Paper: Johson, NS, Swink, WD, Brenden, TO (2017). Field study suggest that sex determination in sea lampreys is directly influenced by larval growth. Proceedings of the Royal Society B. 284: 20170262. http://dx.doi.org/10.1098/rspb.2017.0262
What determines if an animal is a male or female? For humans, it’s simple (for the most part). It comes down to the presence or absence of a Y chromosome. Human DNA is packaged into 23 pairs of chromosomes and one of those pairs, codes for sex. Two X’s makes you a female, one X and one Y makes you a male. For other animals, the mechanisms that determine sex are not as straightforward. Environmental cues such as temperature, social context and density of offspring can both determine or influence the adult sex. For example, hotter temperatures lead to more female sea turtles and when the largest female clownfish that tends to her harem of males dies, the next largest clownfish in the harem turns into a female. That’s right–in Finding Nemo, Marlin would have turned into Marlene.
For sea lampreys (Petromyzon marinus), environmental factors and population density have been suggested as a trigger for determining sex; however, a new study suggests a new surprising factor: growth rate.
Sea lampreys are jawless, parasitic fish that look like vampire eels from hell (although they are not closely related to eels). They are best known for their scary set of pearly whites–rings within rings within rings of sharp, hooked teeth reminiscent of cat claws. Native to the North Atlantic Ocean, sea lampreys use a suction-cup mouth to stick onto an unsuspecting fish. Then, they sink their claw-like teeth into their host for grip and use a rasping tongue to bore into flesh and even secrete an enzyme to keep the host’s blood from clotting and body fluids flowing. In the Great Lakes, they are an invasive species with no natural predators that wreak havoc on native fish populations such as trout, sturgeon and salmon. Either the host fish die from having their fluids drained out of them, or from the resulting wounds, thus threatening commercial and recreational fisheries worth billions of dollars. Today, sea lamprey populations in the Great Lakes are kept at around 10% of historic peaks and efforts continue to keep their populations in check. Any insight into what influences their life cycle and development, especially reproduction and sex ratios could be the key to unlocking the most effective management strategies.
To shed more light on lamps, between 2005-2007, scientists from the U.S. Geological Survey and the University of Michigan tagged larval sea lampreys with coded wire tags, then released them back into the environment. Some were released into productive, fast moving streams and rivers leading into Lake Huron and Lake Michigan. Others were released into unproductive, slower moving areas where streams meet lakes with lower food availability and quality. The male:female ratio of larval sea lampreys was similar when released into different environments and higher than the untagged, incumbent population of sea lampreys likely due to the stress of handling and displacement involved in tagging. However, 3 years after release, when lampreys had time to metamorphose into their parasitic adult stage, 56% of tagged lampreys in productive areas were male, whereas 78% were male in unproductive areas. One exception was Carp River, an unproductive area where the probability of larvae becoming male decreased over time.
So what is the mechanism linking growth rate to a higher likelihood of becoming an adult male lamprey? While it’s not entirely clear, a hypothesis is that productive streams offer less competition for habitat and space so lamprey larvae can recover from tagging. In unproductive streams with higher competition and slower growth rate, higher energetic demands to produce eggs versus sperm could lead to male bias. Furthermore, if females are more likely to delay metamorphosing into their final adult stage, those additional years as larvae could cause them to succumb to higher predation rates. If this is the case, could the higher male-skewed population in the unproductive site a result of higher female mortality and not growth rate at all? While this is a possibility, and perhaps they are connected, scientists found that the overall survival rates between the two sites were not different, and different rates of mortality and metamorphosis between males and females have not been previously found in other species of lampreys–time for more research!
This study gives us a glimpse into why sea lampreys and perhaps other fish have variable population sex ratios–a quandary that researchers have been grappling with for years. Pending future studies to offer further support for this finding, this information could be used to develop sex-based management strategies to get an edge on curbing this damaging invasive species in the Great Lakes.