Baseline ecology and environmental data are urgently needed to accurately illustrate the conservation status of P. chaishanensis and predict how further development of a liquefied natural gas receiving terminal will affect this status17. Our demographic census confirmed the occurrence of P. chaishanensis in the Datan Algal Reef, southern Taoyuan, Taiwan. Both the number of coral colonies (84 in total) and colony size indicate that P. chaishanensis forms a stable population in the Datan Algal Reef, reiterating the necessity for conservation action.
Our study confirmed the previous finding that P. chaishanensis mostly inhabits shallow waters1. All Polycyathus species, except P. chaishanensis and P. fulvus18, are azooxanthellate, and P. fulvus (30–50 m), as well as all Polycyathus species except P. chaishanensis, are found in deep waters; e.g., P. mullerae contributes to the formation of the mesophotic reef at 55 m deep in the Mediterranean19. P. mayae and P. senegalensis are found at 127–309 and 12–143 m deep, respectively, in the Gulf of Mexico5. In Colombia, both species are recorded from 75–217 and 73–152 m deep, respectively4. At the holotype locality in Chaishan, Kaohsiung, P. chaishanensis is distributed in an intertidal pool estimated to be less than 300 cm deep during high tide and − 50 cm during low tide, according to the Taiwan Vertical Datum 2001 (TWVD 2001). P. chaishanensis was found at a similar tidal level in the Datan Algal Reef (below − 160 cm), confirming that P. chaishanensis can adapt to a shallow-water environment with high turbidity and a water column with a high concentration of sand and particles due to erosion, waves, and tide1.
We hypothesized that, unlike other Polycyathus species, P. chaishanensis has adapted to shallow tidal pools or the intertidal area and is symbiotic with the Symbiodiniaceae Cladocopium (C1). Corals in disturbed environments tend to associate with stress-tolerant symbionts in the genus Durusdinium and some Cladocopium species (C3 and C15). In contrast, corals from highly sedimented environments in Hong Kong and Singapore were also found to associate with Cladocopium C120,21. The reason it associates with Cladocopium C1—even in environments with high sediment—is that, although stress tolerant, Durusdinium sp. is considered a selfish genus that does not provide efficient nutrient translocation to its coral host. However, Baker et al.22 showed that Cladocopium sp. might be involved in efficient nitrogen assimilation and carbon translocation to the hosts. On the other hand, high turbidity might provide protection from light stress (elevated levels of irradiance), thereby reducing the impact of thermal bleaching events20,23. Highly turbid environments might also help corals obtain nutrients in the form of suspended particulate matter, thereby making the coral less dependent on the symbiont (irrespective of the type of symbiont) for its energy requirements and allowing it to exist in shallow and turbid environments.
Polycyathus chaishanensis in the Datan Algal Reef was distributed in the lower intertidal zone, and larger colonies were found towards the sublittoral zone. It was suggested that two abiotic gradients—disturbances from wave damage and light availability—determine the distribution and growth of reef-building corals in tropical waters24,25 and species diversity in the rocky intertidal zone26,27. Similar abiotic factors plus substratum availability can explain the distribution pattern of P. chaishanensis in the Datan Algal Reef. The algal reef stretches along the intertidal sand flat towards the sublittoral zone along the Taoyuan coast, and the Datan Algal Reef its best-developed section14 (Fig. 1). High sedimentation rate caused by strong winds and waves could easily bury the reefs in the upper intertidal zone. Thus, even if new P. chaishanensis juveniles could settle, they would also be buried. By contrast, in the lower intertidal zone, particularly in Datan G1 and Datan G2, the algal reef structure is more continuous and not interrupted by sand or cobbles (Fig. 1c,d), and small grooves or caves on the porous algal reef provide shelter for P. chaishanensis to be submerged underwater during low tide. In addition, the monthly spring low tide occurs at midnight, early morning, late afternoon, or after sunset along the Taoyuan coast, suggesting that P. chaishanensis prefers tidal zones less affected by desiccation from sun exposure (see Supplementary Fig. S1 online). These scenarios were also found to support several stress-tolerant scleractinian coral species at a similar tidal range—Porites sp., Goniopora sp., Pseudosiderastrea tayami, and Oulastrea crispata14.
The sedimentation rate was significantly correlated with wind speed on the intertidal algal reef in Taoyuan. Coral communities in the fringing reefs of Magnetic Island, northeastern Australia are also influenced by wind regimes, and the height of locally produced, short-period wind waves control the magnitude of near-bed suspended sediment concentrations28. The sedimentation rate9 in a healthy coral reef is around 10 mg cm−2 day−1, and the rate in the worst case scenario of coral degradation10 is estimated to be above 50 mg cm−2 day−1. Although sedimentation rates vary depending on the method and the sediment collected by sediment traps seems to be around 1/3 of in nature habitats29, the sedimentation rate in the algal reef ranges from 3,818.26 to 29,166.88 mg cm−2 day−1 and is over 300-fold the minimum in a coral reef. Hence, it is clear that, except for P. chaishanensis and a handful of stress-tolerant species, no corals can survive in an environment with so much sediment or build a coral-dominant reef structure. In contrast, crustose coralline algae (CCA) might be more tolerant to sand scouring or burial in a high sediment environment30,31. A study using the CCA collected at rocky sites along the northwestern coast of the USA showed that CCA can survive lengthy anoxic burial for three months with only slightly discoloration32. A similar experimental approach can be conducted to examine how long CCA can survive in the Datan Algal Reef under sand scouring or burial conditions. Furthermore, it is vital to understand how burial conditions affect the growth rate of CCA, the fundamental reef builder in the algal reef.
In conclusion, our study confirms the existence of a second population of P. chaishanensis in Taiwan in the Datan Algal Reef, which has proven to be an unprecedented ecosystem not only for Taiwan, but around the world12,14,33. The P. chaishanensis population in Datan is the only existing healthy and stable population in Taiwan, since the holotype locality of P. chaishanensis, Chaishan, was destroyed by coastal development in Kaohsiung34. In addition, the recent discovery of an unexpected large colony of P. chaishanensis (110 cm long and 80 cm wide) at the − 250 cm tidal level in Datan G2 confirms the urgent need for conservation action35, including measures that allow the public to participate in decisions involving energy-related construction issues and better advising around the government’s policy decisions36. Unfortunately, the Datan Algal Reef is currently facing destruction from the development and construction of liquefied natural gas (LNG) receiving and storage terminals and ports17, which also severely threaten the survival of the merely extant local population of P. chaishanensis. For example, there are at least seven colonies of P. chaishanensis located within 200 m of the scheduled trestle bridge in the LNG terminal in Datan G1, with the closest one being only 52 m away (see Supplementary Table S2, Fig. S2 online). The Datan Algal Reef has been designated a Hope Spot by the marine conservation NGO Mission Blue33, and the overall strength, willingness to pay (WTP), for protecting the algal reef is higher than for the gas receiving station in Taiwan36; nonetheless, both the Datan Algal Reef and P. chaishanensis face regional extinction if serious conservation action is not taken against coastal development.
Source: Ecology - nature.com
