Abstract:
Fishes of the family Holocentridae have a morphologically distinctive postlarval ''rhynchichthys'' stage characterized by extensive head spination, including a prominent serrate rostral spine. In most species the rhynchichthys physiognomy persists until settlement, which occurs within a size range of about 30-50 mm SL. The two species of Holocentrus have a uniquely protracted pelagic period during which they transform from the rhynchichthys at about 40 mm SL to a distinctive, unornamented, and relatively streamlined prejuvenile ''meeki'' stage that may reach 74 mm SL prior to settlement. Analysis of daily growth increments in lapilli of recently settled juvenile and adult holocentrids confirms that the pelagic stages of H. ascensionis and H. rufus grow more rapidly and settle at significantly greater ages and larger sizes than other holocentrids. A microstructural transition zone in the otoliths of these two species probably corresponds to the rhynchichthys-meeki morphological transformation of these pelagic stages. In other holocentrids examined a similar microstructural transition corresponds to the rhynchichthys-juvenile morphological transformation associated with settlement onto reefs. The average age and size of small juveniles of H. rufus collected at their earliest appearance during censuses of shallow reefs was 67 days and 66 mm SL, but the average pretransition increments in their lapilli was only 44. Limited data for H. ascensionis are similar, but differ in average age (59 days) and size (56 mm SL) at settlement and number of pretransition increments (48). The size of specimens found in museum collections also substantiates that H. rufus is larger at settlement (and has a larger maximum size meeki stage) than H. ascensionis. The two meeki stage specimens for which we have otolith data (for H. rufus, both slightly less than 50 mm SL) lack the expected transition mark. However, their ages at capture (38 and 43 days) suggest that they are newly transformed from the rhynchichthys stage, and marginal transition marks are difficult to identify. Daily growth increments were validated in one meeki stage H. rufus and two rhynchichthys stage Sargocentron coruscus that were maintained, respectively, for five and nine days in aquaria after strontium infusion of their otoliths. The rapid morphological transformation of the meeki stage into a benthic juvenile within two days of capture, including overnight change in coloration from blue above and silver below to orangish red with pale stripes, is described.
