Something Darwin didn't know about barnacles: spermcast mating in a common stalked species

Area of Science:

• Marine Biology
• Reproductive Biology
• Invertebrate Zoology

Background:

• Free-living barnacles are typically hermaphroditic, with fertilization assumed to occur via pseudo-copulation or self-fertilization.
• The gooseneck barnacle, Pollicipes polymerus, was believed to exclusively cross-fertilize, with no known capacity for self-fertilization or external sperm acquisition.

Purpose of the Study:

• To investigate the reproductive mechanisms of Pollicipes polymerus, specifically addressing how isolated individuals outside of typical mating range produce fertilized eggs.
• To test the hypotheses of self-fertilization versus sperm acquisition from the water column in Pollicipes polymerus.

Main Methods:

• Collection of isolated Pollicipes polymerus individuals and pairs, some bearing egg masses.
• Genotyping using 16 single nucleotide polymorphism (SNP) markers to determine parentage and fertilization source.
• Analysis of egg masses from both isolated individuals and those with adjacent partners.

Main Results:

• A significant percentage of eggs in Pollicipes polymerus were fertilized by sperm captured from the surrounding water.
• Sperm capture was confirmed in 100% of isolated individuals and 24% of individuals with an adjacent partner.
• Fertilized eggs resulting from sperm capture were found distributed throughout individual egg masses.

Conclusions:

• Sperm capture, or "spermcast" mating, is a viable reproductive strategy in Pollicipes polymerus, supplementing pseudo-copulation.
• These findings challenge long-held assumptions about barnacle reproductive capabilities and sperm transfer mechanisms.
• The study suggests the potential for sperm capture in other aquatic species and raises questions about prior self-fertilization claims in barnacles.