Gamete concentrations and timing and success of fertilization in a rocky shore seaweed

Abstract

Whether sperm limitation or sperm competition occurs during fertilization in the sea has drawn increasing attention because of its importance in understanding population structure and recruitment. Inferences have been based upon both artificial and natural studies of in situ fertilization success, because natural gamete concentrations have been difficult to measure. Here, gamete concentrations and the timing of fertilization were examined during the tidal cycle in the fucoid alga Fucus vesiculosus L. on the Maine coast at Pemaquid Point and in the Narraguagus River estuary. Gamete concentrations were assessed by collecting samples from the water column throughout the tidal cycle; sperm were identified by indirect immunofluorescence using a F. vesiculosus-selective monoclonal antibody raised against sperm from a Maine population of this species. Eggs were detected visually, and fertilization was assessed by the presence and position of a sperm pronucleus within the egg. Gamete release occurred only on calmer days (Pemaquid Point) or calmer periods of the tidal cycle (Narraguagus estuary), and sperm:egg ratios near the time of fertilization ranged from 10:1 to 70:1. Fertilization success was 100% in both locations. Nearly all eggs were fertilized within a 1 h interval of the 6 to 7 h long high tide, 2 to 3 h after algae were covered by seawater. Gamete release occurred at low tide after a prolonged period (3 wk) of stormy weather that inhibited gamete release during high tide; this has implications for dispersal and interspecific hybridization. Archival environmental data from the US National Oceanic and Atmospheric Administration (NOAA, 1982 to 2000) were used to predict the frequency of successful reproduction during high tide and the potential for gamete release at low tide. These studies demonstrate that sperm are not limiting to fertilization in F. vesiculosus.