The characterization of the internal microenvironment of symbiotic marine invertebrates is essential for a better understanding of the symbiosis dynamics. Microalgal symbionts (of the family: Symbiodiniaceae) influence diel fluctuations of in host O₂ and pH conditions through their metabolic activities (i.e., photosynthesis and respiration). These variations may play an important role in driving oxygen budgets and energy demands of the holobiont and its responses to climate change. In situ measurements using microsensors were used to resolve the O₂ and pH diel fluctuations in the oral arms of non-calcifying cnidarian model species Cassiopea sp. (the “upside-down jellyfish”), which has an obligatory association with Symbiodiniaceae. Before sunrise, the internal O₂ and pH levels were substantially lower than those in ambient seawater conditions (minimum average levels: 61.92 ± 5.06 1SE μmol O₂ L^–1 and 7.93 ± 0.02 1SE pH units, respectively), indicating that conditions within Cassiopea’s oral arms were acidified and hypoxic relative to the surrounding seawater. Measurements performed during the afternoon revealed hyperoxia (maximum average levels: 546.22 ± 16.45 1SE μmol O₂ L^–1) and internal pH similar to ambient levels (8.61 ± 0.02 1SE pH units). The calculated gross photosynthetic rates of Cassiopea sp. were 0.04 ± 0.013 1SE nmol cm^–2 s^–1 in individuals collected at night and 0.08 ± 0.02 1SE nmol cm^–2 s^–1 in individuals collected during the afternoon.

Persistent oxygen-depleted zones in the ocean are known primarily from enclosed basins in temperate regions or the open ocean (including oxygen minimum and limiting zones) (1). However, little is known about the possibility of such zones forming in tropical coastal domains, even though the combination of warmer temperatures and complex geological features in some tropical regions makes their occurrence more likely (1, 2). Here, we report two subsurface oxygen-depleted zones within deep (>490 m) depressions of the Red Sea's Difaht Farasan—a carbonate platform hosting the world's third largest contiguous tropical coral reef system. One zone maintains suboxic oxygen levels (∼11–14 µmol O2 kg⁻¹), while the other sustains oxygen levels below detection (21 °C). Targeted exploration of deep tropical coastal environments is crucial for determining whether similar zones exist beyond the Red Sea and understanding their potential responses to climate change.

Immobilization of marine invertebrates for research purposes has been commonly used and is often necessary to obtain high-quality findings. Despite these approaches being standard procedures, they can affect the specimens, and their responses as well. The effectiveness of magnesium chloride (MgCl₂) to “narcotize” marine invertebrates has been recognized since 1946. Here, we determine the concentration of MgCl₂ that immobilizes Cassiopea sp., at jellyfish stage, effectively and quickly, without causing mortality. The specimens were exposed to different concentrations of MgCl₂, until the range was narrowed, and the most effective concentration was found (0.092M). In the final experiment, the jellyfish (~ 8.68 cm ± 1.70 SD of diameter) were exposed to a 0.092M MgCl₂ solution for 2 and 24 hours. Before, during, and after the exposure we monitored the following biological responses: (1) cessation of bell pulsations, (2) responsiveness to bell-tap stimulus, (3) photochemical efficiency, (4) bell pulsation, (5) recovery time, and (6) survival after 24 and 48 hours from exposure. Environmental conditions (pH and dissolved oxygen, dO₂) were monitored as well. Our findings indicate that, despite being an effective concentration for immobilization of Cassiopea sp. at jellyfish stage, their bell pulsations were significantly affected both after 2 and 24 hours when comparing the values measured in the “pre-incubation” and “immediate post-incubation” phases, whereas bell size was significantly affected only in the 24 hours incubation. In conclusion, we suggest that this concentration can be used to immobilize Cassiopea sp.at jellyfish stage for short-term analyses. However, the limitations related to the use of this chemical should be reported, as it could lead scientists to misleading conclusions.