John Walsh <U33695@uicvm.uic.edu> (05/30/91)
This question came up in conversation recently (don't ask how): Are jellyfish animals, or are they colonies of plants or animals, like coral or sponges? We checked some textbooks but couldn't seem to get a satisfactory answer. What is the definitional distinction between an animal and a colony? Just curious. John Walsh U33695@UICVM.BITNET
freeman@argosy.UUCP (Jay R. Freeman) (05/31/91)
I'm sure someone will hollar if I am wrong, so I will venture that the
vaguely bell-shaped, betentacled marine entities commonly called
"jellyfish" are indeed single animals; but that their life cycle is
complicated: Many of them exhibit alternation of generations, in which
the immediate adult offspring of a jellyfish may look like something
else entirely, and may be one or another form of sessile, colonial
critter. In such cases, it is not until these colonial forms'
offspring reaches adulthood that another generation of "jellyfish"
emerges.
Not all jellyfish exhibit strict alternation of generation.
The phylum (cnidaria) which includes these animals also includes
corals, which are largely colonial and have no free-living adult
stage, and sea anemones, which if not strictly colonial are often
found in groups, and which also have no free-living adult stage.
Also in the phylum are assorted free-living colonial animals, like the
Portugese Man O'War (_Physalia_). Each Portugese Man O'War is a
colony of separate individual animals of the same species, but having
many different shapes; that is, the species is polymorphic. When
people speak of a "colonial jellyfish", critters like these are
probably what they had in mind, although they do not look like
jellyfish -- there's no "bell" -- and they behave somewhat differently
-- they float at the surface rather than immersed.
-- Jay Freeman
<canonical disclaimer -- I speak only for myself>squirrel@behind.caltech.edu (Patricia M. White) (05/31/91)
>This question came up in conversation recently (don't ask how): Are jellyfish >animals, or are they colonies of plants or animals, like coral or sponges? We >checked some textbooks but couldn't seem to get a satisfactory answer. What >is the definitional distinction between an animal and a colony? >Just curious. >John Walsh >U33695@UICVM.BITNET My text, _The Invertebrate World_, Barth and Broshears, Saunders College Publishing, 1982, states that most jellyfish are members of the class Scyphoza, although the famed Portugese man-of-war is in the related class Hydrozoa. These are two of the classes included in the phylum Cnidaria, commonly called coelenterata. Coelenterata are radially symmetric, which means they are rather simple organisms. They possess a gastrovascular cavity, an internal space for the digestion and distribution of nutrients, "making them far more complex than sponges." For a text like this, normally as dry as the Gobi, the authors get nearly poetic about the creatures, likening them to a flexible bottle with the cork pulled. In answer to your question, the medusae, which are the jellyfish, are the sexually reproductive form in the life cycle of the organisms. Most of the time they are polyps, which are stationary and frequently colonial. Polyps expand (it's hard to say reproduce) in an asexual fashion. These two forms, polyps and jellyfish, seem very different, but they are the same animal in different life stages. Remember that there are about 8900 species in the phylum, and so there are many variations on this scheme. Some very large colonies are supposed to contain both medusae and polyps. I'm not a taxonomist, but it seems to me that a colony is a group of organisms that can survive if you disassociate them into their respective parts, whereas an animal dies if you dissect it. Pat, squirrel@above.cs.caltech.edu
sarima@tdatirv.UUCP (Stanley Friesen) (06/01/91)
In article <91150.145201U33695@uicvm.uic.edu> U33695@uicvm.uic.edu (John Walsh) writes: > >This question came up in conversation recently (don't ask how): Are jellyfish >animals, or are they colonies of plants or animals, like coral or sponges? They are animals, but so are corals, or rather corals are colonies of animals. [In fact corals are colonies of forms closely related to jellyfish]. Sponges are *not* considered colonies, they are considered individual animals. > We >checked some textbooks but couldn't seem to get a satisfactory answer. What >is the definitional distinction between an animal and a colony? This is difficult to answer. There are different definitions among different groups of biologists. One frequently cited definition is a form with cell differentiation and specialization (except that just distinguishes between colonies of protists on the one hand and animals and plants on the other). Also, generalizing this to apply to component parts would treat the Portugese Man-of-War as a single individual rather than a colony, since the component polyps (hydra-like forms) are quite differentiated, and are all specialized for different functions. -- --------------- uunet!tdatirv!sarima (Stanley Friesen)