Abstract
For animals that reproduce in water, many adaptations in life‐history traits such as egg size, parental care, and behaviors that relate to embryo oxygenation are still poorly understood. In pipefishes, seahorses and seadragons, males care for the embryos either in some sort of brood pouch, or attached ventrally to the skin on their belly or tail. Typically, egg size is larger in the brood pouch group and it has been suggested that oxygen supplied via the pouch buffers the developing embryos against hypoxia and as such is an adaptation that has facilitated the evolution of larger eggs. Here, using four pipefish species, we tested whether the presence or absence of brood pouch relates to how male behavior, embryo size, and survival are affected by hypoxia, with normoxia as control. Two of our studied species Entelurus aequoreus and Nerophis ophidion (both having small eggs) have simple ventral attachment of eggs onto the male trunk, and the other two, Syngnathus typhle (large eggs) and S. rostellatus (small eggs), have fully enclosed brood pouches on the tail. Under hypoxia, all species showed lower embryo survival, while species with brood pouches suffered greater embryo mortality compared to pouchless species, irrespective of oxygen treatment. Behaviorally, species without pouches spent more time closer to the surface, possibly to improve oxygenation. Overall, we found no significant benefits of brood pouches in terms of embryo survival and size under hypoxia. Instead, our results suggest negative effects of large egg size, despite the protection of brood pouches.
Highlights
One striking difference between aquatically and terrestrially reproducing animals is the size of eggs produced in each environment
Using four pipefish species, we tested whether the presence or absence of brood pouch relates to how male behavior, embryo size, and survival are affected by hypoxia, with normoxia as control
We found a significant interaction between oxygen treatment and group (Table 1) with E. aequoreus showing a more pronounced negative effect of hypoxia on embryo survival (Fig. 2A) and on embryo length (Fig. 2B), but not on embryo dry weight, compared to the other groups (Fig. 2C)
Summary
One striking difference between aquatically and terrestrially reproducing animals is the size of eggs produced in each environment. Aquatic environments differ from terrestrial ones in multiple ways, including in having lower and more variable levels of dissolved oxygen (Keister et al 2000; Nilsson and O€ stlund-Nilsson 2008). A developing embryo acquires oxygen through the egg membrane and the perivitelline fluid (Jones 1986; Strathmann and Strathmann 1995). This process is based on diffusion and both the egg membrane and the perivitelline fluid act as barriers to embryo oxygenation
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