Pyrosomes, Pyrosoma atlanticum: Highlighting plankton as an important food source for coral reefs in Timor-Leste

Coral reefs in Timor-Leste are a hotspot of marine biodiversity within the Coral Triangle, a region encompassing six member states in Southeast Asia and the Pacific, housing the highest levels of marine biodiversity globally. Timor-Leste lies between Indonesia and Australia in the Indonesian Throughflow, a significant oceanographic feature connecting the Pacific and Indian Oceans. The Throughflow plays an essential role in regulating global climate, and we claim that it provides food to coral reefs in Timor-Leste and creates conditions favorable for corals in the context of ocean warming. The Ombai and Timor Straits, sitting to the north and south of Timor-Leste, are important outflows of the Throughflow with a respective 3-year mean transport of 4.9 and 7.5 Sv (1 Sv = 10⁶ m³ s⁻¹) from 2004 to 2006 (Gordon et al., 2010).

Coral reef surveyors (The University of Queensland) identified a bloom of pyrosomes along the north coast of Timor-Leste in September–October of 2019 (Figure 1a, Video S1). The water was filled with spiky, white and red pyrosomes, Pyrosoma atlanticum (Péron, 1804) (Tunicata, Thaliacea), an open water (pelagic) and planktonic, free-living colonial tunicate as far as the eye could see. Pyrosome blooms were identified at two different sites, Ataúro Island and Be'hau, about 2 weeks apart. At both sites, high current conditions swept the pyrosomes up onto shallow (<10 m) coral reefs. At Be'hau, pyrosomes were spotted “stuck” onto two individual corals of two different species: Hydnophora cf. pilosa and Duncanopsammia peltata (Arrigoni et al., 2014; Kelley, 2022). There was considerable extension of mesenterial filaments of H. pilosa (Figure 1b) which are used to digest food inside or outside of the coral mouth within a polyp. At both sites, pyrosomes were numerous, drifting from the deep blue depths to the surface (Figure 1a, Video S1).

The pyrosomes were likely swept up to shallower depths through intrusive upwelling of the Indonesian Throughflow current that raised the local oceanic thermocline above the depth of the shelf break without breaking the sea surface (Furnas, 2011). Because intrusive upwelling does not reach the sea surface, it is undetectable via satellites, resulting in an underestimation of the importance to thermal energy flow and marine biology. Pyrosomes are typically found in global oceans from 50° N to 50° S, primarily in deeper ocean environments >75 m depths (Figure 2; Appendix S1; Kim, 2025). They follow a typical planktonic daily migration pattern, ascending during the night to feed and migrating a vertical distance of nearly 1 km each day. During the day, pyrosomes are found at >75 m depths (Andersen & Sardou, 1994), and observations in Timor-Leste were during the day on shallow reefs, contrary to the established daily migration. The observations coincide with seasonal upwelling in Timor-Leste, demonstrated by satellite-derived chlorophyll a data (Figure 3; Appendix S2; Kim, 2025). Anecdotal evidence from dive operators suggests that it is common to see pyrosome blooms around Ataúro Island, seasonally from September to November (A. S. Bin Haron, personal communication, November 16, 2023; Figure 1a). The pyrosome bloom further supports upwelling as P. atlanticum prefers water temperatures <18°C (Lilly et al., 2023) and the annual temperature range on shallow reefs is 21.3–32.2°C (Kim et al., 2022; PIFSC, 2017). Kuo et al. (2015) reported pyrosomes on reefs in Taiwan where densities up to 1500 per m² were observed at the “sandy bottom of a reef edge at 10 m depth.” The authors posit that the pyrosomes were swept onto reefs from a typhoon, or tropical cyclone, indicating that waves and storms can sweep pyrosomes onto shallow reefs. It is unlikely that similar storm activity would sweep pyrosome blooms shallower in Timor-Leste as its equatorial location prevents cyclonic activity reaching its coasts (Kim, 2021). Thus, the oceanography of the region is a more probable cause.

During pyrosome blooms, pyrosomes can be an energy-rich food source in nutrient-limited ocean environments based on photographic and video observations. For example, when pyrosomes die, they quickly sink and accumulate in deep ocean bottom habitats where organisms including crabs, seastars, sea spiders, urchins, and anemones have been observed to feed on pyrosomes (Archer et al., 2018; GBIF.org, 2023; Lebrato & Jones, 2009). Pyrosomes have the highest dry mass of carbon for gelatinous plankton (Lebrato & Jones, 2009), and organisms take advantage of this easy, energy-rich food source. Pyrosome blooms and falls have the potential to play a significant role in oceanic carbon cycling (Lebrato & Jones, 2009; Lilly et al., 2023).

Could pyrosomes be an important seasonal food source for shallow reefs in Timor-Leste? The existence of such biodiverse ecosystems such as coral reefs in nutrient-poor waters is a conundrum that even Darwin pondered (Darwin, 1889). Corals receive food from their photosynthesizing, symbiotic dinoflagellate algae and heterotrophically by filtering food particles from the water using their coral polyps with anemone-like tentacles. Pyrosomes are larger than corals' typical heterotrophic food sources, which are usually in the pico- to meso-plankton range (0.2 μm to 1 mm; Houlbrèque & Ferrier-Pagès, 2009), while the observed pyrosomes were up to 20 cm long. Our observations of corals consuming pyrosomes are consistent with reports of corals eating larger organisms, such as sea slugs, salps, and jellyfish (Table 1). Consequently, the food size range for coral heterotrophic feeding should be expanded.

Corals can digest food externally (extracoelenteric digestion) through their mesenterial filaments, as observed here, which seems to provide corals with additional nutrients (Andersen & Sardou, 1994). Reviewing images along 15-m surveys conducted during the same field trip, pyrosomes were observed on four transects between both sites, and only on those transects photographed on the same days, the pyrosome blooms were observed (Appendix S3). A density of 58.9 pyrosomes per 100 m² was calculated, much more than the 1.8 per 100 m² from video surveys of the coastal to deep-sea environments off the Atlantic Ivory Coast (Lebrato & Jones, 2009). However, much more area was covered on the video surveys (13,000 m²) across a diversity of habitats compared to the 165 m² assessed photographically in Timor-Leste. On one transect in Be'hau, two more pyrosomes were observed interacting with hard and soft corals (Appendix S3: Figure S1) despite Be'hau having lower coral cover at 15.6 ± 5.2% (mean ± standard error) compared to 54.2 ± 5.4% at Haruina on Ataúro Island (unpublished data Kim). When scaling up the three observations reported here (Figure 1) to the whole reef, an estimated 0.91 pyrosomes are consumed per 100 m² of reef per hour during blooms (Appendix S3: Table S2). This is comparable to the pyrosome consumption rate calculated from video surveys of 0.48 pyrosomes per 100 m² (Lebrato & Jones, 2009). The presence of a bloom of pyrosomes is indicative of a plankton-rich environment as pyrosomes themselves are voracious consumers of phytoplankton with some of the highest clearance rates recorded (Drits et al., 1992). A bloom of pyrosomes suggests ample food availability for the pyrosomes, as demonstrated by the chlorophyll data (Figure 3), which also supports a community of zooplankton that are not easily visible in situ (Video S1) that the corals can take advantage of. Corals are voracious predators of zooplankton consuming 0.5–2 prey items per polyp per hour of ingestion (Sebens et al., 1996), and further research is needed to quantify in situ feeding rates of corals consuming plankton in Timor-Leste.

This seasonal availability of planktonic food may enable Timor-Leste corals to overcome stressors such as marine heatwaves. Heterotrophic feeding has been shown to support corals during bleaching (Grottoli et al., 2006), and experimental work demonstrates that fed corals have faster recovery rates from bleaching, twofold calcification rates, and twofold greater photosynthetic rates per unit skeletal area (reviewed in Houlbrèque & Ferrier-Pagès, 2009). Timorese reefs have not experienced mass bleaching and subsequent coral mortality in the last decade, unlike other reef regions such as the Great Barrier Reef and the Caribbean (Hughes et al., 2018). Comparisons between in-water temperatures and satellite sea surface temperatures in Timor-Leste show a divergence during the austral summer, where in-water temperatures are sometimes >1°C cooler, meaning coral bleaching stress is less than predicted (Kim et al., 2022). In this case of Timor-Leste, the oceanographic context of the Throughflow helps regulate water temperatures to prevent mass coral bleaching, in addition to providing nutrient-rich waters and food, further supporting corals to deal with stressors. Stable isotope analysis could identify the energy contributions from feeding on plankton compared to photosynthates from the algal symbionts.

The upwelling of cooler deeper waters in association with the Throughflow brings both positive and negative impacts to reef functioning. The negative effects on reef physiology start with the cooler, deeper waters being more acidic (hypercapnic) than surface waters. This acidity makes it more difficult for corals to grow their calcium carbonate skeletons, which dissolve in acid. Timor-Leste had lower than average seawater pH and aragonite saturation state values, in addition to one of the lowest calcification rates (0.045–0.091 g CaCO₃ cm⁻² year⁻¹) compared to 180 sites across the Pacific (0.024–3.776 g CaCO₃ cm⁻² year⁻¹) surveyed by the US National Oceanic and Atmospheric Administration (PIFSC, 2017). Feeding has been demonstrated as a way corals cope with more acidic conditions (Houlbrèque et al., 2015; Towle et al., 2015). The acidic conditions could impact corals through slowing growth rates. Research examining coral cores, growth experiments, and/or calcification measurements is fundamental to establish the impacts of the Throughflow in terms of creating a more acidic environment.

Feeding on pyrosomes represents a mechanism that may make Timorese coral reefs more resilient in an uncertain future. Reefs in Timor-Leste and the wider Indonesian region have been identified as less impacted by coral bleaching, cyclones, and projected future conditions (Beyer et al., 2018). The Indonesian Throughflow provides both water movement that regulates temperature and a seasonal food source of plankton that may contribute to the reefs' resilience from impacts such as coral bleaching and ocean acidification. While the impacts of climate change on the Throughflow are yet to be established, climate-related changes could alter its oceanographic processes. There appears to be a negative correlation between El Niño Southern Oscillation and chlorophyll abundance where La Niñas were associated with lower chlorophyll with a 6-month lag (Appendix S2: Figure S4b; Kim, 2025). Identifying the impacts of climate change on this unique oceanographic system, and the planktonic community it supports, is critical to fully understand the resilience of Timorese reefs and global climate. Timor-Leste's location within the Throughflow could buy more time for its reefs.

These observations of pyrosome blooms in the Indonesian Throughflow are distinctly new and important, both as a signal of abundant planktonic food for coral reefs in Timor-Leste and the potential for the oceanography to serve as a protective factor against ocean warming. These aspects warrant further research to establish whether feeding by corals increases resilience to the negative impacts of acidic conditions and the extent to which the Throughflow may create a climate refugium. More research is needed to determine whether the net benefit will remain in the future and challenges scientists to identify other refugia in reef regions. For example, the Great Barrier Reef comprises over 3000 individual reefs, and two regions have been identified as climate refugia based on modeling and satellite data (Sun et al., 2024). Identifying and managing localized reef refugia includes determining whether these upwelling areas provide more food for corals, all of which is fundamental to understanding how to sustain reefs in the future.

Catherine J. S. Kim made the observations, captured media, conducted analyses, and wrote the paper. Russell Kelley identified the corals pictured and contributed to the paper.

The authors declare no conflicts of interest.