Perturbations associated with earthquakes, such as changes in atmospheric pressure and gravity, ground deformations, acoustic signals and vibrations due to micro-crack generation, and the concurrent emission of gases and chemical substances, have been documented to cause unusual animal behaviors or UABs (^{Hayakawa, 2013}). Moreover, ^{Freund & Stolc (2013)} showed that electronic charge carriers (called positive holes) were activated during the build-up of stress deep in the Earth before earthquakes. These positive holes can travel fast and far into the surrounding rocks, generating ultralow frequency electromagnetic waves, which may ionize air and generate hydrogen peroxides at the sea surface (^{Freund & Stolc, 2013}). The latter authors suggested that the momentary disappearance of the toad Bufo bufo Linnaeus, 1758 in central Italy about 5 days before the L’Aquila earthquake of April 2009 (^{Grant et al. 2011}) was triggered by chemical cues in the water, such as the release of hydrogen peroxide products resulting from an influx of hydron (cationic form of atomic hydrogen). Other variables related to earthquakes may be at play in the ocean, such as telluric currents produced through electromagnetic induction by the geomagnetic field or whenever a conducting body (e.g. seawater) moves because of tides or other processes across the Earth’s permanent magnetic field (^{Lanzerotti & Gregori, 1986}). These low-frequency currents travel large distances near the surface of the Earth (^{Lanzerotti & Gregori, 1986}). In addition, amplitude anomalies of the telluric field have been detected before shallow earthquakes with magnitudes ≥ 4.5. These amplitude anomalies reached about 100-300 mv km^-1 3-16 days before the earthquake (^{Myachkin et al. 1972}). All the above-mentioned earthquake-related changes (^{Nagao et al. 2002}) have been identified as sources of UAB (^{Freund & Stolc, 2013}), including in aquatic animals (^{Tributsch, 1982}). For instance, geomagnetic sensitivity was related to stranding of dolphins and whales (^{Kirschvink, 1990}), and of Humboldt squids (^{Than, 2009}).

Here we describe for the first time an UAB in a member of the Echinodermata phylum and a side effect recorded post-earthquake. Monitoring the seafloor down to 5 m occurred during daily fishing activities in Belize (Central America) that included sea-cucumber harvests south of Laughing Bird Cay and in Port Honduras in 2009 and 2018, respectively. On January 9, 2018 at 21:51, an earthquake of magnitude 7.6-7.8 was recorded 200 km off the Caribbean coast of Belize. Aftershocks of magnitude 4.2-5.0 followed in the evening and early the following day. The earthquake had a shallow focus (~10 km) which amplified its effects, i.e., increased shaking/vibration levels (^{USGS, 2018}). No tsunami was generated. In the morning of January 10, around 07:00, two species of coral-reef sea cucumbers, Isostichopus badionotus Selenka, 1867 and Holothuria mexicana Ludwig, 1875, underwent a mass-spawning event involving thousands of individuals (from the shore to ~5 m depth) in the entire Fore Reef (about 3.2 km²). This spawning was uncharacteristic in that it was outside the normal reproductive season and occurred at sunrise rather than at sunset, as is usually the case (^{Mercier et al. 2007}). While these sea cucumbers normally shelter in sand, seagrass, or among corals (especially from early morning to late afternoon), most individuals had emerged and displayed their typical spawning posture while broadcasting gametes in the water column (Fig. 1) in full daylight (the inverse of normal behavior). Inside a shoal area measuring 3.2 km long by about 1 km wide (16°12’05.61” N; 88°40’04.97” W), which was composed of patch reef and seagrass, nearly 100% of females and males (representing a maximum of 1-3 ind m^-2) spawned simultaneously for a duration of about 2 h (sexes determined based on the distinct appearance of male and female gametes in the water column). The water temperature was 30.9 °C, the salinity 33.6, with a recorded rainfall of 34.5 mm over the previous 24 h. There was no correlation with the full or new moon (as spawning after the earthquake occurred 2 days after the third moon quarter), or with the low or high tide or any Sea cucumbers spawning after earthquake broad environmental factor known to coincide with the spawning period of those species in the study region, including a phytoplankton bloom (^{Rogers et al. 2018}).

Figure 1 Close-up view of a spawning a male and b female of H. mexicana Ludwig, 1875 (photos Arlenie Rogers); c broader view of a spawning male of H. Mexicana (photo Víctor Alamina). Scale bar: 3 cm in a, 1.7 cm in b, 5 cm in c 

Despite a growing number of reports indicating that certain terrestrial and marine species can display unusual behaviors before and during earthquakes (^{Bhargava, 2009}), to our knowledge no impact on the timing of reproduction has ever been documented in the animal kingdom. The present observation may also be the first to show the influence of an earthquake on a non-vertebrate marine benthic taxon. While explaining correlations of this magnitude is complex, it may be suggested that sea cucumbers are able to detect seismic events directly or indirectly, through anomalous telluric current changes, changes in atmospheric pressure, and changes in gravity, acoustic signals, vibrations, or ultralow frequency electromagnetic waves generated by earthquakes (^{Hayakawa, 2013}). Exposure to physical stress is a common technique used to artificially induce sea cucumbers to spawn in captivity, especially in aquaculture facilities (^{Mercier & Hamel, 2009}), and it can be assumed that earthquakes generate similar stressors. Despite the present spawning event was only confirmed from one area of the extensive coral reef system of Belize, it is not impossible that the phenomenon occurred in other locations along the coast. An anecdotal correlation between mass spawning of the same sea cucumber species and an earthquake was observed in Belize ~4 hours after the 7.3 magnitude earthquake that occurred at 03:24 on May 28, 2009 (Héctor Saldívar, pers. comm., September 2011), supporting this assertion and the potential disruptive impact of earthquakes on sea cucumber reproduction. If non-mature gametes are released in full daylight under out-of-phase conditions, development failure and higher predation pressure are likely to exacerbate propagule mortality rates.

Overall, the reaction to earthquake documented here may be interpreted as a strategy developed by slow-moving species to survive dangers by releasing dispersive pelagic propagules. It remains unclear how sea cucumbers might detect seismic events, although electric or electromagnetic sensitivity are probable candidates.