygen, followed by a de- 1986) and both vertical and horizontal migrations are strategies to crease between 1.0 and 0.5 ml l−1 DO (Haselmair et al., 2010). In the prolong survival. In the Neuse River Estuary (North Carolina, USA), brackish-water shrimps Palaemonetes varians (Hagerman and Uglow, trawl collections showed that the blue crab Callinectes sapidus 1984)andCrangon crangon (Hagerman and Szaniawska, 1986), ~50% 30 K. Pretterebner et al. / Journal of Experimental Marine Biology and Ecology 411 (2012) 23–33

A B C DE

5 140

120 ) -1 ) 4 -1 A 100

(ml l 3 80 (µmol l -1

normoxia -1 2 B 60 mild 40

1 D S conc. h DO conc. h moderate 20 2 C E H severe hypoxia 0 0 11224 36 48 60 78 anoxia (h)

Fig. 5. Sequence of behavioural responses to oxygen thresholds, anoxia duration and H2S development: (A) normal body posture, (B) crab extended from upright shell, (C) crab extended from overturned shell, (D) crab outside shell, and (E) mortality. Hours in pictograms are mean values from onset of anoxia. oxygen saturation caused restless swimming followed by immobility at approximately 24 h after anoxia. Extension increased (percentage of b10 mm Hg DO and 30% saturation, respectively. Hermit crabs atop bio- observations) up until anoxia, when nearly 50% of all crabs were herms and decreased locomotion were concurrent events at this extended. The number of overturned shells increased throughout an- threshold. The relatively small clumps (decimetre range), restrict loco- oxia. Before emergence, crabs showed only body movements, but no motion if the crabs want to stay on them. We therefore interpret de- locomotion. Emergence (after a mean of 26.9 h after anoxia) was creased locomotion and immobility as a combination of saving energy associated with a locomotion spurt or “escape movement” in almost and the relatively small elevated sites. all shell-less individuals. Such spurts lasted an average of 3.7 h. The overall activity pattern changed with decreasing oxygen con- Hermit crabs expend energy in carrying a shell (Herreid and Full, centrations. P. eremita is diurnal, being mostly active during the day 1986). During slow locomotion, for example, the land hermit crabs and moving less at night: lengthy stops during the night suggested Coenobita compressus in shells require twice as much oxygen as a resting phase (Stachowitsch, 1979). Thus, during a 12-h daytime shell-less individuals. We therefore interpret emergence along with phase, the number of individuals moving within the camera's field increased locomotion as a last attempt to escape in an energy- of vision in that study was fivefold compared to nighttime. The pre- saving mode. sent study confirms this activity pattern at normoxia. At the begin- Hermit crabs emerge from their shells when exposed to various ning and end of oxygen decline – during mild hypoxia and anoxia – environmental stresses: hypoxia (Riedel et al., 2008b; Stachowitsch, this day/night rhythm disappeared, but for different reasons. In the 1984), extreme thermal stress (Bertness, 1982), high temperature former case we interpret this to reflect activity both day and night and desiccation (Taylor, 1981), when pursued (Greenaway, 2003) to avoid the unfavourable conditions. In the latter case, moribund and during dredging (Young, 1979). The intertidal hermit crab animals during anoxia also showed no day/night difference. Altered Pagurus samuelis is commonly subjected to sedimentation, which activity patterns have been reported in other crustaceans in this com- can lead to oxygen deficiency (Shives and Dunbar, 2010). When bur- munity, for example P. longimana. That crab has a cryptic lifestyle and ied with the shell aperture facing upward, P. samuelis emerged from emerges only during nighttime (Haselmair et al., 2010). From moder- the shell in a laboratory study. In our study, nearly two-thirds of the ate hypoxia on, however, P. longimana was exposed and visible both emerged P. eremita did so from normally positioned shells, although day and night. shell position may be less important when shells are on the sediment As opposed to locomotion, body movements steadily increased as surface. Hypoxic conditions can also impact other shell-related be- DO values fell from 2.0 to 0.5 ml l− 1. Thereafter, during severe hypox- haviours, for example shell selection: crabs spend less time investi- ia and anoxia, such movements decreased. General body movements, gating new shells and inhabit significantly lighter and smaller shells combined with movements of the pleopods (Lancester, 1988) or ab- (Côté et al., 1998). This reduces shell volume, increases the predation domen (Gerlach et al., 1976), create water currents to deliver gill risk and impacts reproduction. Nonetheless, these drawbacks are respiratory current in the hermit crab Pagurus bernhardus; pleopod apparently outweighed, at least temporarily, by the reduced energy movements also remove faeces from inside the shell. Hypoxia triggers costs. increased pleopod beating in the Norway lobster N. norvegicus to ven- tilate the burrow (Gerhardt and Baden, 1998). Brief injection of hyp- 4.2. Housing-specific responses oxic water (2 ppm DO), however, did not change the pleopod beating rate in the hermit crab Dardanus arrosor (Innocenti et al., 2004). Shell size and weight affected avoidance behaviour onto multi- Certain crustaceans, in addition to pleopod movements, increase the species clumps and the number of interactions. Crabs in S. domuncula, frequency of scaphognathite beating when oxygen concentrations de- for example, spent the most time on such clumps and also had the cline (McMahon, 2001). Accordingly, body movements in P. eremita most interactions, followed by small and then large individuals. are interpreted as a response to hypoxic stress but may also create ad- Large shells were overturned already during severe hypoxia, while ditional water movement inside the shell. small shells and S. domuncula-covered shells were first overturned Decreasing oxygen initiated a sequence of different postures and during anoxia. This may reflect the difficult-to-overturn form of the an altered shell position: extension from the shell during mild hypox- A. pespelecani shells occupied by smaller crabs or different tolerances ia, overturned shells during severe hypoxia and emergence beginning of juvenile versus adult crustaceans to hypoxia (although juveniles K. Pretterebner et al. / Journal of Experimental Marine Biology and Ecology 411 (2012) 23–33 31 are typically more sensitive; Eriksson and Baden, 1997). Weight may 4.4. Mortality and tolerance also play a role (Bridges and Brand, 1980): larger individuals (with heavier shells) climbed less onto elevated substrates. They may re- Among the hypoxia-sensitive crustaceans (Gray et al., 2002; quire less oxygen and also save energy by remaining on the sediment. Theede et al., 1969), P. eremita is relatively tolerant of oxygen defi-

The pronounced upward movement of S. domuncula-inhabiting crabs ciencies and H2S. Hermit crabs first died during prolonged anoxia may be an attempt to keep the sponges alive. This escape onto bio- (mean=37.7 h). During a mass mortality event in 1983 in the Gulf herm refuges may explain the increased interactions (see below). of Trieste, few crabs were alive on the 5th day after the onset of The opposite occurred in C. parasitica-covered (mostly larger) shells: hypoxia/anoxia, and on day 7 no living individuals were observed normally, the sea anemone not only provides protection (Williams (Stachowitsch, 1984). Previous studies with hermit crabs have mainly and McDermott, 2004) but may also reduce interspecific interactions. involved laboratory experiments and dealt with short-term exposure The responses of S. domuncula occupants differed the most, to low DO, focusing on survival. Pagurus spp. survived 0.6 mg l− 1 DO including intensified body movements, higher frequencies of exten- for one hour and was then returned to normal oxygen conditions sion from the shell as well as an earlier and more constant decrease (Marshall and Leverone, 1994). Other hermit crabs survived burial of locomotion. The oxygen content in the sponge tissue is only 50 to (and the related hypoxia) for 12 h (Shives, 2010). The intertidal 60% that of the surrounding water (Gatti et al., 2002) and the “aper- hermit crab Clibanarius vittatus typically survived 5.5 h in oxygen- ture” is typically tight fitting; this may reduce water exchange and free seawater (Wernick and Penteado, 1983). prompt the crab to extend out more often during moderate hypoxia A general critical DO concentration for the onset of benthic mor- and anoxia and increase body movements during mild, moderate tality is 1 ml l− 1, with wide-ranging mortality at about 0.5 ml l− 1 and severe hypoxia. (Diaz and Rosenberg, 1995). These thresholds, however, might be underestimated for crustaceans: recent approaches suggest a median − 1 lethal concentration (LC50) of 2.45 mg l DO (SD=0.14), with a me- 4.3. Atypical interactions dian lethal time (LT50) of 55.5 h (SD=12.4) (Vaquer-Sunyer and Duarte, 2008). Generally, crustaceans and fishes are more sensitive Interactions with other species increased during hypoxia but to hypoxia, whereas molluscs, cnidarians and priapulids are relatively then decreased during anoxia. Hypoxia dampened reactions upon tolerant (Vaquer-Sunyer and Duarte, 2008). Beyond this broad an encounter (no avoidance, shorter flight distances of other pattern, tolerance differs among species within a particular taxonom- organisms), initiated interactions with normally hidden or buried ic group. Bioherm-associated crabs such as P. longimana and Galathea organisms, increased interactions with other organisms aggregating spp. are sensitive, while E. tuberosa and N. pinnotheres are more on elevated substrates, and caused organisms to climb onto crab- tolerant, the latter two dying last (after 34.2 and 78 h of anoxia, occupied shells. respectively; Haselmair et al., 2010). Interestingly, interaction peaks did not correspond with crab lo- Mortality was significantly dependent on the duration of anoxia comotion peaks. During normoxia, most interactions occurred at but highly significantly on H2S levels. Tolerance to oxygen depletion night (crabs day-active). At mild hypoxia, interactions increased alone is difficult to study because anoxia is generally accompanied and remained high throughout severe hypoxia. Crab locomotion, in by H2S(Vismann, 1991). This additional impact reduces survival contrast, decreased during mild hypoxia and again during severe time by an average of 30% (Vaquer-Sunyer and Duarte, 2010). Hermit − 1 hypoxia. At anoxia, interactions were halved and most crabs were crabs died at a mean of 128.1 μmol l H2S. In the examined benthic moribund. Most interactions were apparently initiated by nocturnal community, however, mortalities could often be attributed to oxygen organisms, by hypoxia-induced increased activity of other organisms depletion alone: many organisms, including the crab P. longimana, during the day, and by the artefact of long-lasting contact with and died before H2S developed. Others such as N. pinnotheres tolerated between moribund organisms. higher H2S concentrations (Haselmair et al., 2010) or are physiologi- Finally, the range of potential interactions was broadened by emerging cally adapted to hypoxic environments with high sulphide concentra- infauna species, e.g. a hermit crab climbing onto S. canaliferus (Pados, tions (sediment-burrowing Thalassinidea) (Johns et al., 1997). 2010) and by the occupied shells themselves becoming an atypical sub- Tolerance depends on behavioural and physiological adaptations stratum for other species. The brittle stars Ophiura lacterosa and O. (Wu, 2002). In our study, the former included migration to better- quinquemaculata normally escape when hermit crabs approach, oxygenated bioherms and reduced energy consumption (immobility pointing to predator–prey relationships (Stachowitsch, 1979; Wurzian, and emergence from shells). Physiologically, hermit crabs may have 1982). During severe hypoxia and anoxia, however, these brittle evolved – in analogy to sediment-burrowing crustaceans – to be stars positioned themselves on the shells. The crab P. longimana also somewhat more tolerant to low DO concentrations than other, aggregates on any available elevated structures, such as ascidians free-living crustaceans because they live in tight-fitting, imperme- (Haselmair et al., 2010). Such hypoxia-induced movements to and able calcareous shells. C. vittatus, for example, is an “oxygen con- aggregations in oxygenated refuge habitats that increase the former” that lowers its oxygen uptake when values fall (Wernick number of biological interactions have been reported elsewhere and Penteado, 1983). P. samuelis meets the demand for aerobic respi- (Lenihan et al., 2001). The ultimate effect of such spatial overlaps is ration with anaerobic fermentation, which is indicated by increasing determined by the relative tolerance of predator and prey (Breitburg lactate levels in the hemolymph, after burial-induced hypoxia et al., 1994; Kolar and Rahel, 1993). In the Northern Adriatic, for (Shives, 2010). example, the sea anemone C. parasitica consumed less tolerant, moribund O. quinquemaculata in our deployments (Riedel et al., 2008a). 4.5. Symbionts Crustaceans, however, are more sensitive than anemones, and P. eremita was never observed to consume moribund prey at hypoxia. Conversely, During normal oxygen conditions, the tentacle crown of the well- hermit crabs were not consumed by other organisms in our enclosed ex- known hermit crab symbiont C. parasitica is open and directed down- periments. In a more extensive, natural hypoxia/anoxia event, however, wards, towards the sediment, sweeping the bottom as the crabs move a much wider range of potential larger predators would be present and (Fig. 6 in Stachowitsch, 1980). The anemone responds to hypoxia by predation in certain phases cannot be excluded. directing the open tentacle crown upwards (Riedel et al., 2008a; Finally, during anoxia, interactions decreased and involved 2008b). As oxygen decreases, individuals start rotating their crown. Dur- more tolerant gastropods (H. trunculus) and emerged sea urchins ing anoxia, the crown is contracted and again faces down. Ultimately, (S. canaliferus). some individuals detach from the shell (Jørgensen, 1980; Riedel et al., 32 K. Pretterebner et al. / Journal of Experimental Marine Biology and Ecology 411 (2012) 23–33

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