Hermit crabs and their symbionts

Einsiedlerkrebse spielen aufgrund ihrer Häufigkeit, Vielzahl an Symbionten und Strukturierung der gesamten Lebensgemeinschaft in der Nordadria eine wesentliche Rolle. Mit einem speziellen Unterwasserinstrument (EAGU – experimental anoxia generating unit) wurde auf einem sublitoralen Sedimentboden in 24 m Tiefe im Golf von Triest kleinflächig Hypoxie und Anoxie erzeugt, um so die Auswirkungen von abnehmendem Sauerstoff auf das Verhalten des Einsiedlerkrebses Paguristes eremita aufzuzeichnen. Diese Verhaltensänderungen wurden mit fünf Sauerstoffgrenzwerten in Bezug gesetzt und stellten eine Abfolge von atypischen Reaktionen, letztlich Mortalität dar. Bei Normoxie (≥2,0 ml l-1 DO) wurde ein für Einsiedlerkrebse typisches Verhalten festgestellt. Während milder Hypoxie (2,0 bis 1,0 ml l-1 DO) versuchte P. eremita geringe Sauerstoffkonzentrationen zu vermeiden und flüchtete auf höher gelegene, sauerstoffreichere Bioherme. Dieses Verhalten wurde von einer Reihe von Reaktionen begleitet: verringerte lokomotorische Aktivität, erhöhte Körperbewegungen und ein Herausstrecken aus der Schale, sodass Teile des weichen Carapax' sichtbar wurden. Zuvor initiierte Verhaltensänderungen waren auch während moderater Hypoxie (1,0 bis 0,5 ml l-1 DO) sichtbar. Fortgeschrittene Hypoxie (0,5 bis 0,01 ml l-1 DO) war von einer subletalen Phase gekennzeichnet: Die meist unbeweglichen Krebse befanden sich in umgedrehten Gehäusen mit nach oben gerichteten Öffnungen und reduzierten ihre Körperbewegungen. Einsiedlerkrebse verließen ihr Gehäuse während Anoxie (0 ml l-1 DO) und versuchten zu flüchten. Der Tag-Nacht-Rhythmus der normalerweise tagaktiven Einsiedler war während milder Hypoxie und Anoxie verändert. Atypische interspezifische Interaktionen wurden beobachtet z.B. von milder Hypoxie bis Anoxie war die Krabbe Pisidia longimana meist auf Einsiedlerkrebsgehäusen aggregiert. Nach längerer Anoxie (~1,5 Tage) und einer erhöhten Schwefelwasserstoffkonzentration (~128 µmol) lösten diese beiden Faktoren den Tod der Krebse aus. Einsiedlerkrebse sind im Vergleich zu den meist wenig hypoxieresistenten Crustaceen relativ tolerant gegenüber Sauerstoffkrisen. Dies unterstreicht die Bedeutung der Einsiedlerkrebse als Überlebende kurz andauernder Hypoxie und Wiederbesiedler der beeinträchtigten Gebiete.Hermit crabs play an important role in the Northern Adriatic Sea due to their abundance, wide range of symbionts, and function in structuring the benthic community. Among the crustaceans, which are a sensitive group to oxygen depletion, the hermit crab Paguristes eremita is relatively tolerant. A specially designed underwater device (EAGU – experimental anoxia generating unit) was deployed to artificially create and document small-scale hypoxia and anoxia (0.25 m2) on a sublittoral soft bottom in 24 m depth in the Gulf of Trieste and enabled studying the behaviour and mortality of the hermit crab P. eremita. The crabs exhibited a sequence of atypical responses and ultimately mortality, which was correlated with five oxygen thresholds. Initially, at mild hypoxia (2.0 to 1.0 ml l-1 DO), hermit crabs showed an avoidance response by moving onto better oxygenated, elevated substrata. This was accompanied by a series of responses including decreased locomotory activity, increased body movements and extension from the shell. During a moribund phase at severe hypoxia (0.5 to 0.01 ml l-1 DO), crabs were mostly immobile in overturned shells and body movements decreased. Anoxia triggered emergence from the shell, with a last brief “escape movement” of shell-less crabs. The activity pattern of normally day active crabs was altered during mild hypoxia and anoxia. Atypical interspecific interactions were initiated: the crab Pisidia longimana increasingly aggregated on hermit crab shells from mild hypoxia to anoxia. Response patterns partially varied according to the housing of crabs. Mortality occurred after extended anoxia (~1.5 d) and increased hydrogen sulphide levels (H2S ~128 µmol). This study emphasizes the important role of relatively tolerant hermit crabs in this benthic community as potential survivors and recolonizers of affected areas

Effect of hypoxia and anoxia on invertebrate behaviour: Ecological perspectives from species to community level

Coastal hypoxia and anoxia have become a global key stressor to marine ecosystems, with almost 500 dead zones recorded worldwide. By triggering cascading effects from the individual organism to the community-and ecosystem level, oxygen depletions threaten marine biodiversity and can alter ecosystem structure and function. By integrating both physiological function and ecological processes, animal behaviour is ideal for assessing the stress state of benthic macrofauna to low dissolved oxygen. The initial response of organisms can serve as an early warning signal, while the successive behavioural reactions of key species indicate hypoxia levels and help assess community degradation. Here we document the behavioural responses of a representative spectrum of benthic macrofauna in the natural setting in the Northern Adriatic Sea (Mediterranean). We experimentally induced small-scale anoxia with a benthic chamber in 24 m depth to overcome the difficulties in predicting the onset of hypoxia, which often hinders full documentation in the field. The behavioural reactions were documented with a time-lapse camera. Oxygen depletion elicited significant and repeatable changes in general (visibility, locomotion, body movement and posture, location) and species-specific reactions in virtually all organisms (302 individuals from 32 species and 2 species groups). Most atypical (stress) behaviours were associated with specific oxygen thresholds: arm-tipping in the ophiuroid Ophiothrix quinquemaculata, for example, with the onset of mild hypoxia (< 2 mL O2 L-1), the emergence of polychaetes on the sediment surface with moderate hypoxia (< 1 mL O 2 L-1), the emergence of the infaunal sea urchin Schizaster canaliferus on the sediment with severe hypoxia (< 0.5 mL O 2 Lg-1) and heavy body rotations in sea anemones with anoxia. Other species changed their activity patterns, for example the circadian rhythm in the hermit crab Paguristes eremita or the bioherm-associated crab Pisidia longimana. Intra-and interspecific reactions were weakened or changed: decapods ceased defensive and territorial behaviour, and predator-prey interactions and relationships shifted. This nuanced scale of resolution is a useful tool to interpret present benthic community status (behaviour) and past mortalities (community composition, e.g. survival of tolerant species). This information on the sensitivity (onset of stress response), tolerance (mortality, survival), and characteristics (i.e. life habit, functional role) of key species also helps predict potential future changes in benthic structure and ecosystem functioning. This integrated approach can transport complex ecological processes to the public and decision-makers and help define specific monitoring, assessment and conservation plans. © 2014 Author (s).This study was financed by the Austrian Science Fund (FWF; projects P17655-B03 and P21542-B17) and supported by the OEAD Bilateral Slovenian Austrian Scientific Technical Cooperation project SI 22/2009Peer Reviewe

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

Hermit crabs play an important role in the Northern Adriatic Sea due to their abundance, wide range of symbionts, and function in structuring the benthic community. Small-scale (0.25 m2) hypoxia and anoxia were experimentally generated on a sublittoral soft bottom in 24 m depth in the Gulf of Trieste. This approach successfully simulates the seasonal low dissolved oxygen (DO) events here and enabled studying the behaviour 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, which are a sensitive group to oxygen depletion, P. eremita is relatively tolerant. Initially, at mild hypoxia (2.0 to 1.0 ml l− 1 DO), hermit crabs showed avoidance by moving onto better oxygenated, elevated substrata. This was accompanied by a series of responses including decreased locomotory activity, increased body movements and extension from the shell. During a moribund phase at severe hypoxia (0.5 to 0.01 ml l− 1 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