The color of shrimp seeds is different in pandalus borealis in different seasons.

1. The shrimp stomach is broken.

When salvaging pandalus borealis, shrimp are pulled up from the bottom of the sea with a big trawl. Sudden changes in atmospheric pressure may lead to stomach rupture. pandalus borealis's stomach grows on its head, and with its transparent shell, the black or brown plankton or algae eaten by the shrimp will spread, and it looks like its head is black.

2. There are mature shrimp eggs in the ovary.

Female pandalus borealis sometimes has mature shrimp seeds in her ovaries and dark green on her head. All these may be the reasons for the appearance of "blackheads" in pandalus borealis.

pandalus borealis grows on the seabed 15 meters or even below 25 meters, and preys on zooplankton in the deep sea for a living. These deep-sea zooplankton are brown and black because they don't see sunlight all year round. Pandalus borealis's stomach is not in the abdomen like ordinary marine fish, but in the head.

After preying on these zooplankton, their heads (i.e. stomachs) naturally appear faint brown or black, especially when they are full. Some female shrimps even have mature shrimp seeds in their ovaries, which makes their heads appear dark green.

pandalus borealis is directly fished from the deep sea with large trawls. Sometimes, due to the rapid trawling speed, the water depth in pandalus borealis has changed greatly in a short time. Due to the rapid change of atmospheric pressure, the shrimp head (stomach) is easy to break, and the zooplankton in it also spreads.

In this way, the faint black stomach contents (zooplankton) appear, which is the real reason for the so-called "blackhead". Therefore, "blackheads" are zooplankton in the deep sea. Because they also grow in the pure and pollution-free Arctic and North Atlantic, they are as harmless as pandalus borealis.