The ability of a reef fish to survive in the long term is determined by the amount of adversity it faces as a larva, according to a study published in the most recent issue of the Proceedings of the National Academy of Sciences.
The study by Scott Hamilton, James Regetz, and Robert Warner focused on the Bluehead wrasse (Thalassoma bifasciatum) living in St. Croix, US Virgin Islands.
The researchers used otolith (ear stone) elemental profiles of lead (Pb) to assign recent settlers to a group that developed in waters elevated in lead concentrations throughout larval life (i.e., nearshore signature) and a group that developed in waters depleted in lead (i.e., offshore signature), potentially dispersing from upstream sources across oceanic waters.
The authors found that contrary to expectations that fishes leading a more sheltered life as larvae (i.e. those growing in nearshore waters) would make up most of the adult fish living on the reef, only 23% of survivors after the first month displayed a nearshore otolith profile.
This is despite the fact that local production (i.e. fish from nearby inshore waters) contributed heavily to settlement: at least 45% of settlers developed nearshore.
The reason for this discrepancy is probably due to the fact that offshore developers were found to grow slowly initially, but compensated with fast growth upon entering nearshore waters and metamorphosed in better condition with higher energy reserves.
Lead poisoning was ruled out as the cause of the selective mortality, because the elevated levels of lead from nearshore waters in the study site are far below levels considered toxic for larval fish and have not been known to cause any adverse physiological effects in them.
The authors also speculate that symmetries in the strength of selection between groups that experienced distinct larval histories may arise from differences in the timing of selective mortality. We collected only fish that survived the larval period, and mortality may have been particularly intense in offshore compared with nearshore waters during the planktonic period. In that case, only the best-performing larvae that developed offshore were able to pass through this selective filter and survive until settlement.
In contrast, fish that spent their larval life in a high-Pb water mass developed in a less selectively intense environment, and both high- and low-quality nearshore fish successfully settled, potentially because of the increased likelihood of encountering suitable adult habitat at the end of the larval period, but poor-performing individuals were subsequently selectively removed.
For more information, see the paper: Hamilton, SL, J Regetz and RR Warner (2008) Postsettlement survival linked to larval life in a marine fish. Proceedings of the National Academy of Sciences 105, pp. 1561"1566.