A Closer Look
Our Closest Invertebrate Relatives By Heidi Gartner, Invertebrate Collection Manager
Invertebrates are animals without backbones. Though this does not necessarily mean they lack a skeleton (think of crabs with their hard outer shells). When you walk along a BC coastal beach most of the animals you are seeing – the crabs, barnacles, bivalves – are all invertebrates.
The pharynx basically acts as a large sieve. As water passes across the pharynx the miniscule food particles are filtered out from the water. From there, the food particles pass through a digestive tract (esophagus, stomach, and intestine). Waste products and gametes are transported by the water as it exits the tunicate body through the atrial/ excurrent siphon. Having a better understanding of their lifestyle, doesn’t the common name ‘sea squirt’ seem fitting?
Tunicates are a group of animals within the invertebrates. These animals are basically filter-feeding ‘sacks’. Their whole body is encased in a protective covering, called the tunic, which has two siphons – one for drawing water into the body and one for expelling it from the body. Water, carrying food and oxygen, is drawn into the body through the oral/incurrent siphon into a large feeding structure called the pharynx.
Along with this filter-feeding lifestyle, tunicates are primarily sessile (not moving) and benthic (living along a substrate). They can be found at all ocean depths worldwide attached to any substrate. So how are tunicates able to spread and move throughout an area or along a coast? This movement is primarily accomplished through reproduction. As part of their reproductive process, tunicates have swimming tadpole larvae that
unicates, or sea squirts, are some of my favourite invertebrates. I find beauty in their many varieties of shapes, sizes and arrangements.
can swim through the water column, and be carried by currents, to new habitats. When they reach a suitable substrate these swimming larvae settle and metamorphose into the adult tunicate form. The larvae of tunicates are fascinating, not only for their ability to aid in the dispersal of the animals, but also because they represent a unique connection to the human world. Invertebrates are animals without backbones and we, as humans, are ‘chordate’ animals with backbones. Well, tunicates belong to a unique subphylum of invertebrates called the Urochordata where they have chordate traits but only in their larval form! In the larval stage tunicates possess distinctive chordate traits: a notochord, a dorsal tubular nerve cord, a post anal tail, and pharyngeal clefts or pouches. These chordate traits are not all retained in the adult