Friend or Foe Part 12 Friend or Foe TRISTAN LOUGHER B.Sc. PHOTOS BY THE AUTHOR UNLESS graduated from Manchester OTHERWISE STATED The Tunicates University in 1992 with a degree in Zoology. He has worked at Cheshire f you read the last article in this series Waterlife for five years. you may recall that I alluded to the fact Ithat this article would centre on a group A notochord (essentially a “backbone” – of invertebrates that represent our closest though not necessarily made of bone) relatives in the non-back-boned world. Why A post anal tail – vestigial in many The sub-phylum Urochordata is sub- is this? Well, this months subjects are the vertebrates including man but there all the divided into three classes. sea squirts of the sub-phylum Urochordata. same. The latter is a sub-division of a larger A dorsal hollow nerve cord. Class Ascidea: The tunicates or sea squirts. phylum called the Chordata of which the Gill slits – obviously refined in higher Class Larvacea: Usually very small predominant number of species are vertebrates, lost in embryonic development. (<5mm) Free-swimming forms resemble encapsulated in the sub-phylum Vertebrata ascidean tadpole larvae. – the vertebrates – of which, the human, The urochordates demonstrate all these Class Thaliacea: Salps. Often colonial, Homo sapiens is but one example. Some of characteristics but not in the adult form. planktonic urochordates usually forming you, probably those who are familiar with Larval urochordates resemble miniature chains of individuals. the form of salps, sea squirts and tadpoles and have all the prerequisites for larvaceans, may find it hard to believe these classification as Chordates, but they are As the only class of urochordates we are creatures have anything in common with short-lived. Most species settle within a few likely to encounter in marine aquaria are the vertebrates. The criteria for admittance hours of release and undergo their the ascidean sea squirts we shall into the esteemed company of vertebrates metamorphosis into the sessile encrusting concentrate on them here. are as follows: or vase-like animal. A large tunicate with Here is the same tunicate out siphons open. This is a of water with the siphons commonly encountered closed. I include my fingers for species and although not scale. the brightest its size makes it very obvious. Note also that the surface of this squirt is smooth unlike the granular texture of sponges. 36 • MARINE WORLD –AUGUST/SEPTEMBER 2003 Friend or Foe The scarlet blobs in this image are a tunicate species probably belonging to the genus Diademnum. This image was taken in the These beautiful tunicates appeared on some base rock that was Red Sea but the genus is common throughout the tropical Indo- home to some button polyps. Here we can see the tunicates in the Pacific. company of encrusting algae and sponges. A closer examination of the two larger specimens reveals the structure of the pharyngeal basket through the semi-transparent tunic. Each specimen aquarium, with the enables the casual observer to separate this measured less than 10mm and they reproduced asexually over a exception of sponges organism from similar looking animals such number of weeks to form a colony of twenty or so individuals. and if the aquarist as sponges. In colonial tunicates the searches for the key inhalant aperture is shared with each There are approximately 1300 identification points separate zooid (“individual”) possessing its urochordate species currently classified of even these should not result in any own exhalent siphon. which the vast majority are ascidians. We confusion. Ascidians are filter-feeding As previously stated, perhaps the only tend to refer to such animals as sea squirts organisms. They have an inhalant (buccal) animals that may be mistaken for tunicates due to the fact that when removed from and exhalent (atrial) siphon aperture are the sponges as they too are commonly water they quickly contract and expel water through which water enters and leaves. The found in tubular or encrusting forms. from their body cavity. The commonly used inhalant siphon is usually larger than the However, the feeding apparatus of sponges term tunicate is derived from the fleshy exhalent and in free-living forms the former is extremely primitive in comparison to surround to the animal which protects is usually situated at the top of the animal tunicates. As I briefly mentioned in the internal organs and also provides support with the latter usually to one side and lower introduction to this article, one of the for the animal. When you know what you than this. The fact that free-living forms prerequisites for inclusion in the phylum are looking for sea-squirts are unlikely to possess two obvious siphons of differing Chordata is gill slits. These are obvious in be confused with any other animals in the diameter is an important feature, which animals like fish but not so apparent in the after Barnes 1980 after Barnes 1980 This sponge is a simple tube lined with The tunicate, superficially similar to the asconoid sponge from the outside, shows a much specialised cells. Compare this relatively greater specialisation with tissue arranged into organs simple design with the Ascidian tunicate MARINE WORLD –AUGUST/SEPTEMBER 2003 • 37 Friend or Foe the more familiar difficult to spot, particularly if they become corals. In a reef covered with other encrusting organisms or aquarium you can algae, but there are several, highly coloured increase your chances species that are equally difficult to miss. of success with these Many various species are common fascinating animals by accidental arrivals and are particularly supplementing with associated with mushroom rock, button iodine and trace polyps and on the bases of soft corals, elements – tunicates although they can conceivably turn up have been shown to almost anywhere. They should be store such compounds considered harmless and even welcomed by in their tissues – but marine aquarists. Unlike sponges which can do not be too also be highly colourful tunicates are downhearted if the seldom offered for sale so the aquarists best sudden appearance of chance of keeping such creatures is to be a colony is followed vigilant and alert to their presence on coral some months later by base rock. their equally abrupt There are several genera of ascidian demise. Many species that may appear in reef aquaria. These have short life-cycle include Polycarpa, Didemnum, Ciona and phases lasting only a Ascidia. However, the aquarist is unlikely few months although to separate these. Even assigning a certain species are specimen to the three orders, longer lived and may Phlebobranchia, Aplousobranchia and survive for years. Stolidobranchia, is not easy without internal For the purposes examination of the organisation of the of this article we will tissues. sub-divide the So far we have centred on those Ascidians into two species showing a pair of siphons, one further groups – those usually being larger than the other. I have that appear to be previously mentioned the possibility of individual specimens confusion with sponges and this is certainly and those that are the case with Diademnum spp. This genus definitely colonial. has a single exhalent siphon but water enters via a number of “pores” scattered Free-living over the tunicates surface. Several species may occur but if you encounter a green Ascidians specimen it is very likely to be home to Free-living ascidians symbiotic algae and therefore it requires This tunicate resembles some of the small orange sponges we find strong lighting. The tunic of this genus is associated with coral base rock are separate entities in their own right able to be perforated without losing its resembling, in many rigidity because it is hardened by ascidians. Ascidians have a specialised cases, vase-like structures that may be less calcareous spicules. feeding apparatus called the pharyngeal than 10mm to several centimetres tall. basket contained inside the body ball Although not truly colonial, like the (known as the tunic – hence the common tunicates we will look at later in name for the group). Water enters the this article, they may occur in pharyngeal basket via the inhalant siphon groups of many individuals. In Botrylloides sp. The genus Botrylloides is typified and leaves through the pharyngeal slits other situations we may notice by large inhalant siphon surrounded by double (=gill slits) leaving behind sieved what appear to be groups of rows of individual animals. If you look into the planktonic organisms, detritus etc which is inhalant siphon in this image you can catch a separate individuals that are fed into the stomach. Water does not enter glimpse of the pharyngeal basket. actually connected at their base. the animal passively, instead being driven Where these seem to by cilia that line the pharyngeal slits. The spontaneously appear asexual pharyngeal basket is able to capture very reproduction is likely to have small prey items (one micron or so) due to taken place. Certain species the secretion of a specialised mucus which, actually have different phases to when examined with a microscope, their reproduction where one resembles a very fine mesh net. This is generation multiplies asexually continually secreted by a specialised organ and the next sexually and so on. called the endostyle and is eventually For the purposes of this article we “spun” into a strand and channelled into the will centre on all tunicates that stomach. can be perceived to be complete Some species found on tropical reefs animals, connected or not, as supplement their filter feeding with the free-living. products of photosynthesis, facilitated by Some individuals may be symbiotic algae in much the same way as 38 • MARINE WORLD –AUGUST/SEPTEMBER 2003 Friend or Foe Colonial Tunicates The image of Botrylloides shows a next article in this series we will look at white colony but you might encounter some unrelated groups of animals that can Although some of the vase-shaped tunicates green, orange or, indeed, many different be referred to as the unsegmented worms.