October 11- October 17, 2003
October 17, 2003
Charles Messing writes:
Perhaps the most interesting aspect of the cruise was that apparently similar habitats at similar depths on the same seamount support significantly different assemblages of benthic organisms. During the single dive on Pioneer Seamount, organisms collected between 1786 and 1588 m were chiefly octocorals and echinoderms. The octocorals include two species of bamboo coral (Isididae), a fine-meshed sea fan with polyp calices like stalked stars, and the bubble-gum coral, Paragorgia. Echinoderms include a variety of brittle stars (Ophiuroidea), the most abundant organisms observed, feather stars (Crinoidea Comatulida), and sea stars (Asteroidea) including a long-armed filter-feeding brilliant orange brisingid. Among the crinoids, we retrieved Psathyrometra fragilis, a species that had not been collected off California in a hundred years. Other organisms include the limacean bivalve, Aceste, and scale worms (Polychaeta). Two apparently parasitic specimens were discovered when they climbed out of the mouth of the brisingid sea star after it had been preserved.
The first of four dives on Rodriguez Seamount was in much shallower water and we collected organisms in 709 to 639 m. These include a variety of sea stars and octocorals. Among the latter is a deep maroon sea pen with long polyps, unusual in that it clings to rocky substrates instead of anchoring in sediment like most sea pens. We also collected a large, brilliant yellow barrel sponge that housed a large population of scale worms and brittle stars. The second dive began in 2117 m and ascended several hundred meters before descending again. The highlights of this dive were the numerous yellow stalked crinoids, including a new species of Ptilocrinus, and the enormous numbers of feather stars belonging to at least four species. Brittle stars sometimes carpeted the sea floor. We also collected a variety of octocorals, a black coral (Antipatharia), a stalked tunicate (Urochordata), and an unusual membranous purple sea star. The third dive on Rodriguez Seamount collected specimens from 1199 to 805 m. Here we collected the bivalve Aceste again along with several octocorals, including some that we had not seen before. Unlike the previous dives, we encountered for the first time large pale purple and white pancake urchins (Echinoidea, Echinothuridae) and armor-plated sea cucumbers of the family Psolidae that cling to the rocky substrate and extend their finely branched arms upward to capture passing plankton and detritus. On our final dive, we collected specimens from depths of 1107 to 733 m. Psolids and echinothurids were again present, but not in the numbers found on the preceding dive. During the last two dives we found enormous trumpet sponges and lacy, lobed glass sponges. Many, however, were dead or in various stages of dying.
Although these few explorations barely scratch the surface of this remarkable ecosystem, we have been able to observe that organism distributions are sometimes highly patchy, both locally over distances of a few meters, and more broadly from cone to cone over distances of kilometers. Ophiuroids may cluster densely on a boulder and be sparse elsewhere within the same field of view. Psolid sea cucumbers may be abundant on one cone yet completely absent during a transect up another cone at the same depth.
--Charles Messing, Nova Southeastern University