Carcharhinus isodon by DOTkamina.

Drawing of Carcharhinus isodon, made with a rapidograph and markers. For this work, I used photographs from the Smithsonian Tropical Research Institute and Bryan Huerta as references. The colors used are purely artistic, they do not represent the actual color of this animal! The image quality probably isn't the best. I'm sorry my camera isn't the greatest. I don't think I'll ever move forward (or have the desire) to make a digital version. I feel better drawing "by hand" away from any screen, I'll always repeat that.

You can find this image hosted on: Wikimedia Commons, Twitter, Tumblr, Threads, Pinterest, DeviantArt, Pixiv.

Some facts about Carcharhinus isodon.

Attention. I wrote almost none of the following information. It's a combination of various pieces of information that weren't taken from Wikipedia. Sources at the end!

Description: Blue-grey on top, greyish on sides, white below, white band on sides, fins unmarked. Body relatively slender; eyes large, round, nictitating eyelids present; snout pointed, length before mouth < mouth width, ~1.1-1.3 time width between nostrils; anterior nasal flaps very reduced; upper lip furrows short, not very noticeable; top and bottom teeth nearly symmetrical, with non-oblique narrow points, lower teeth smooth, upper teeth weakly and irregularly serrated; 12-16 rows of teeth on side of top jaw; spiracle absent; gill slits very long (3rd slit longest, > height of anal fin), last two over pectoral, no papillae; first dorsal relatively small, front margin a little convex, rear margin concave, its origin over or just behind free rear tip of pectoral fin; second dorsal relatively large, with slightly concave rear margin, its origin over or slightly behind that of the anal fin; anal fin with deeply notched rear margin; pectoral fins small, narrow, with angular front edge; no ridge on top of body between dorsal fins; no keel on tail base; tail fin strongly asymmetrical, with well developed lower lobe, undulating ridge along dorsal surface of top lobe, and a notch under tip of that lobe.

Image credit: FAO.

Distinguishing Characteristics

1. Snout pointed, shorter than the width of the mouth.
2. Fins unmarked, the undersides of the pectoral fins are pure white.
3. First dorsal fin originates behind the insertion point of pectoral fins along inner margin.

Small in size with a long pointed snout and fairly large eyes. The mouth is broadly rounded in the front with well-defined labial furrows around the corner of the mouth. The gill slits are very long, about half the length of the base of the first dorsal fin. The origin of the first dorsal fin is over or just slightly posterior to the insertions of the pectoral fins. The first dorsal fin is small with a short rear tip. The trailing edge of the first dorsal is falcate with a rounded apex. The second dorsal fin is moderately large with a short rear tip. The pectoral fins are small. Pelvic fins have narrowly rounded tips. The upper edge of the caudal fin is just about straight with a narrowly rounded tip. There is no interdorsal ridge on the finetooth shark.

Species appearing similar to the finetooth shark include the lemon shark (Negaprion brevirostris), blacktip shark(Carcharhinus limbatus), and spinner shark (Carcharhinus brevipinna). The lemon shark can be distinguished by its second dorsal fin that is almost as large as its first dorsal. The blacktip shark and spinner shark have black-tipped fins while the finetooth shark lacks any distinguishing markings on the fins.

Coloration. This shark is dark bluish-grey or bronze above, paling to grayish then to a white underside. There is an inconspicuous white band along the flank. There are no distinguishing marks on the fins.

Dentition. This shark is named for its very small, clear, fine teeth. The upper teeth are narrow and weakly serrated or smooth with erect to slightly oblique cusps. The lower teeth are erect with smooth cusps and transverse roots. Dermal denticles are small and overlapping. Each is a bit broader than long with blades containing three ridges, each with three short teeth with the median tooth longest.

Biology: Commonly found close inshore. Forms large schools. Feeds on small bony fishes and cephalopods. Distinct pairing with embrace. Viviparous (with a yolk-sac placenta), 1 - 6 to 13 young per litter. The embryos are nourished through a placental connection. Gestation is approximately 12 months with the females moving into nursery areas in late May. Birth occurs from late May to mid-June.

Typically hovering around 10 m (33 ft) deep, the deepest they go is 152 m (490 ft). Adults and juveniles are common in shallow coastal waters off South Carolina during the warm summer months and migrate south when surface water temperatures drop below 20 °C (68 °F). This population of ringtooth sharks winters in waters off the coast of Florida. These sharks are often found near estuaries, lagoons, bays, and off river mouths, as long as the salinity is within their range.

Predators: Larger sharks, including the dusky shark (Carcharhinus obscurus).

Parasites: The parasite Phoreiobothrium triloculatum is known to sometimes inhabit finetooth sharks. This parasite also can affect other related sharks in the Carcharhinus genus. It is found in the spiral valves of the sharks.

Importance to humans: In the western Atlantic, this shark may be incidental by-catch on floating longlines in inshore waters but is considered to have little overall economic importance to the commercial shark fishery. However, local fishermen may target them with gillnets close to shore. In the northern Gulf of Mexico, they are taken occasionally by rod and reel, and the meat is presumably marketed and eaten either fresh or dried-salted. While this shark is not involved in any reported injuries, care should be taken when handling these sharks.

Size: average 129 cm, range 119-139 cm; max 160-200 cm. Size at birth 51 to 64 cm.

Depth: 0-20 to 40 m.

Distribution: Western Atlantic: North Carolina, USA to Cuba, the Gulf of Mexico, and southern Brazil to Uruguay. Occurrence in the eastern Atlantic, specifically in Senegal and Guinea-Bissau have not been confirmed, and may be based on Carcharhinus brevipinna.

IUCN status: Near Threatened (NT) (A2bd). Date assessed: 01 July 2019.

References and sources:

• Carcharhinus isodon at IUCN database. 2019.
• FishBase s.f. Carcharhinus isodon (Valenciennes, 1839). Finetooth shark.
• Raimundo D & Bowling Tyler. 2020. Finetooth Shark. Prepared by: Cathleen Bester. Florida Museum. 
• Shark References s.f. Carcharhinus isodon (Valenciennes in Müller & Henle, 1839). Finetooth shark.
• Smithsonian Tropical Research Institute. 2024. Species: Carcharhinus isodon, Finetooth Shark. Shorefishes of the Greater Caribbean online information system.

Why are bauxite and aluminum toxic to organisms?

We all know that aluminum has no apparent use for living beings. Its toxicity lies in its ability to solubilize and diffuse rapidly into the environment, entering and interfering with biomolecules in tissues and organs. One of the largest sources of aluminum is bauxite, which can release this element through acid mine drainage. Learn more about how this happens in this text.

Keywords: bauxite toxicity, aluminum toxicity, geobiology

Try reading the original documents! You'll also find the references used in the writing.

• English. Available too on ResearchGate.
• Spanish.

Image source: こTは

Any rock enriched with mostly aluminum hydroxides is called bauxite. These hydroxides follow the general formula of AlOx(OH)3-2x, according to Wikipedia, and the “x” should be equivalent to any value between 0 and 1. Honestly, I do believe this formula because it makes sense with all the minerals that can make up bauxite (at least from what I have managed to understand, I explain it below, sorry if my deductions are wrong). This formula is also equivalent to another one that is often circulating out there, Al2O3 · nH2O, but this one is somewhat incorrect because it oversimplifies the structure of bauxite minerals, and also makes it seem like they are oxides, not hydroxides.

There are 3 main aluminum hydroxides that can make up bauxite:

• Gibbsite: Al(OH)3, which according to the general formula, would be the same as: AlOo(OH)3-2(0), which by calculation, would mean that the oxygen in “AlO” does not really exist, and on the part of the hydroxide, since it is clear that “3 – 2(0) = 3 – 0 = 3”. And that is why it is simplified to Al(OH)3. But as I said, this deduction may be wrong. The following is more reliable: according to the oxide formula, it would be Al2O3 · 3H2O. If you do the multiplication in the water molecule, you would get that 3H2O = H6O3 = O3H6, to put it in hydroxide mode, and adapting it to the general formula, it would be: Al2O3 (O3H6). Even more simplified, it would be: Al2O6H6. Wanting to leave it as hydroxide: Al2 (OH)6. And we can summarize it even further, dividing the subscripts by the same value (in this case, 2), finally leaving: Al(OH)3.

• Bohmite and diaspore: γ-AlO(OH) and α-AlO(OH), respectively. But it's clear that the general formula for both is AlO(OH). The same logic applies: according to the oxide formula, it would be Al2O3 · H2O. Trying to add it all up, it would be: Al2O4H2. Expressed as hydroxide: Al2 (O4H2). If we divide all the subscripts by 2, we get: Al(O2H). And this way of expressing hydroxide is unconventional; ideally, it should remain as a clean "OH". What we can do to fix it is simply remove one of the oxygens and place it next to the aluminum. This gives us the formula under the general scheme: AlO(OH). If instead we wanted to arrive at that formula by starting directly with the general formula, I think it would be using x=1: AlO1(OH)3-2(1), which would give: AlO(OH)3-2, leaving again: AlO(OH).

But anyway, discussing the chemical formulas of aluminum hydroxides in bauxite was not the purpose of this article. Bauxite is dangerous to organisms primarily because of its aluminum, especially the Al3+ ion, which is highly reactive with the biomolecules of living organisms (meaning it can disrupt their order and cause their functions to fail). In wastewater treatment bacteria, its presence decreases the absorption of other essential metals such as calcium, magnesium, or iron, in addition to hindering cell membrane formation and slowing bacterial growth.

If you think about it, it's strange. Aluminum is one of the most abundant elements in the Earth's crust; in fact, it is the third most abundant and ubiquitous metal in the Earth's crust, accounting for 7% of its mass after oxygen and silicon. It has no known biological use, but it is clearly present in living beings simply because it is a common element in soils. In mineral terms, it is usually incorporated into soils in precipitated form as gibbsite (one of the components of bauxite) or in the form of harmless aluminosilicates. When soils become acidic (pH below 5), gibbsite and other ions can decompose and release Al ions, especially Al3+, which can then solubilize in the soil and disperse more quickly, expanding their toxic effect. This is why acidic soils with some degree of aluminum contamination are so detrimental to plant growth: dissolved aluminum quickly enters plant tissues, interfering with the cytosol, cytoplasm, and cell walls. The effects are usually noticeable in the roots, which do not grow as they should.

It seems that the problem that causes the increase in aluminum solubility is soil acidification. Mining is precisely one of the activities that most acidifies soil. Especially in those where sulfide-rich minerals are extracted, the oxidation of these compounds releases metals (many of them heavy) and sulfuric acid (responsible for acidification) into the soil, and, worse still, into nearby waters. This phenomenon is known as acid mine drainage (AMD). AMD can also be mediated by acidophilic bacteria naturally present in the environment. Acidity can drop to a pH of 3, which is definitely lower than the 5 required for aluminum's toxic effect.

Sulfides are common in almost all soils, one of the most abundant being pyrite (FeS2). The alteration of soils containing sulfides, for example, during mining by exposing them to air, makes them susceptible to oxidation, which results in AMD, as I mentioned in the previous paragraph. Sulfides can be associated with bauxite ores, such as pyrite itself (with gray bauxite, for example), and this is not only an environmental problem but also a technical one, because if it exceeds 0.8%, it already affects the quality of the alumina (Al2O3) obtained from bauxite using the Bayer process, which consists of dissolving bauxite in hot sodium hydroxide (NaOH).

In conclusion, bauxite mining can expose sulfides to oxidation, which causes AMD, which can then attack other compounds in the soil, for example bauxite itself, releasing Al3+ ions that are poisonous to organisms. Another toxic consequence of bauxite mining is the Bayer process used: in addition to the oxides, it also leaves behind a soluble solution of sodium aluminate [NaAl(OH)4], along with residual sludge rich in iron oxyhydroxide (BRR). Both increase alkalinity (pH from 11 to 13), reduce the solubility of essential plant nutrients, and generally create uninhabitable environments, in addition to the excessive amount of Na+ ions.