Posts tagged with biology...
Producing eggs that hatch outside the body. Amphibians, birds, and most insects, fish, and reptiles are oviparous
- Ovuliparity: fertilization is external (in arthropods and fishes, most of frogs)
- Oviparity: fertilization is internal, the female lays zygotes as eggs
- Ovo-viviparity: or oviparity with retention of zygotes in the female’s body or in the male’s body
- Histotrophic viviparity: the zygotes developed in the female’s oviducts, but find their nutriments by oophagy or adelphophagy (eating the other eggs in the womb, or live young).
- Hemotrophic viviparity: nutriments are provided by the female, often through placenta.
Land-dwelling animals that lay eggs, often protected by a shell, such as reptiles and insects, do so after having completed the process of internal fertilization. Water-dwelling animals, such as fish and amphibians, lay their eggs before fertilization, and the male lays its sperm on top of the newly laid eggs in a process called external fertilization.
Almost all non-oviparous fish, amphibians and reptiles are ovoviviparous, i.e. the eggs are hatched inside the mother’s body (or, in case of the sea horse inside the father’s).
(Photo found here)
That long yellow thing in the middle is the Chinese Trumpetfish (Aulostomus chinensis), a species of reef-dwelling fish in the family Aulostomidae. They occur on protected reefs from the eastern coast of Africa, through the Indian Ocean, Australasia, and the Pacific Ocean from Japan and China to the coast of the Americas. They feed on small fishes and shrimps, relying on camouflage and stealth to obtain prey. They occur in three basic color phases: uniformly brown to green, mottled brown to green, or uniformly yellow. They reach a maximum of 80cm.
A polyp in zoology is one of two forms found in the phylum Cnidaria, the other being the medusa (see this post). Polyps are approximately cylindrical in shape and elongated at the axis of the body. In solitary polyps, the aboral (surface opposite the mouth) end is attached to the substrate by means of a disc-like holdfast called the pedal disc, while in colonies of polyps it is connected to other polyps, either directly or indirectly. The oral end contains the mouth, and is surrounded by a circlet of tentacles.
In the class Anthozoa, comprising the sea anemones and corals (see these posts), the individual is always a polyp; in the class Hydrozoa, however, the individual may be either a polyp or a medusa, with most species undergoing a life cycle with both a polyp stage and a medusa stage. In class Scyphozoa, the medusa stage is dominant, and the polyp stage may or may not be present, depending on the family. In those scyphozoans that have the larval planula metamorphose into a polyp, the polyp, also called a “scyphistoma,” grows until it develops a stack of plate-like medusae that pinch off and swim away in a process known as strobilation. Once strobilation is complete, the polyp may die, or regenerate itself to repeat the process again later. With Cubozoans, the planula settles onto a suitable surface, and develops into a polyp. The cubozoan polyp then eventually metamorphoses directly into a Medusa.
The body of the polyp may be roughly compared in a structure to a sac, the wall of which is composed of two layers of cells. The outer layer is known technically as the ectoderm, the inner layer as the endoderm (or gastroderm). Between ectoderm and endoderm is a supporting layer of structureless gelatinous substance termed mesogloea, secreted by the cell layers of the body wall. The mesogloea may be a very thin layer, or may reach a fair thickness, and then sometimes contains skeletal elements formed by cells which have migrated into it from the ectoderm.
The sac-like body built up in this way is attached usually to some firm object by its blind end, and bears at the upper end the mouth which is surrounded by a circle of tentacles which resemble glove fingers. The tentacles are organs which serve both for the tactile sense and for the capture of food. Polyps extend their tentacles, particularly at night, containing coiled like stinging nettle-cells or nematocysts which pierce and poison and firmly hold living prey paralysing or killing them.
(Photo found here)
The coconut crab, (Birgus latro), is a species of terrestrial hermit crab (see this post), also known as the robber crab or palm thief. It is the largest land-living arthropod ( an invertebrate animal having an external skeleton, a segmented body, and jointed appendages) in the world, and is probably at the upper size limit of terrestrial animals with exoskeletons in today’s atmosphere at a weight of up to 4.1 kg (9.0 lb). It is found on islands across the Indian Ocean and parts of the Pacific Ocean. It has been extirpated (extinct in certain areas, but not in others) from most areas with a significant human population, including mainland Australia and Madagascar. The coconut crab is the only species of the genus Birgus, and is related to the terrestrial hermit crabs of the genus Coenobita. Like hermit crabs, juvenile coconut crabs use empty gastropod shells for protection, but the adults develop a tough exoskeleton on their abdomen and stop carrying a shell. Coconut crabs have organs known as “branchiostegal lungs”, which are used instead of the vestigial gills for breathing. They cannot swim, and will drown if immersed in water for long. They have developed an acute sense of smell which they use to find potential food sources. Mating occurs on dry land, but the females migrate to the sea to release their fertilized eggs as they hatch. The larvae are planktonic for 3–4 weeks, before settling to the sea floor and entering a gastropod shell. Sexual maturity is reached after about 5 years, and the total lifespan may be over 60 years. Adult coconut crabs feed on fruits, nuts, seeds, and the pith of fallen trees, but will eat carrion and other organic matter opportunistically. The species is popularly associated with the coconut, and has been widely reported to climb trees to pick coconuts, which it then opens to eat the flesh. Coconut crabs are hunted wherever they come into contact with humans, and are subject to legal protection in some areas.
(Photo by Alan James)
Basking sharks (Cetorhinus maximus) are recognized by their huge sizes, conical snouts, extremely large gill slits, and dark bristle-like gill rakers inside the gills (present most of the year). Basking sharks are the second largest fish, only surpassed by the whale shark. Their average size is 6.7-8.8m. The largest measured basking shark was 9.75m, and a 9.14m long individual was recorded that weighed 3,900kg. There are also unconfirmed reports of basking sharks up to 13.7m long. The basking shark can open its cavernous mouth up to 1.2m wide, allowing water to pass over the gill rakers, which strain small fishes and invertebrates out of the water. They are often seen feeding near the surface. Basking shark populations have been declining since the 1970s; they never fully recovered from the large scale commercial fisheries of the 1950s and remain over-fished in the North Atlantic. Though you might want to steer clear of its mouth, the basking shark is not considered dangerous.
(photo by Scott Gietler)
Giant 15 foot purple-striped jellyfish (see this previous post).