New observations of the reproductive biology of crabs living around hydrothermal vents help explain their distribution and provide clues about the selection pressures prevalent in these hostile environments.
Hydrothermal vents were first discovered along the Galapagos Rift in 1977 and are home to at least 500 animal species about whose biology much remains to be learnt. One such animal group is the family Bythograeidae - the only family of crabs truly endemic to deep-sea hydrothermal vents. The first bythograeid vent crab to be described was Bythograea thermydron in 1980. This is the dominant crab species on most vent sites along the East Pacific Rise and on the Galapagos Rift. More recently, two other species, B. laubieri and B. vrijenhoeki, have been discovered. In addition to alvinellid tube worms, these two crab species dominate the hydrothermal vent faunas of the Pacific-Antarctic Ridge.
Professor Paul Tyler of the National Oceanography Centre, Southampton (NOCS), together with Dr Ana Hilário of both NOCS and the University of Aveiro and other Portuguese colleagues have been studying the reproductive biology of vent organisms for many years. They have studied ovary development in females of B. laubieri and B. vrijenhoeki collected on the Sebastian’s Steamer vent site (37° 47.48’ S, 110° 54.85’W; 2204 m depth) - the southernmost vent area known on the Pacific-Antarctic Ridge. The specimens were collected during an expedition aboard the American ship the RV Atlantis in 2005. The crabs were lured into a baited trap moored in position using the deep-sea submersible Alvin, which is operated by the Woods Hole Oceanographic Institution, Massachusetts.
The researchers tested the hypothesis that B. laubieri and B. vrijenhoeki would have reproductive patterns generally similar to those of B. thermydron because of their close evolutionary relationship. And indeed, as in B. thermydron, the ovarian microstructures in B. laubieri and B. vrijenhoeki were similar to those of shallow water brachyuran crabs. The paired ovaries were found beneath the carapace and overlying the hepatopancreas, an organ that provides functions similar to those of the mammalian liver and pancreas.
The researchers also show how immature eggs (oogonia) proliferate and develop into mature yolk-containing eggs. The general process of gamete formation in the two species studied was similar to that reported for B. thermydron, with some minor differences. No egg-bearing females were found in the samples. This is consistent with previous suggestions for B. thermydron that egg-bearing females retreat to less hostile places away from the active vent chimneys - hydrothermal fluids are rich in sulphide and heavy metals that may be toxic to the embryos and larvae.
But there were also some crucial differences in the reproductive patterns of B. laubieri and B. vrijenhoeki as compared to B. thermydron. The last species shows reproductive synchrony and seasonality linked to the formation of phytoplankton blooms in surface waters. However, females of B. laubieri and B. vrijenhoeki collected simultaneously from the Pacific-Antarctic Ridge contained oocytes (egg cells) in all stages of development, suggesting a lack of synchrony in reproduction for the population as a whole.
The researchers point out that the geographical distribution of B. laubieri and B. vrijenhoeki falls within the boundaries of the South Pacific Subtropical Gyre, the most uniform and seasonally stable region of the open oceans. They suspect that the absence of phytoplankton blooms and constant low concentration of dissolved nutrients may preclude environmental cues for seasonal reproduction, and that this oceanographic feature may also function as a dispersal barrier. Thus, they suggest, the biogeography of vent crabs may be determined by “contrasting oceanographic regimes that influence the reproductive patterns observed in different species.”
Materials provided by National Oceanography Centre, University of Southampton. Note: Content may be edited for style and length.
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