Within-female sperm allocation in Enoploteuthis chunii is associated with polyandry and male-biased sex ratio

Abstract

One of the unusual reproductive features in cephalopods is the flexibility of sperm deposition sites in females. Recently we reported that female Enoploteuthis chunii possess not only a major sperm receptacle (MSR) but also a cryptic sperm pocket (CSP) and CSP utilization increases towards the end of the reproductive season. This may occur due to male-male competition over insemination sites, as a male-biased sex ratio and CSP utilization are tightly correlated. The microsatellite-based genotyping revealed that both MSR and CSP are used by the same males, a similar phenomenon previously found in Loliolus sumatrensis , which is herein hypothesized to be a strategy associated with sperm allocation.

Figure 1. Males allocate their sperm investment to different storage sites within a female

A, a dorsal view of a mated female with sperm sacs stored both in the MSR and the CSP. B, inset shows an enlarged image of the boxed area in A. C, the usage patterns of two sperm deposition sites (MSR and CSP) in female individuals. Most females that used the CSP, had more than 100 sperm sacs in the MSR. Females with ( orange closed circles ) or without ( blue closed circles ) sperm sacs in the CSP. Furthermore, the numbers of individuals divided by a threshold of 100 sperm sacs in the MSR ( a broken red line ) are shown in the corners. D, the pattern and number of sperm genotypes in three females that had sperm sacs in both the MSR and CSP. In each pie chart, "the total number of sperm sacs successfully genotyped / the total number of sperm sacs found attached" is displayed. Each color in the same female experiment represents an identical genotype.