One of the functions of blood is to carry oxygen around the body. In vertebrates this is accomplished by the haemoglobin of red blood cells, which binds to oxygen molecules and delivers them to wherever in the body they are needed. It is this protein that gives vertebrate blood its red colour. All vertebrates have haemoglobin, except one small group of icefish (the Channichthyidae family).
Currently, the ocellated icefish (Chionodraco rastrospinosus) is making headlines due to its seemingly-bizarre characteristics. As well as the transparent blood, it possesses no scales and an enlarged heart and blood vessels. Though standard for members of the Channichthyidae family to which it belongs, that doesn't make these traits any less strange.
These fish live around a kilometre (3200 ft) deep in icy Antarctic waters, and only one pair are held in captivity. This means not much is known about them, but happily this pair mated in January and it's hoped their offspring will reveal more about their unusual physiology.
It's thought that icefish use blood plasma to carry oxygen around their bodies, explaining their enlarged hearts and blood vessels. It's also thought that the lack of scales may allow it to absorb oxygen directly through its skin - the polar waters are more oyxgen-rich than warmer waters.
It's not properly known why icefish have evolved away from using haemoglobin. It's clear they used to possess it by the remnants of haemoglobin-producing genes seen in icefish genomes. It's especially strange considering the costs of circulating blood are higher when haemoglobin is absent. Hopefully the occelated icefish offspring will hold some answers.
Photo credit: YouTube.
(Journal article) The loss of haemoglobin in icefish: http://bit.ly/10PVHXR
(Journal article) Key steps in haemoglobin loss in icefish: http://bit.ly/12F5JeA