Again it is about crustacean parasites. Somebody ever heard about Tantulocarida? And again taxonomists had their problems with a taxon from the beginning of the 19th century until the 1980’s.? Not until then Tantulocarida where recognized as a discrete group of ectoparasites within the crustaceans. But as so many other “unlinkable” groups they can be found in the Maxillopoda today, following Schminke (2007). Unfortunately most of these articles I mention under “Reference” are not freely accessible or even not accessible trough the library online-system of my institute. But one of the leading scientists on Tantulocarida, Geoffrey Allan Boxshall, has written a summary on Answers.com.
These tiny crustaceans have crustacean hosts only and can be found sucking on Copepoda, Ostracoda, Cumacea, Tanaidacea and Isopoda in all oceans on earth.
The larva is called “Tantalus”. It is these larvae which have to find a new hosts. They attach to the hosts with a mouth disk and penetrate the host using an internal stylet which can be protruded out through the mouth opening. They suck haemolymph to feed or, as Boxshall points out, may have an “absorptive rootlet system” extending from the oral disc to penetrate the internal tissues of the host (Sounds a little bit like the rhizocephalan strategy).
It is known, that there are two enigmatic and separated reproduction strategies. A sexual lifecycle was inferred from the occurrence of a male and a female but in practice a reproduction as well as living adults were never seen.
Also, in sexual females the development from larva to adult was never observed. But it is assumed to happen like this: Soon after attaching to the host, the larva sheds its thorax. Instead, a huge sac develops on the Cephalon, out of which the adult female will hatch. (So basically it is a mixture of the developments of males and the second female type I will describe later.) Adult females have no appendages for swimming. The existence of a single big claw-like seta on the tip of each of the only two pairs of legs is interpreted as an organ to hold tight the male.
In males the larva does not shed its thorax but a strange metamorphosis takes place in a sack which develops behind the 5th or 6th segment of the larva. The sac is filled with a cluster of differentiated larval cells which reorganise to form an adult male. While the Cephalon is still attached to the host, an umbilical chord originating from the head supplies the male with nutrients from the host. By rupturing the sac wall the male is released. The adult male is able to swim, has 6 pairs of swimming legs, but it has no limb to hold the female. Males swim to find a female, it is assumed, and when he has found a receptive female, she grabs him. They have special sensory systems for that purpose, Aesthetascs, which possibly are derived antennae. Using their large penis, the male inseminate the female through a single ventral copulatory pore (Boxshall).
Inside a colossal carapace the female carries her eggs. Males and females – as it is usually implied – produce offspring in a sexual manner. It is assumed that the offspring is the Tantalus. Both, adult male and female have no anus so they do not seem to feed at all after hatching.
But there is also another type of female, which has a very different appearance compared to the sexual females. The so called “parthenogenetic female” is attached to the host permanently. The trunk of the larvae is most certainly shed after the attachment to the host, as it is in the sexual females. At least there is a scar found on the head. A sac-like new and unsegmented thorax develops behind the head. In this sac, which expands dramatically, tantalus-larvae are formed – without any molting - and released to infect new hosts, again without molting.
Whether the sexual or asexual phase is encountered and what controls it is unknown. Where is the link between these two phases? It is assumed that the asexual phase is encountered more frequently than the sexual phase, like in cladocerans or other Branchiopoda. And nobody knows what happens with the sexual female after its hatching. Does it hatch before or after a male has found it? When it hatches before: How does it avoid sinking to the sea floor?
- Huys, R., G.A. Boxshall and R.J. Lincoln (1993); The Tantulocaridan Life Cycle: The Circle closed? Journal of Crustacean Biology, Volume 13, Issue 3, pp. 432–442.
- Boxshall, G.A. & R.J. Lincoln (1987); The Life Cycle of the Tantulocarida (Crustacea). Philosophical Transactions of the Roya Society of London. Series B, Biological Sciences, Volume 315, Issue 1173, pp. 267-303.
- Schminke,H.K. (2007); „Crustacea“ in Westheide, W. & R. Rieger’s „Spezielle Zoologie Teil 1. Einzeller und Wirbellose. Elsevier. Spektrum Akademischer Verlag 555-637.
No comments:
Post a Comment