The Rarape Parribacus perlatus Holthuis 1967, and two other slipper lobsters (Crustacea, Decapoda, Scyllaridae) on the coral Porites lobata from the coastal marine ecosystem of Rapa Nui (Easter Island)

Deep in the Pacific Ocean, far from any continent, lies Rapa Nui, better known as Easter Island. The waters surrounding this isolated land are home to a unique community of marine life, including several species of lobster. While the spiny lobster, known locally as ura, has long been the focus of local fishing, a different kind of crustacean has recently drawn the attention of scientists and the island's community. This creature is the Parribacus perlatus, a slipper lobster with a flattened body and broad, shield-like antennae, known to the Rapa Nui people as the rarape. Unlike its spiny cousin, the rarape has not been formally managed as a fishery resource, despite its growing popularity in local cuisine and tourism. Because the island's ecosystem is fragile and isolated, understanding how these animals live, reproduce, and interact with their environment is critical. Without this knowledge, the increasing demand for the rarape could lead to overfishing, threatening a species that is endemic to the region and plays a vital role in the health of the coral reefs it calls home.

A team of researchers, working alongside the Rapa Nui Ocean Council, set out to fill this gap in knowledge. They focused their efforts on the coral reefs formed by Porites lobata, a hard coral that creates a complex, three-dimensional habitat along the island's coast. Over several years, from 2019 to 2023, the team collected 104 specimens of the rarape from two specific coastal areas, Hanga Nui and Hanga Ho'onu. They gathered these animals by hand during night dives, carefully measuring their bodies and weighing them to understand their size and growth patterns. To ensure they were studying the correct species, the scientists also performed genetic tests, comparing the DNA of their samples against known sequences to confirm their identity. Alongside the rarape, they also found two other types of slipper lobsters in the same coral habitats: the Arctides regalis and the Scyllarides haanii. These two species are not native to Rapa Nui and appear to be recent arrivals, adding a new layer of complexity to the local ecosystem.

The study revealed distinct differences between male and female rarape. On average, the females were slightly smaller and lighter than the males, though both sexes ranged in size from about 11 to 18 centimeters in total length. The researchers also examined what these lobsters eat by looking inside their stomachs. They found that the rarape is a generalist eater, consuming a wide variety of small creatures and organic matter found on the reef floor. Their diet includes tiny shellfish, worms, pieces of coral, and even the remains of other crustaceans. This diverse menu suggests that the rarape acts as a scavenger, cleaning up the reef while also serving as a food source for larger fish. The coral itself, Porites lobata, appears to be essential for the lobster's survival, providing not just food but also a safe place to hide from predators and a secure environment for reproduction.

One of the most significant findings concerned the reproduction of the rarape. The team identified female lobsters carrying eggs, known as ovigerous females, and counted the number of eggs they were holding. These females, which had a shell length between 5.8 and 6.8 centimeters, were carrying between 25,631 and 47,678 eggs. The eggs were all at a similar early stage of development, suggesting that the females had recently spawned. The researchers noted that the rarape likely reaches sexual maturity when it reaches a shell length of about 5.5 centimeters. Interestingly, the number of eggs carried by these Rapa Nui females was lower than that of a related species found elsewhere in the Pacific, which may indicate that these are young mothers having their first batch of eggs. The presence of a spermatophore, a packet of sperm, on one of the females confirmed that the reproductive season is active in the region, likely spanning from late spring through the summer months.

The discovery of the two non-native slipper lobsters, Arctides regalis and Scyllarides haanii, in the same coral habitat raises important questions about the future of the local ecosystem. These species, which are native to other parts of the Pacific, are now living alongside the endemic rarape. While they are currently caught only occasionally for local consumption, their presence introduces the possibility of competition for space and food. The researchers suggest that managing the harvest of these non-native species could be a practical way to protect the native rarape from potential threats. The study concludes that the rarape is a species of high conservation concern, vulnerable to overfishing due to its limited range and specific habitat needs. Based on these findings, the Rapa Nui Ocean Council and Chilean fisheries authorities have begun to implement management measures, including restrictions on catching egg-carrying females and setting minimum size limits, to ensure the population remains healthy. This work highlights the delicate balance of life on the reef and the importance of scientific knowledge in guiding the sustainable use of marine resources.