A Sacculina carcini barnacle parasitizing a swimming crab (Liocarcinus holsatus). The barnacle is yellow in contrast to the pale color of the crab and is highlighted by a circle around the crab's abdomen, brighter than the rest of the image.
Credit: Hans Hillewaert, https://commons.wikimedia.org/wiki/File:Sacculina_carcini.jpg
A pentastomid (Linguatala serrata), a long worm-like parasite with a segmented body. It is mostly a pale gray color, with a darker streak along the front of its body flowing lengthwise. It is placed against a blue background.
Credit: Dennis Tappe & Dietrich W. Büttner, https://commons.wikimedia.org/wiki/File:Linguatula.jpg
A tongue-eating louse (Cymothoa exigua) in the mouth of a sand steenbras (Lithognathus mormyrus), held in a human hand. The fish is a metallic yellow-green color and only the head is visible, while the isopod is pale grey and just barely visible in the fish's mouth.
Credit: Marco Vinci, https://commons.wikimedia.org/wiki/File:Cymothoa_exigua_parassita_Lithognathus_mormyrus.JPG
Three life stages of the salmon louse (Lepeophtheirus salmonis). At top is a mature female, trailed by two long, thin, pale egg strings. Its cephalothorax (to the right) is brown and patterned, while its genital segment (between the cephalothorax and the egg strings) is a darker reddish-brown. Below this female is another adult female, but without egg strings. It is similar in appearance to the female above it. At the bottom is an immature sea louse, smaller than the other two and with a much smaller genital segment, but otherwise similar in color. They are all against a white background.
Credit: Thomas Bjørkan, https://commons.wikimedia.org/wiki/File:Salmonlouse.jpg
On this day of #Crustmas and #InverteFest, we're inviting you to learn about the strange and specialized crustacean parasites out there! Some will be instantly recognizable, but others are so specialized to parasitism that they don't resemble their free-living relatives in the slightest!