Description of a Male Urogenital Papilla in the California Grunion, Leuresthes tenuis, a Beach-spawning Marine Silverside Fish

Abstract

The California grunion, Leuresthes tenuis (Ayres, 1860), like its sister species, the Gulf grunion, L. sardina (Jenkins & Evermann, 1889), engages in unusual reproductive behavior in which males and females aggregate on sandy beaches to spawn (Thompson 1919; Walker 1949, 1952; Thomson and Muench 1976; Martin 2015). Spawning by California grunion occurs at night during semilunar spring high tides in March-August, from Magdalena Bay, Baja California, Mexico, to Tomales Bay, California (Walker 1952; Moffatt and Thomson 1975; Roberts et al. 2007; Martin et al. 2013). Females deposit eggs ∼5-10 cm deep in the sand while males release sperm as they surround females at the sand surface. The externally fertilized eggs develop within the sand until competent to hatch. Hatching is triggered when the embryos are washed out of the sand during a subsequent spring high tide series, and the swimming larvae are washed into the ocean (Thompson 1919; Griem and Martin 2000; Martin et al. 2011). While extracting gametes for other studies, we noticed a structure associated with gamete release in California grunion males, but not females. In this study, we used dissections, magnetic resonance imaging (MRI), paraffin histology and light microscopy to characterize that structure as a muscular urogenital papilla, a sexually dimorphic character that can be used to distinguish the sexes noninvasively. We also propose a function for the urogenital papilla in external fertilization in this beach-spawning species. Adult California grunion were collected during spawning runs on Cabrillo Beach, San Pedro, CA (GPS coordinates: 33o42′33′′N, 118o16′59′′W), and Doheny State Beach, Dana Point, CA (33o27′43′′N, 117o41′14′′W), under California Department of Fish and Wildlife Scientific Collecting Permits (SC-3211, SC-4783, SC-10567, and SC-10585). Whole fish were placed on ice or dry ice, or euthanized in tricaine methanesulfonate (MS-222; 0.2 g L−1 of seawater) and fixed in 10% phosphate-buffered formalin, and then transported to California State University Fullerton (CSUF). Iced or frozen and thawed whole males (N = 6) were dissected to examine the urogenital papilla and its relationship with other organs, and several females were also dissected to examine the ovary and oviduct. One fixed male and one female were prepared for MRI, and segments of other fixed individuals (15 males and 8 females) were processd for histology. All work was approved by the CSUF Institutional Animal Care and Use Committee under protocols 08-R-07, 11-R-07, 14-R07, and 17-R-06.