During February and April 2024 we dove into the depths of the Salas & Gómez Ridge, a submarine mountain range whose peaks that emerge above the sea surface are known to all as Rapa Nui and Motu Motiro Hiva. The journey began on February 24th in Valparaiso, where a group of scientists and representatives of the Rapa Nui people embarked aboard the RV Falkor (too), of the Schmidt Ocean Institute.
This adventure was going to be different from our previous trips, since this time we could explore the depths of the mountains and islands in full HD, under the mesophotic layer, where the light no longer reaches, depths unexplored until then. But the most incredible thing is that the images could be shared live to the world thanks to the ROV SuBastian that transmitted live all the dives.
In March 2024 we arrived in Rapa Nui, our objective was to observe and discover the diversity below 600 m depth, reaching 1200 m near the Motu Motiro Hiva slopes. The tour included five seamounts, three of them unexplored until then. The first time we were able to see the depths of Rapa Nui, Mount Mana and especially Pukao was breathtaking. The colors and shapes of Mount Pukao have no comparison being one of the most diverse mountains we were able to rediscover in this expedition.
When the SuBastian reaches the seafloor it illuminates an unexplored world, revealing bright yellow, violet, red, green and white colors belonging to habitat-forming species such as corals, glass sponges and anemones. These species harbor a great diversity of many other invertebrate and vertebrate species that find shelter and food under its overhang. But, throughout our adventure, one of the most frequent and abundant types of organisms were squat lobsters, which are beautifully colored anomuran crustaceans. These small and sometimes not so small crustaceans, appeared everywhere; on corals, sponges, sea urchins, among shrimps, on rocks, inside galleries in soft sediments or on them, in short, wherever we looked we could find them. They are fascinating crustaceans, not only for their diversity and abundance, but also for their behavior. Those that lived in galleries and among the rocks were not intimidated by the great SuBastian, when they saw or felt it approaching, they raised their long chelipeds (legs with chelae) as a sign of attack and defense.
Although these organisms are diverse and abundant very little is known about how many species exist along the Salas & Gomez Ridge, only a couple of species are known to inhabit shallower waters, above 100 m. We were able to see at least 41 different morphospecies of squat lobsters on 16 SuBastian dives within the Motu Motiro Hiva Park and the Rapa Nui Marine Multipurpose Marine Protected Coastal Area Sea (AMCPMU-Rapa Nui), all of them new records, and probably most of them even potential new species. In order to describe them as such we needed to obtain at least 3 specimens of each morphospecies, which was not an easy task.
Squat lobsters are fast moving, territorial and very aggressive crustaceans, but what we did not know was how strong they can be. The sampling method we used was a “vacuum cleaner” that the SuBastian has, which can suck without problems animals such as gastropods (snails) and starfish that are strongly attached to the rocks, even some small fish, but the small prawns could stoically withstand the suction force, this to the regret of the SuBastian pilots, that for them it was a challenge to collect these powerful prawns.
When the “vacuum cleaner” was positioned over the squat lobsters, they took position, lowered their body so that their legs were closer to the bottom, while holding tightly to the rock and crevices, using their dactyls (the last segment of their 3 pairs of non-chelate legs, called periopods). The dactyls are shaped like a curved tip, each possessing a row of several movable spines on its inner margin that increase the grip to rocks and crevices very efficiently, like a climber using “cat feet”. Finally, using the “vacuum cleaner” we were able to capture 28 morphospecies of shrimp of the Family Munididae and 4 of the Family Galatheidae, orange and red, white. The Galatheidae are very small and were found on or near sponges, going unnoticed because of their white color.
We found another type (form) of squat lobsters that live associated with corals, they differ from the other families because their chelipeds are twice as long as their bodies or even longer. They were collected next to the coral colonies using one of SuBastian’s arms, which carefully cut off an arm of the colony as if it were a flower. The squat lobsters living on the colonies belonged to two families: Chrirostylidae and Eumunidae, both of which have different relationships with corals. Squat lobsters of the genus Uroptychus (Family Chrirostylidae) have hooked dactyls to attach themselves to the arms of coral colonies of the genus Chrysogorgia, this allows them to remain attached to the colonies even if the current is very strong. The association between the two genera is so close that most Chrysogorgia colonies had one or more Uroptychus on them.
The largest prawns we saw in this area were those of the Eumunidae Family, being able to recognize two genera Eumunida (3 morphospecies) and Pseudomunida (5 morphospecies). The associations with coral colonies were not as close as the one we saw with Uroptychus and Chrysogorgia, these prawns could be found on corals of the genera Acanthogorgia, Primnoides and Hemicorallium, and of the family Plexauridae, even sometimes we could observe them on rocks, but close to large coral colonies.
