What does your date of birth say? Maybe you feel that it reveals your character or even your destiny. For male spear squid, it can tell us a lot about their love life. Male spear squid plays different roles depending on when they were born and has always been so. The latest research has found that the day a male spear squid hatches determines which mating strategy it will adopt throughout its life. The spear squid (Heteroligo bleekeri) that hatches earlier will become "gangsters" and compete for mating opportunities. Squid that hatches later become "stalkers" and use more stealthy mating strategies.
The researchers found that mating strategies, determined by date of birth, were fixed throughout the squid's lifetime. Understanding how mating strategies are influenced by date of birth and environmental conditions at the time helps researchers consider the possible impacts of climate change on squid and the implications for marine resource management.
A team of researchers in Japan found that the mating strategy of the spear squid was largely influenced by its date of birth. Depending on the mating technique, the spear squid can be divided into two categories: the "squid", which actively fights off their opponents, puts sperm into the female squid, and then protects the female squid when she lays eggs, and the "sneak attackers", as the name suggests, steal mating opportunities and secretly place sperm on the outside of the female squid's spawning place in the hope of fertilizing the egg.
The scales inside the squid's body help maintain balance. They have a thick layer of sediment called a "hatch check" that marks the day the squid is born. Thereafter, the squid grows once a day, so its age can be estimated by counting the squid.
Interestingly, this study found that once squid's mating strategy was determined by their date of birth, they persisted. Male squid born between early April and mid-July will become large in size before they begin to breed and become mates. Squid that hatches late, i.e., squid born between early June and mid-August, are smaller at the start of the mating season and become "sneak attackers." However, even if an early-born squid grows large enough to become a "gangster" during the early breeding season, it will delay maturity and continue to grow until it is old enough to become a "gangster". Squid born at the beginning of July are half the chance to use both strategies.
The effects of environmental conditions on squid
Associate Professor Yoko Iwata of the Institute of Atmospheric and Oceanic Research at the University of Tokyo said: "Our findings suggest that the date of incubation determines the entire life trajectory of this species. Differing hatching dates mean that squid experience different environmental conditions early in life, which may affect their growth trajectory. If extreme environmental events such as marine heatwaves occur during the hatching season, it may affect the mature size of the squid and subsequent mating strategies. This can also have a huge impact on commercial catches. "
In total, the team analysed more than 350 squid, either caught by themselves or from commercial fishing between 2020 and 2023. This is the first reported result supporting the "date of birth hypothesis" in aquatic invertebrates. This hypothesis suggests that the date of birth influences the reproductive strategy of males and has only been recorded in fish before. This new finding suggests that it may actually occur in more species of animals than previously thought.
Native to the western Pacific off the coast of Asia, spear squid has a lifespan of only one year and is a popular food in Japan. Image credit: 2024 Shota Hosono
A surprising finding was that the squid grew at a different rate than the research team expected. Previous studies have shown that squid are very sensitive to environmental conditions, especially water temperature. Therefore, the research team expects this to affect the growth of squid. However, the results of the study suggest that although squid with two mating strategies grew up in different seasons, there was not much difference in their growth rate early in life. Researchers believe this may be due to complex environmental factors.
Now, the team is analyzing a structure called "statolith" in squid. Squid stones grow every day, and in the current study, the age of the squid is estimated by counting the number of layers of the squid stone, similar to tree age determination. In addition to estimating age, the trace elements contained in the statistic stone can also be used to estimate the marine conditions experienced by the squid when a portion of the statistic stone was formed. For example, the strontium element can be used to estimate the water temperature at that time. By analyzing these trace elements, the researchers hope to determine what kind of environment individual squid experienced in their early life and gain a clearer understanding of how this may have affected their mating strategies.
"I'm interested in the evolution of animal survival and reproductive strategies, in other words, the life history of animals, and phenotypic plasticity, which is how individuals respond to changes in their environment," Iwata said. For several reasons, I consider spear squid to be an ideal creature to study how environmental and ecological conditions affect life history. "
编译来源：ScitechDaily