by Erika Meerhoff, Armando Mujica, María Luisa Nava & Beatriz Yannicelli – Esmoi
One of the most notable crustaceans of Rapa Nui is the endemic lobster Panulirus pascuensis, called Urain the Rapanui language. “In the past, there was such an abundance of this crustacean that they could be caught and eaten at any time of the day. Lobsters were even used for bait by the fishermen. Today it’s very hard to find. If in 1980 it was possible to have 100 lobsters in any given cave, today there might be only one,”says Michel García, a diver who has lived on the Island since 1979.
This species has been important in the diet of the Island residents for a very long time and is part of the mythology and culture of Rapa Nui, with a figure of a lobster found among the pictograms of the Rongo-rongo, leading archaeologists to associate it with the moon goddess. Today, it is periodically captured, preferably with traps and in nighttime dives with flashlights. The first studies of lobster capture were done between the decades of the 50s and 70s, showing an overfishing of this resource (CORFO, 1979) which indicates that a management plan is needed to maintain sustainability of harvesting. To properly establish this, we need to improve our scientific knowledge of the lobster. A key part of that is the larval stage, one of the most enigmatic stages of the life cycle of the lobster.
The lobster’s life cycle is complex. The females carry the eggs, which, when they eclose, give rise to larvae called phyllosomas. These larvae form part of the marine plankton, a large variety of organisms which live suspended in the ocean, floating between the suface and the sea floor subject to the movement of the currents. The phyllosomas live among the plankton, dispersed from 100 to thousands of km (60 to thousands of miles) off the coast, where they are carried by the currents into the open sea. This larval phase can last for a year until they find an adequate place to settle on the sea floor and undergo metamorphosis, passing through a transitional stage called puerulus, in which they become morphologically similar to an adult, but in miniature.
In order to learn more about this larva and the depths at which it is found, the scientific group ESMOI held two oceanographic expeditions to Rapa Nui in April and in October of 2015. Both included sampling with special nets for collecting zooplankton at various depths (Tucker nets). The Chilean naval ship, “Tokerau”, gave logistical support and the Universidad Católica del Norte offered its laboratories for analysis of the samples. In addition, with the support of the ORCA Diving Center, lighted traps were placed at depths of 5 meters (16 feet) and 15 meters (50 feet), in an attempt to capture larvae and/or juvenile lobsters, since they are attracted to the light. To date, these experiments have been unsuccessful, due to the difficulties of following the larvae in the vast ocean around Rapa Nui.
During April of 2015, among the plankton of Rapa Nui, at a spot near the Apollo seamount, phyllosoma larvae of the Panulirus pascuensis lobster were found in the strata between the surface and 200 meters (656 feet) of depth. The presence of this seamount seems to modify the currents in the area, which allows the zone to retain the eggs and larvae of different fish and crustaceans, as well as increase the production of the first trophic level of the sea (primary production of phytoplankton).
This discovery and its formal description is a major addition to the knowledge of the species and its populations and to the understanding of the connectivity that might exist between Rapa Nui and the Motu Motiro Hiva Marine Park around Salas y Gómez Island, 415 km (260 miles) distant from Easter Island. The study of connectivity between marine organisms around oceanic islands is a determinant in the management of Protected Marine Areas. One of the objetives of this study is to see if larvae freed by lobsters of Rapa Nui and of Salas y Gomez Island could be mixing and contributing to an increase in the population of juvenile lobsters between these islands. We are taking the first steps in the study of connectivity: learning the horizontal and vertical distribution of phyllosomas, their different stages of development and the ocean currents which could be carrying them from Motu Motiro Hiva Marine Park to Rapa Nui. Identifying the zones where these larvae arrive on their return to Rapa Nui after their long oceanic voyage would be a fundamental pillar for sustainable management. It is already evident that we need to take many more samples of plankton, which are planned for the near future.