Do fish have difficulty breathing?
Fish are breathing with their gills, but this is literally, for many of them, only half of the story. The difficulty of extracting oxygen from water is the single most defining force that shapes fish evolution. Land animals are mostly exposed to the same oxygen concentration, but fish need to operate in waters of vastly different and rapidly changing oxygen content. Moreover, water contains much less oxygen than air, and the diffusion of oxygen in water is magnitudes slower than the diffusion of oxygen in air.
Therefore, fish evolution was forced to come up with creative ways to
- top off the blood's oxygen content by means that do not involve the gills and
- prevent oxygen loss from red blood cells when exposed to low-oxygen water.
In our most recent review, we stipulate that one of these adaptions — the so-called secondary vascular system (SVS) — originally evolved to achieve both these tasks. Later, some fish evolved into land animals. In these animals, the successor of the SVS is now known as the lymphatic vascular system. In mammals like humans, the lymphatics reabsorb fluid that is leaking from blood vessels due to the high intravascular pressure. Fish do not have such high blood pressure as mammals do, and their bodies don't experience the high hydrostatic pressure differences that humans do. Do they need lymphatics at all?
The lymphatic system is important in all infectious diseases, as well as cancer and cardiovascular diseases. Because biomedical scientists are conducting an increasing amount of research on zebrafish, it appears important to understand the anatomy and functioning of their vascular systems!
Around 1980, fish scientists started to quarrel: some denied that fish have a lymphatic system, while others provided evidence that they do. This controversy is actually much older, with controversial findings more than 100 years ago. The question of whether fish have lymphatics and how these lymphatics relate to the secondary vascular system of fish pops up regularly in vascular biology meetings. At the same time, there is not much documentation about this controversy in the scientific literature. Good scientists are naturally cautious, and everybody has always been waiting for more data before taking a stand.
We finally had enough and decided that we ought to document the status quo. Together with our colleagues from Germany and Sweden, we laid down a consensus interpretation of the data. Even though we are not 100% sure about everything, our model of a co-existing and partially overlapping SVS and lymphatic vascular system is compatible with most of the available data. Importantly, the model helps in the design of new research. We are happy to announce that this review on the fish vasculature has now been published in the excellent journal Biological Reviews: https://doi.org/10.1111/brv.13114. Thanks to everybody involved in this project! With 26 pages, 8 figures and 263 references covering the years 1845-2024, this is, to date, our most extensive review.