Biodiversity

Wednesday, April 26, 2006

DIVERSITY OF MARINE INVERTEBRATES

From the first living cells some 3 billion years ago, multicellular organisms developed. Among them were the marine invertebrates, with the segmented marine invertebrates becoming the most successful of all marine creatures. These all form part of the Phylum Arthropoda (1) and Phylum Annelida(2). They have managed to inhabit almost all spheres of the oceans, vary from minutely small to very large. Some are carnivorous, others herbivores or omnivores and some even parasites. They drift, swim, crawl, are anchored or burrow. Some, like the Trilobites have gone extinct, but many more have continued adapting to the changing environments of the oceans. Here I have tried to give a short summery of all the groups of living segmented marine invertebrates.

The annelids are segmented worms of which leeches and earthworms are well known examples. Polychaetes (bristle worms) are a class of annelid worms, which are mainly marine species with over 10000 species described. Polychaeta means many-bristled and worms in this class indeed have many bristles which are born on two fleshy lateral outgrowths (parapodia) of each body segment. They use these parapodia for locomotion and they also function as external gills (3). More than 70 families are classed under the polychaetes. They have a wide variety in size, habits and habitats. For example, the Pompeii worm (Alvinella pompejana) is a deep sea species that is only found around hydrothermal vents in the Pacific Ocean. They can tolerate temperatures of up to 80 degrees Celsius (4). Another species recently discovered is called the bone-eating snot flower (Osedax mucofloris) which is also a deep sea species that feeds on whale fat which is digested through bacteria.(5) More common polychaetes are the lugworms (Arenicola marina)(6) and sandworms (Nereis)(7). Lugworms inhabit inter-tidal zones of beaches and can measure up to 25cm. Sandworms are free swimming scavengers that feed at the bottom of shallow marine waters, with some species exceeding 1m in length.

Other classes that contain marine annelids are the Myzostomida (8), a group of small parasitic worms that live on sea lilies (crinoids) and the Echiura (9), known as spoon worms that inhabit mostly shallow waters, where they mostly live in burrows. The spoon worms do not have a segmented structure, but they are grouped under the annelids.

The phylum Arthropoda(1) is the biggest in the animal kingdom, comprising some 80% of all living animals. It includes the spiders, insects, horseshoe crabs, sea spiders, crustaceans and others. Arthropods have a hard exoskeleton and a segmented body with appendages on each segment. The crustaceans are the largest marine group that belongs to this phylum.

Common members of the crustaceans are lobsters, crabs, shrimps and barnacles, but they are not the only ones. The Branchiopoda(10) is a group with over 900 species which include the water fleas (Daphnia)(11) and the brine shrimps (Artemia) (12). Daphnia are widely spread and primarily planktonic. They inhabit a wide range of aquatic environments. The brine shrimps can be found worldwide in saltwater, although not in oceans. Both are used commercially for aquarium food.

The Remipedia(13) are blind crustaceans that live in deep caves with only a dozen or so living species. They are colourless, swim on their backs and have poisonous glands to kill their prey.
The Cephalocarida(14) are known as the horsehoe shrimps with only 9 known species. They are benthic, quite primitive and feed on organic waste material from plants and animals .

The Maxillopoda(15) is a group that includes the barnacles and copepods. They are characterized by a reduction of the abdomen and appendages. Many of these species have planktonic larvae. Some are minute ectoparasites others are bigger ectoparasites like the fish lice (Branchiura). Some live in sedimentary environments on the sea floor and some like the copepods inhabit nearly all freshwater habitats as well as the sea.

The Ostracoda(16) are small round crustaceans also known as seed shrimp. They inhabit sea beds or can be part of zooplankton.

The Malacostraca(17) is the biggest group and contains amongst others, the better known crustaceans, namely the crabs, lobsters and shrimps. More than 22,000 species have been identified. The group is so diverse, filling many marine niches. Some interesting examples are: The Mantis shrimp, which is neither shrimp nor mantis, but gets its name from resembling a shrimp and striking like a mantis. It has powerful claws, formed like jackknives with which it can dismember its prey (18). The Japanese Spider Crab, which is the largest crab that can reach a leg span of almost four meters and can weigh up to 20 kg (19). And the Krill, a tiny shrimp like crustacean which forms part of the zooplankton(20).

Another group of Arthropods are the Merostomata(21), which include the horse shoe crabs and the extinct eurypterids, the largest arthropod known to have lived, that could grow up to two meters. The horseshoe crabs are closer related to spiders, than to crabs, but they live beneath the sand in the ocean and only migrate to the shore to mate once a year (22).

Lastly the sea spiders (Pantopoda) are marine arthropods with more than a 1000 species known. There size varies from a tiny 1mm up to 90cm. They inhabit from inter tidal zones up to deep sea beds as deep as 7,000m (23).

As you can see, these creatures have truly managed to inhabit all nooks and crannies of marine habitat. I have been fascinated by the diversity of these segmented invertebrates and the immense success they have had in persevering and adapting to their ever changing environments over millions of years.

References:

1. Wikipedia contributors. Arthropod [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 23, 20:09 UTC [cited 2006 Apr 26]. Available from: http://en.wikipedia.org/w/index.php?title=Arthropod&oldid=49800523.
2. Wikipedia contributors. Annelid [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 25, 17:25 UTC [cited 2006 Apr 25]. Available from: http://en.wikipedia.org/w/index.php?title=Annelid&oldid=50114575.
3. Wikipedia contributors. Polychaete [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 25, 18:33 UTC [cited 2006 Apr 25]. Available from: http://en.wikipedia.org/w/index.php?title=Polychaete&oldid=50124195.
4. Wikipedia contributors. Pompeii worm [Internet]. Wikipedia, The Free Encyclopedia; 2006 Mar 22, 15:33 UTC [cited 2006 Apr 26]. Available from: http://en.wikipedia.org/w/index.php?title=Pompeii_worm&oldid=44958957.
5. Wikipedia contributors. Osedax [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 9, 11:38 UTC [cited 2006 Apr 26]. Available from: http://en.wikipedia.org/w/index.php?title=Osedax&oldid=47687465.
6. Wikipedia contributors. Lugworm [Internet]. Wikipedia, The Free Encyclopedia; 2006 Mar 15, 20:49 UTC [cited 2006 Apr 25]. Available from: http://en.wikipedia.org/w/index.php?title=Lugworm&oldid=43941648.
7. Wikipedia contributors. Sandworm [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 11, 23:59 UTC [cited 2006 Apr 25]. Available from: http://en.wikipedia.org/w/index.php?title=Sandworm&oldid=48037966.
8. Wikipedia contributors. Myzostomida [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 22, 02:57 UTC [cited 2006 Apr 26]. Available from: http://en.wikipedia.org/w/index.php?title=Myzostomida&oldid=49539768.
9. Wikipedia contributors. Echiura [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 25, 14:55 UTC [cited 2006 Apr 26]. Available from: http://en.wikipedia.org/w/index.php?title=Echiura&oldid=50093587.
10. Wikipedia contributors. Branchiopoda [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 24, 07:40 UTC [cited 2006 Apr 26]. Available from: http://en.wikipedia.org/w/index.php?title=Branchiopoda&oldid=49885680.
11. Wikipedia contributors. Daphnia [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 18, 09:17 UTC [cited 2006 Apr 26]. Available from: http://en.wikipedia.org/w/index.php?title=Daphnia&oldid=48975694.
12. Wikipedia contributors. Brine shrimp [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 25, 13:53 UTC [cited 2006 Apr 26]. Available from: http://en.wikipedia.org/w/index.php?title=Brine_shrimp&oldid=50085444.
13. Wikipedia contributors. Remipedia [Internet]. Wikipedia, The Free Encyclopedia; 2006 Jan 31, 11:33 UTC [cited 2006 Apr 26]. Available from: http://en.wikipedia.org/w/index.php?title=Remipedia&oldid=37503769.
14. Wikipedia contributors. Cephalocarida [Internet]. Wikipedia, The Free Encyclopedia; 2006 Jan 30, 14:41 UTC [cited 2006 Apr 26]. Available from: http://en.wikipedia.org/w/index.php?title=Cephalocarida&oldid=37361281.
15. Wikipedia contributors. Maxillopoda [Internet]. Wikipedia, The Free Encyclopedia; 2006 Feb 21, 10:04 UTC [cited 2006 Apr 26]. Available from: http://en.wikipedia.org/w/index.php?title=Maxillopoda&oldid=40549062.
16. Wikipedia contributors. Ostracod [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 23, 16:16 UTC [cited 2006 Apr 26]. Available from: http://en.wikipedia.org/w/index.php?title=Ostracod&oldid=49768943.
17. Wikipedia contributors. Malacostraca [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 24, 07:47 UTC [cited 2006 Apr 25]. Available from: http://en.wikipedia.org/w/index.php?title=Malacostraca&oldid=49886186.
18. Wikipedia contributors. Mantis shrimp [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 8, 14:56 UTC [cited 2006 Apr 25]. Available from: http://en.wikipedia.org/w/index.php?title=Mantis_shrimp&oldid=47558389.
19. Wikipedia contributors. Japanese spider crab [Internet]. Wikipedia, The Free Encyclopedia; 2006 Mar 15, 09:48 UTC [cited 2006 Apr 25]. Available from: http://en.wikipedia.org/w/index.php?title=Japanese_spider_crab&oldid=43871517.
20. Wikipedia contributors. Krill [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 10, 09:24 UTC [cited 2006 Apr 25]. Available from: http://en.wikipedia.org/w/index.php?title=Krill&oldid=47777968.
21. Wikipedia contributors. Merostomata [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 22, 20:48 UTC [cited 2006 Apr 26]. Available from: http://en.wikipedia.org/w/index.php?title=Merostomata&oldid=49646997.
22. Wikipedia contributors. Horseshoe crab [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 22, 20:49 UTC [cited 2006 Apr 25]. Available from: http://en.wikipedia.org/w/index.php?title=Horseshoe_crab&oldid=49647185.
23. Wikipedia contributors. Sea spider [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 8, 11:23 UTC [cited 2006 Apr 25]. Available from: http://en.wikipedia.org/w/index.php?title=Sea_spider&oldid=47540757.

Karen Marais
BCB Hons NISL student
University of the Western Cape
Private Bag X17
Bellville

E-mail 2657211@uwc.ac.za
Web http://brit-journal.com/karen2006bcbnisl/