It's been known for some time that cuttlefish have contractile veins, but only from dissections; nobody had ever seen them contract in a live,free-swimming cuttlefish. To investigate this remarkable feature, Alison King and her colleagues at Dalhousie University and the Scripps Institution of Oceanography developed an ingenious set-up to see cuttlefish circulatory systems in action (p. 2071).
King wanted to know how blood returns to the cuttlefish heart. Most of our veins are conveniently located in muscles, and their contractions squeeze blood back to our heart. But the large cuttlefish veins aren't surrounded by muscle. Instead, they sit in a body cavity enclosed by the mantle - a big,muscular body wall that ventilates the cuttlefish's gills. It's generally assumed that the large veins are compressed by increased pressure in the cavity caused by the mantle's contractions, and that this pushes blood back to the cuttlefish's heart. But is this really what happens? Radiologist Matthias Schmidt astutely suggested that King could try using ultrasound to find out.
King and Schmidt began experimenting, but soon hit a snag; they found that the cuttlebone in cuttlefish backs is opaque to ultrasound. Realising that she'd have to go underneath the cuttlefish to see anything, King designed a plastic cylinder propped up with struts. To get ultrasound images of the insides of the cuttlefish happily resting in the cylinder, King pressed an ultrasound transducer against the bottom of the cylinder and hoped the creature would sit still long enough for her to get 30-second video clips. It worked; King could finally take a peek inside cuttlefish. But she had to make sense of the seething mass of cuttlefish insides. Poring over an atlas of cuttlefish body parts, she struggled to decipher her upside-down moving images. It was worth it. `For the first time, we could see blood vessels changing shape in live cuttlefish,' King says. `It was magical seeing physiology in action.'
So, does increasing mantle cavity pressure compress the veins, pushing blood to the heart? If it does, the contractions of the anterior and lateral venae cavae (two of the major veins that deliver oxygen-depleted blood to the heart) should be in sync with the mantle's contractions. But King saw that the lateral venae cavae and the mantle contract at different rates. Since they are out of sync, the mantle can't be compressing the lateral venae cavae. Taking a closer look at her real-time images, King saw peristaltic waves moving along the anterior and lateral venae cavae; the veins contract on their own! So it's unlikely that pressures created by the mantle compress the veins, because then`we'd expect the veins to collapse as a unit, rather than progressively along their length,' King says. She suggests that `actively contracting veins aid the return of blood to the cuttlefish heart.'
But King also noticed that the anterior and lateral venae cavae contract at different rates, which could spell disaster. The two veins are connected, so if one relaxes and expands while the other contracts, blood could flow in the wrong direction. Cuttlefish solve this potential problem in the same way we do; King discovered a new valve (which she has dubbed the Wells valve) sitting between the venae cavae, which ensures that blood always flows towards the heart.