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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 . 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 (essentially a “backbone” – of 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- Urochordata is sub- is this? Well, this months subjects are the 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 cord. Class Ascidea: The tunicates or sea squirts. phylum called the Chordata of which the Gill slits – obviously refined in higher Class : Usually very small predominant number of species are vertebrates, lost in . (<5mm) Free-swimming forms resemble encapsulated in the sub-phylum Vertebrata ascidean larvae. – the vertebrates – of which, the human, The urochordates demonstrate all these Class : . 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 and have all the prerequisites for larvaceans, may find it hard to believe these classification as , 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 into the sessile encrusting concentrate on them here. are as follows: or vase-like .

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 .

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The scarlet blobs in this image are a tunicate species probably belonging to the 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 such number of weeks to form a 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 (“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 . 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 but not so apparent in the

after Barnes 1980 after Barnes 1980

This 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

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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 , , and phases lasting only a . 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 , and survive for years. , 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 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 is likely to have small prey items (one micron or so) due to taken place. Certain species the secretion of a specialised 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 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 , facilitated by Some individuals may be symbiotic algae in much the same way as

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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. may be colonial in nature the species we colour variations in specimens of this will centre on here are colonial in the sense genus. The specimen shown grew from the that they share an inhalant siphon and have base of a Platygyra sp. hard coral and individual buccal siphons. Species that within only a couple of weeks, three further Bibliography and conform to this description are commonly colonies had sprung up of equal size found associated with all manner of corals suggested further reading (approximately 2cm square). Previous – hard or soft – and various polyps. Their Barrett and Yonge. (1985) Guide to the experiences with this genus have resulted in ability to reproduce asexually is incredible Seashore. Collins. colonies of 20-30 square centimetres on and several species can cover many square Debelius. (1999) Crustacea Guide of the rocks and even aquarium glass. They don’t centimetres in a matter of weeks. Colonies World. Ikan Publishing suffer physical damage very well and can are highly variable in colouration and Debelius and Baensch. (1994) Marine collapse if disturbed. Taking “cuttings” patterning. I have seen green, turquoise, Atlas Vol I. Mergus Publishing. from a colony may be very risky and result orange, red and white species and Debelius and Baensch. (1994) Marine in the death of the parent and daughter sometimes combinations of these. If you Atlas Vol III. Mergus Publishing colonies. The best way to obtain a specimen are lucky enough to have some of these Fossa & Neilson. (2000) The Modern like these is to purchase the substrate upon varieties already in your aquarium, or are Aquarium. Vol 4. Birgit which it is growing. Of course this is observant enough to spot them nestling in Schmettkamp Verlag. impossible in the case of glass-growing and amongst some polyps they can prove J.G.Walls (Ed.). (1982) Encyclopaedia colonies but good colonies can be found, very rewarding to keep. However, in my of . T.F.H. even on live rock. experience they have very short life-spans, Publications This brings our look at the tunicates to possibly due to a lack of suitable trace Ruppert and Barnes. (1994) an end. I hope that I have encouraged you element addition or, and this is more likely, Zoology Sixth Edition. to seek out these fascinating animals on any their natural life-cycles mean that colonies Saunders College Publishing specimens that you buy. Presence of a will go into a sexual phase after the initial Sprung. (2001) Invertebrates – A quick tunicate can certainly make my mind up burst of . reference guide. Ricordea publishing whether to purchase or coral or not. In the

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