Sea squirts come under the microscope
Science Centres: Aquatic Biodiversity and Biosecurity
Clavelina sp. An undescribed species collected from the Three Kings Islands. Individuals are connected to each other by stolons to form a social colony. (photo: Sean Handley)
The common colonial sea squirt Botrylloides leachi. (Photo: Mike Page)
Sea tulips are tough leathery ascidians that form clusters of tulips supported on long stalks. They are common in southern New Zealand and in the Chatham Islands. (photo:Malcolm Francis)
The stalked ascidian, Pycnoclavella kottae, forms large clusters in dark archways and under overhangs. It is abundant at the Three Kings Islands, and has not been found elsewhere. (photo: Malcolm Francis)
Marine biodiversity scientists recently got the chance to learn about the biology and identification of New Zealand’s ascidians (generally known as sea squirts) at an intensive 5-day NIWA workshop presented by Dr Patricia Kott of the Queensland Museum. Scientists from Otago University, Te Papa, Cawthron Institute, and NIWA attended the workshop run through NIWA’s National Centre for Aquatic Biodiversity & Biosecurity in March this year. Septuagenarian Dr Kott is the only active taxonomist in Australasia working on this important group.
The training workshop has given New Zealand scientists the skills and techniques to identify solitary sea squirts and the more complex colonial species. NIWA and Cawthron provided the specimens. These specimens came from NIWA’s museum collection, a recent marine biotechnology programme collection from the Three Kings Islands (sea squirts are a known source of bioactive compounds), a series of port surveys (for the Ministry of Fisheries), and from the Marlborough Sounds. Three previously undescribed species were found in the small number of specimens examined during the course.
Despite early work on intertidal and continental shelf fauna by the late zoologist New Zealander Beryl Brewin and British zoologist R.H. Millar, only a few collections have been described. Many of the 166 species of sea squirts described in New Zealand are endemic. Dr Kott estimates that at least 150 more new species remain to be collected and described from our Exclusive Economic Zone, including those in shallow-water habitats accessible to divers.
Sea squirts have traditionally been included in the same animal phylum (Chordata) as humans because they have a hollow dorsal nerve cord and a rod-like notochord for at least part of their life cycle. However, in spite of these features, sea squirts are today considered to be an early offshoot of the branch that led to chordates, and are included in the phylum Tunicata. The class name, Ascidiacea, comes from the Greek word askidion, a leather wine flask with a tough outer casing around its body. Sea squirts vary greatly in form, from simple sac-like creatures on the underside of rocks that propel a jet of water when squeezed, to complex brightly coloured colonial species with hundreds of minute individuals.
Immobile sea squirts occupy a lot of space on rocky substrata and sediments, and play a very important role in the ecology of the seafloor environment, including filtering large volumes of water to capture plankton and organic matter. Sea squirts often dominate the fauna that fouls boat hulls, wharf piles, and marine farms because many of the two-dimensional colonial species can spread rapidly to successfully compete for space with other immoble invertebrate taxa. Invasive species introduced on ship hulls may also pose a threat to New Zealand’s biosecurity. The mass settlement of the alien Ciona intestinalis on mussel lines, and the recent irruption of a large, yellow colonial species, Didemnum vexillum, shows the importance of sea squirts to New Zealand’s biosecurity. Although the latter species, which attracted attention in the Auckland–Coromandel region a couple of years ago, is now believed to be native, its sudden rampant growth was similar to that of newly introduced exotics.
I acknowledge the assistance of Dr John McKoy (NIWA), Kathryn Hattersley (Nelson Institute of Technology), Gavin Robinson (Zoology Department, University of Canterbury), and Cawthron Institute.