The Long-Whipped Bryozoan

Zooids of Crepidacantha longiseta, from Tillbrook et al. (2001).

Coral is far from being the only organism involved in the construction of a coral reef. Other calcareous organisms such as coralline algae, foraminifera and molluscs may also be significant. And, of course, there are those delicate artistes known as bryozoans. Many bryozoans tend to be underestimated as reef components because, as well as being relatively small, they often prefer to settle in more cryptic habitats such as around and under coral gravel (Kobluk et al. 1988).

The organism in the SEM photo at the top of this post is one reef-inhabiting bryozoan, Crepidacantha longiseta. This species belongs to the ascophoran bryozoans, i.e. the zooid is protected dorsally by a calcified frontal wall, and each feeding zooid is associated with two long whip-like avicularia (the exact function of bryozoan avicularia is debated, but they are generally believed to be related to defense and/or cleaning the surface of the colony). In the top image, the feeding zooids are represented by the keyhole- or cartoon-fish-shaped openings, while the avicularia are positioned to either side of the main opening. Other Crepidacantha species may have the avicularia in different positions; they also differ in the length of the avicularia and the shape of the primary orifice (Tilbrook et al. 2001).

Crepidacantha longiseta is found in cryptic habitats around coral gravel in shallower waters, but may be found in more exposed positions as the water gets deeper, below about 25 m (Martindale 1992). It has been found in Vanuatu, Brazil, the Caribbean and Mauritius, and is presumably pantropical in its distribution (Tilbrook et al. 2001).

REFERENCES

Kobluk, D. R., R. J. Cuffey, S. S. Fonda & M. A. Lysenko. 1988. Cryptic Bryozoa, leeward fringing reef of Bonaire, Netherlands Antilles, and their paleoecological application. Journal of Paleontology 62 (3): 427-439.

Martindale, W. 1992. Calcified epibionts as palaeoecological tools: examples from the Recent and Pleistocene reefs of Barbados. Coral Reefs 11 (3): 167-177.

Tilbrook, K. J., P. J. Hayward & D. P. Gordon. 2001. Cheilostomatous Bryozoa from Vanuatu. Zoological Journal of the Linnean Society 131: 35-109.

Gunky Lace

Zooids of Arachnidium pilosum. Photograph by Hans De Blauwe.

I've written posts before on the Bryozoa, the aquatic (mostly marine) colonial animals sometimes known as 'lace animals'. On both previous occasions, the bryozoans in question have belonged the the group known as Cheilostomata. For this post, I'll be looking a members of another major bryozoan group, the Ctenostomata. While cheilostomes have their colony enclosed in a calcified body wall, ctenostomes have a soft chitinous or gelatinous body wall. Most ctenostomes do not form the tightly arranged colonies of cheilostomes, and the orifice of each individual zooid is not closed by an operculum as in cheilostomes (but read on). It is the delicate tracings of cheilostome colonies that lead to the name of 'lace animals'; if ctenostomes are to be compared with lace, it is with a mucky forgotten rag that has been allowed to rot in the mud.

The Arachnidiidae are a family of ctenostomes that form trailing colonies across their substrate; the type genus Arachnidia was originally named for its resemblance to a spider's web (Harmer 1915). An individual arachnidiid zooid is roughly teardrop-shaped with the orifice at the bulbous end. The branching pattern of the colony is roughly cross-shaped with the thin stolons of three daughters attached to the bulbous end of their parent. Both marine and freshwater species of arachnidiid have been described. It is primarily their colony structure that distinguishes them from other ctenostome families.


Fossil arachnidiid colony, Arachnidium smithii, preserved on a bivalve shell. From Taylor (1990).

Lacking a calcified skeleton, it is not surprising that the fossil record of ctenostomes is somewhat sparse. Ctenostomes are generally preserved as imprints on other fossils, either as borings or by a process called bioimmuration where another hard-bodied organism (such as a coral or another bryozoan) grows over the top of a ctenostome colony. Ctenostome borings date back to the Ordovician, and arachnidiids preserved by bioimmuration are known from the Jurassic (Taylor 1990). However, although cheilostomes and ctenostomes are generally accepted as more closely related to each other than to other bryozoans, the cheilostome fossil record only dates to the Jurassic. It has therefore been suggested that cheilostomes were derived from ctenostome-type ancestors, making ctenostomes potentially paraphyletic (interestingly, this would imply that calcification arose independently in cheilostomes from Stenolaemata, the other major calcified bryozoan group). Among the Jurassic arachnidiids described by Taylor (1990) was one genus, Carboarachnidium, with a D-shaped orifice that Taylor interpreted as indicative of the presence in life of an operculum. As an operculum is a distinctively cheilostome characteristic, Taylor suggested that Carboarachnidium could represent a stem cheilostome. This could make arachnidiids the closest known relatives of cheilostomes among the 'ctenostomes' (though it has to be admitted that, as supporting links go, it's not the most robust), and potentially crucial in understanding the origins of that significant bryozoan clade.

REFERENCES

Harmer, S. F. 1915. The Polyzoa of the Siboga Expedition – Part I. Entoprocta, Ctenostomata and Cyclostomata. E. J. Brill: Leyden.

Taylor, P. D. 1990. Bioimmured ctenostomes from the Jurassic and the origin of the cheilostome Bryozoa. Palaeontology 33 (1): 19-34.

Taxon of the Week: Lacy Lepraliellidae

Celleporaria sibogae, showing the characteristic hollow colony structure of this bryozoan genus. Image by Bob Fenner.

Lepraliellidae are a family of bryozoans (small colonial animals sometimes known as 'lace animals' or 'moss animals' due to the appearance of the colonies) found around the world with the highest diversity in warmer waters. Different species may grow as encrusting forms or as upright branching colonies and even lunulitiform varieties have been recorded from the Eocene of Australia (Schmidt & Bone, 2002).

Distinction of the various types of bryozoan requires close microscopic examination of the individual zooids (the units of the colony). Lepraliellidae belong to one of the more successful branches of the bryozoans, the Cheilostomata, which have each individual animal in a separate calcified box-shaped chamber. Within the Cheilostomata, they belong to the Ascophora which have an entirely calcified frontal wall on each chamber. Characters used in distinguishing Lepraliellidae from other ascophorans include umbonuloid growth of the relatively smooth frontal wall (i. e. primary calcification of the wall is internal rather than external), widely open ovicells (embryo brooding chambers) and a small spine-encircled ancestrula (the first zooid of a new colony to develop from a metamorphosed larva). In most species, an avicularium is present on the lower edge of the main opening of each feeding zooid though this feature is absent in Kladapheles gammadeka (Gordon, 1993).


SEM image of Buchneria dofleini colony showing individual zooids. The larger openings are those of feeding zooids, with a smaller avicularium opening against each one. Image by Dennis Gordon.

Lepraliellids of the genus Celleporaria include some of the largest of all bryozoan colonies, with individual heads up to a foot in height and nearly a metre in diameter, that may form predominantly bryozoan reefs (Hageman et al., 2003). Branches of upright Celleporaria colonies have a characteristic nodular appearance and are generally thick but hollow as they originally grow around an organic substrate such as a sponge that later dies off after being overgrown. Some species of Celleporaria are significant fouling agents on the bottoms of boats, and have been spread around the world by shipping.

REFERENCES

Gordon, D. P. 1993. Bryozoan frontal shields: studies on umbonulomorphs and impacts on classification. Zoologica Scripta 22 (2): 203-221.

Hageman, S. J., J. Lukasik, B. McGowran & Y. Bone. 2003. Paleoenvironmental significance of Celleporaria (Bryozoa) from modern and Tertiary cool-water carbonates of southern Australia. Palaios 18 (6): 510-527.

Schmidt, R., & Y. Bone. 2002. Eocene bryozoan assemblages of the St Vincent Basin, South Australia. In Bryozoan Studies 2001 (P. N. Wyse Jackson, C. J. Buttler & M. E. Spencer Jones, eds) pp. 293-298. A. A. Balkema.

Colonies on the Move

First off, the latest edition of the plant blogging carnival Berry Go Round is up at A Neotropical Savanna, so go take a look.



Secondly, Susannah of Wanderin' Weeta has presented a nice overview of bryozoans. Bryozoans, for those as don't know 'em, are tiny colonial aquatic animals, and are one of the groups of organisms that I personally think are most unfairly overlooked. A great many people will have probably never heard of them, but they are not uncommon, they or the remains of their colonies are easily found attached to rocks, shells, etc. at any beach, and their delicately structured colonies can be exceedingly attractive, as indicated by their sometimes-used common name of "lace animals".

As a follow-on from Susannah's post, I just thought I'd present you with one of the more unusual and eye-catching types of bryozoan. Modern lunulitiform bryozoans belong to three families, Lunulitidae, Selenariidae and Cupuladriidae. Unlike other bryozoans, lunulitiforms colonies are not anchored to their substrate, but lie unattached on the surface of the sediment. While free-living bryozoan colonies sometimes grew quite large during the Palaeozoic, the rise of a well-developed infauna (burrowing animal assemblage) and the resulting bioturbation rather did it in for the large forms, and modern lunulitiforms are small, only about a centimetre and a half across at most. They are disc-shaped and raised in the centre, so they look not unlike a Southeast Asian fieldworker's hat, with the zooids all emerging on the outer surface. As shown in the picture near the top of this post from Aaron O'Dea, lunulitiform colonies are generally highly integrated, with zooids growing in a strict radial pattern.



So highly integrated are these colonies, in fact, that some are able to move as if they were a single organism, as shown in the photo of Selenaria maculata just above by P. L. Cook. The zooids around the edge of the colony develop exceeding long setiform avicularia (to find out what an avicularium is, read Susannah's post) that the colony can use like legs to walk on. Selenaria maculata colonies can reach speeds of about one metre per hour, which may not sound particularly fast until you consider that most bryozoan colonies don't move at all.

Tiny Flowers of the Sea

The greatest feeling in biology is undoubtedly that which comes with seeing a particular organism for the first time. You can read on it, study specimens of it, think you know it inside out - and then you actually see one, and suddenly you realise that it's so much more than you ever imagined. This is the feeling that drives bird enthusiasts to spend their life's savings on trips to some malaria-infested corner of the third world to try and catch a brief glimpse of the Lesser Spotted Turntwick or some such. This is the feeling that inspires deep-sea biologists spending whole days lying on the bottom of the ocean in miniature submarines in which they can't move more than a few centimetres in any direction, just so that they can watch a tube-worm wave its tentacles at them. It can only be described as a mystical experience, this moment in time that really brings it home to you that this is why you study biology.

I had a brief such moment yesterday, when I encountered representatives of an animal phylum that I'd never seen before. I was tutoring a lab for which some live specimens of encrusting bryozoans had been brought in. Unfortunately, the bryozoans were being very unco-operative and refusing to emerge from their protective skeleton. While scanning the colony under a microscope to try and find some emerging zooids, I saw some soft white structures overgrowing the colony. A quick change of focus brought them into view, and I was presented with my first ever view of an entoproct.



It seems to be quite impossible to find an image available online that does these animals justice. The one at the top of the page comes from here, while the picture above from here shows part of an entoproct colony on the left (the other two animals are a gnathostomulid and a myxosporidian). Entoprocts are minute colonial animals with a body shaped like a wine-glass, topped by a ring of feeding tentacles. The relationships of entoprocts are rather uncertain at the present point in time. In the past they were included in the Bryozoa, but it is now widely agreed that the apparent similarities to Bryozoa in the stricter sense are only superficial, and the two groups are probably not closely related. It is accepted that entoprocts belong somewhere within the Spiralia, the large group of animals that includes, among others, molluscs, annelids and flatworms (and probably bryozoans), but exactly where in this group they sit is very much an open question. The specimens I saw each arose from the substrate on their own individual stalks, which according to the guide book on Australian marine life that was lying in the lab meant that they belonged to the family Loxosomatidae, as opposed to the Pedicellinidae which have multiple zooids budding from a single stalk.

Personally, though, I was stunned by just how beautiful these little animals were. They looked like minute, frilly tulips, balanced ethereally on their long slender stalks. If I looked closely, I could see a line of fine white filigree connecting each individual to its neighbours in the colony, forming delicate tracings over the substrate. I was enthralled, and the thought crossed my mind - "This is why I study biology".