Contact Chemoreception and Its Role in Zooplankton Mate Recognition

Abstract

Physical constraints cause small aquatic animals like copepods, rotifers, and cladocerans to experience their environment much differently than humans. One solution to sending and receiving signals in a vast, well-mixed environment is to bind the signal to the body surface, requiring receivers to use contact chemoreception for detection. Contact signals permit zooplankters to use large, information-rich molecules like glycoproteins as signals, whereas excreting such chemicals would be energetically costly. Lectin-binding experiments have demonstrated the role of surface glycoproteins in rotifer and copepod mate recognition. Female surface glycoproteins provide males with information about sex, age, species identity, and female anatomy. Mate-guarding in harpacticoid copepods was investigated by binding lectins to surface glycoproteins of females. Male decisions about mate guarding and spermatophore placement are typically determined by a series of stroking behaviors where males probe the body surface of females for chemical cues. Protein receptors on male antennules recognize specific carbohydrate moieties of female surface glycoproteins to identify appropriate partners for guarding and mating. Monoclonal antibody affinity chromatography was used to purify surface glycoproteins from female Tigriopus japonicus. A 70-kDa protein was eluted from the column and a partial amino acid sequence was determined using mass spectrometry. This protein has significant similarity to α₂-macroglobulin, a large protease inhibitor found in high concentrations in vertebrate blood and the hemolymph of invertebrates. α₂-Macroglobulin was most highly expressed in late copepodid stage (CV) females, the developmental stage most avidly guarded by males. Based on these observations, a model of the molecular mechanism of T. japonicus mate recognition has been developed. Mate recognition in copepods is compared to other malacostracans, cladocerans, insects, and rotifers. Future research should emphasize identification of signal compounds responsible for mate recognition and life cycle regulation. These critical molecules will be pivotal for understanding the evolutionary dynamics of aquatic invertebrates, including the development of reproductive isolation, speciation, and the rich biodiversity of cryptic species.