Supplementary data to “Alternative reproductive tactics are associated with sperm performance in invasive round goby from two different salinity environments”

During male-male competition, evolution can favor alternative reproductive
tactics. This often results in a dominant morph that holds a resource,
such as a nest for egg laying, competes with a smaller sneaker morph that
reproduces by stealing fertilizations. The salinity environment can
influence male growth rates, e.g. via osmoregulatory costs, which in turn
may influence the use of sneaker tactics for small males competing for
mating opportunities. Salinity can also affect sperm directly; however,
little is known of how salinity influences sneaker tactics through sperm
performance. We sampled males of the invasive round goby (Neogobius
melanostomus) from two environments, a freshwater river and a brackish
estuary. This fish has two male morphs: nest-holding dark males and
non-nest-holding light males. We examined the role of water salinity of 0,
8 and 16 on sperm performance and found that in estuarine males, a
salinity of 0 reduced sperm velocity compared to a salinity of 8 and 16.
Riverine males had low velocity in all salinities. Sperm viability also
decreased by over 30 % in 0 salinity, compared to 8 and 16, for fish from
both environments. Gobies produce ejaculate contents in specialized glands
that could in theory shield sperm in an adverse environment. However,
gland contents did not improve sperm performance in our tests. Body mass
and age estimates indicate that riverine males invested more in somatic
growth compared to estuarine males. Estuarine light morph males had a high
enough gonadosomatic index to indicate sneaker tactics. We propose that
when sperm performance is low, such as for the riverine males, sneaker
tactics are ineffective, and will be selected against or phenotypically
suppressed. Instead, we interpret the increased investment in somatic
growth found in riverine males as a life-history decision that is
advantageous when defending a nest in the next reproductive season.