Male pregnancy is a phenomenon that is unique to syngnathid fishes (e.g. seahorses and pipefish). In mammalian pregnancy, environmental factors such as diet can have time-critical influences in that their impact on development of the offspring can depend strongly on when the environmental change is experienced, with the most sensitive periods referred to as 'critical windows'. However, whether male pregnancy is subject to such time-critical influences has not been established. This project aimed test the hypothesis that male pregnancy in seahorses is subject to critical windows by assessing transcriptomic profiles of newborn seahorse offspring derived from fathers fed a fatty acid-deficient commercial diet, as opposed to a fatty acid-rich 'wild' diet, during specific time periods relative to pregnancy. The experimental groups were as follows: MW (Male Wild control group, administered wild diet throughout experiment), BC (Before Conception commercial diet administered 10 days before mating only), PC (Periconception commercial diet administered for three days either side of mating only), MC (Male Commercial commercial diet administered throughout pregnancy only), and EP (End Pregnancy commercial diet administered for 6 days before the predicted end of pregnancy only).