Sponges do not have nervous, digestive, or circulatory systems. Instead, most rely on maintaining a constant water flow through their bodies for their survival. One spectacular species is the Giant barrel sponge (Xestospongia testudinaria) found in the Philippines, Australia, western and central Indian Ocean, Indonesia and Malaya (picture left, taken at Sipadan). A related Caribbean giant barrel sponge is called Xestospongia muta.
The enormous barrel-shaped sponges help to improve water clarity, support reef regeneration, and provide a habitat for other invertebrates, benthic fish, and bacteria living inside or on the surface of the sponge. Its body is hollow and is held in shape by the mesothyl, a jelly-like substance made mainly of collagen and reinforced by a dense network of fibers also made of collagen. The inner surface of most sponges is covered with collar cells or choanocytes, cells with cylindrical or conical collars surrounding one flagellum (‘whip’: see photo section above).
Pumping to survive The first task of the pumping system of the sponge is to create a flow of water and the second is to capture food items as they pass by these cells. Filter-feeding sponges filter a water volume six times or higher than their volume body per minute. On a daily basis, that is around 50,000 times their own volume in water. The flow generated by the currents pushes the seawater through the pores on the body (ostia) of the sponge, where the nutrients (mainly plankton) are captured, before being ejected from the top opening called osculum (see photo section above). In more detail, the mechanism works as follows. Water enters the sponge through numerous incurrent pores, which line the sponge surface, and then flows through a series of branching and successively narrowing incurrent canals to the water-pumping units, the choanocyte chambers. Choanocytes are versatile cells which extensions called flagellae (‘little whips’) that create the active pumping of water through the sponge, while the collars of the choanocytes are the primary areas that nutrients are absorbed into the sponge (see photo section above). The action of the choanocyte flagellum is to generate a low pressure to draw water through the collar. The basic mechanism that drives water through the sponge's body is believed to be the wave-like motion of the whip-like flagellae working in parallel. From here, water subsequently flows through a series of excurrent canals that merge and empty into the atrial cavity, the spongocoel, then to an excurrent osculum.
In sum: The choanocytes appear to work in parallel and therefore function as the basic pump units. They deliver the moderate pressure rise required to draw water through the inhalant openings (ostia) and to maintain flow through the rather short and open canals and the exhalant openings (oscula) of these species. The flow of water is initiated through the coordinated beating of flagella. Once water enters the sponge through the ostia, it passes through a canal system of lesser or greater complexity, depending on the species, until it reaches the choanocytes.
In biology, the gregarious behaviors of fishes such as shoaling and schooling mean different things. Fishes that stay together for social reasons are shoaling, and if the group is swimming in the same direction in a coordinated manner, they are schooling (*). Schools may, however, vary in density and polarity (direction) with the ‘bait ball’, a densely packed three-dimensional structure as an extreme example. Shoaling fish can shift into a disciplined and coordinated school, then shift back to an amorphous shoal within seconds. Such shifts are triggered by changes of activity from feeding, resting, traveling, or avoiding predators. Schooling fish are usually of the same species and the same age/size while shoaling aggregations can also be mixed.
Examples of schools Many species of fish of the coral reefs, such as jacks, snappers, sweetlips, rabbitfishes, bannerfishes, silversides, and glassy sweepers form schools of different densities and structures. Normally such fish inhabit open water with little opportunity to hide in rocks and weeds or even in the sand. Schools of barracudas can be seen taking shape as a tornado above a sandy plateau. Jacks like big eye trevallies (Caranx sexfaxiatus) and snappers congrgate in dense schools taking the form of balls: a tight spherical formation of numerous fish, or spirals containing more than 1000 individuals. On shallow coral reefs, the silversides often form enormous clouds that move in the same direction and dart away with lightning speed if a dangerous intruder like a coral grouper comes too close. The hardhead silverside occurs commonly in large schools along sandy shorelines and reef margins. It is reported to be a largely nocturnal fish that forms schools numbering from several hundred individuals to aggregations that may be over 100m long and 20m wide. Tight school-like bait-balls are a favorite target for UW photographers, the general principle being the larger en denser the better, the golden rule: get as close as possible to the school, preferably equipped with a fish-eye lens and using natural light. Unfortunately for the schools, modern fishing boats can now easily detect dense schools of herrings or sardines on their ultrasound fishfinders and scoop up thousands in one catch.
Why schooling? Why fishes school is still a matter of debate and controversy. Schools may serve several reproductive functions, like spawning (Bohar snappers are a good example; see insert above), efficient foraging of social groups, and hydrodynamic advantages. Input coming from all the school members in scanning for food or threats will be better than that from an individual fish. Gregarious behavior is also considered as a form of cover-seeking in which each animal tries to reduce its chance of being caught by a predator.* The ‘’oddity effect" posits that any shoal member that stands out in appearance will be preferentially targeted by predators. This may explain why fish prefer to shoal with individuals that resemble themselves. The oddity effect thus tends to homogenize shoals, and even more so by forming dense schools. Accordingly, Hamilton proposed the selfish-herd theory, stating that individuals can reduce the risk of predation by moving to specific positions, in particular toward one another, within the group. In a bait-ball, centripetal instincts in already gregarious species are manifested in choosing the safest positions in the center rather than more risky positions at the outside. From this point of view, schooling is mostly seen as an adaptive strategy against predators developed by natural selection of individuals seeking protection by staying close to its congeners, *** Another important benefit of schooling is increased vigilance (the ‘many eyes’ effect) to detect potential predators. The factor of communal alertness, makes life more difficult for a predator and safer for a gregarious prey, especially if the predator is one that relies on stealth rather than speed.
Is schooling also survival effective? Predators may indeed be inhibited to attack dense schools since they are used to target out single individuals. But some have learned to use countermeasures to undermine the defensive shoaling and schooling maneuvers of prey fish. The sailfish, for example, raises its sail to make it appear much larger so it can herd and scatter a tight school of fish. Swordfish charge at high speed through fish schools, slashing with their swords to kill or stun prey, immobilizing large numbers with blows from its sharp-edged sword. Similarly, thresher sharks use their long tail as powerful whips to frighten and to spread out the group. They then turn and return to consume their "catch". Worse still for the school, an effective strategy of predators against loosely schooling fish may be to first scare them into forming a bait ball, somewhat similar to the way a sheepdog forces a loose group of sheep into a compact flock. A bait ball works much better if the fish school is first brought into a compact form. Dense schools, in particular bait balls of sardines, are easier to detect by fishes like sharks, tunas, and dolphins using their sonar. Dolphins are known to use bubble curtains to isolate and herd groups of sardines. Gannets detect a shallow bait ball of sardines much easier from the sky than individual species and then dart down through the surface to penetrate and consume their catch. Notice that predators also work together to ensure maximum efficiency in attacking the bait balls.
Cost-benefit principle The paradox is although the predators are the greatest winner when they attack bait-balls and cause the most damage in the number of fishes killed, the sheer greater number of fishes or safety in numbers remains a powerful tool. A given predator attack will also eat a smaller proportion of a large shoal than a small shoal. The evolution of the gregarious tendency as a defense against predators may thus still be the best strategy in the long term for prey fishes, even though the result is a considerable lowering of the overall mean fitness during an occasional attack of certain predators.
In sum, the ‘decision’ of fishes to form a school or not depends on the balance between the risk of capture and the benefit of foraging or mating. Guppies are known to engage in high-risk courtship behavior even when a predator is lurking in the neighborhood. Alternatively, fishes may decide to switch from polarized dense grouping to more loose aggregations depending on the conditions of the danger zone.
*Magurran, Anne E. “The Adaptive Significance of Schooling as an Anti-Predator Defence in Fish.” Annales Zoologici Fennici, vol. 27, no. 2, Finnish Zoological and Botanical Publishing Board, 1990, pp. 51–66
Baleen whales are the largest known animals that have ever lived. They feed on minuscule prey by filtering seawater through plates of frayed, bristle-like combs, termed baleen, that are fixed to their upper jaws. Savoca et al. recently reported in their Nature article (1) on the feeding patterns of the baleen whales that all seven species studied consumed up to three times more prey biomass than expected from previous estimates. They were found to eat even more than 30 percent of their own body weight per day.
By eating iron-rich krill and discharging iron-rich fecal plumes in the surface layer, whales were substantially enhancing phytoplankton growth (see also the insert), boosting the availability of food for krill. Thus with their iron-rich droppings, baleen whales indirectly promoted the growth of their own food (krill) in Antarctic waters. A perfect cycle. The krill biomass consumed by whales alone is estimated to have been 190 million tonnes annually, an amount substantially greater than the entire annual world fish catch in modern times.
Krill started declining in large quantities after the decimation of the whales, with the last large-scale surface swarms having been recorded in early 1980. This ‘paradoxical’ decline in krill is consistent with a model in which whale-aided iron cycling supported the growth of krill populations. Thus, the hugely productive ocean pastures dominated by diatoms, described in the past, have since reverted to the classic iron-limited water that is now characteristic of the degraded large areas of the ocean’s surface
In his comments on the article, Victor Smetaceks argued that humans have in their power the means to mimic the iron fertilization mediated by whales to create diatom blooms, to feed the krill, and thereby to feed the whales and perhaps restore their original numbers.
(1) Savoca, M. S. et al. Nature 599, 85–90 (2021)
Birds have large wings, but a light body. Despite their walnut-sized brains, they do have the reputation of being pretty smart. This holds not only for singing birds that produce amazingly complex songs, but also corvids (members of the crow family) and parrots. Avian biologists have been puzzled for a long time how intelligent and complex behavior of birds can be produced by such tiny brains. Avian brains may not have large, but highly compact brains many of these neurons are located in the forebrain, the area that is connected with cognition in humans. They also found that parrots and corvids have forebrain neuron counts equal to or greater than primates with much larger brains.
The amazing quality of seabirds is their ability to navigate over the immense oceans using their smell as their major tool. This holds in particular for Procellariiformes, an order of tube-nosed seabirds that comprises four families: the albatrosses, shearwaters, fulmars, and 2 families of storm petrels (Hydrobates pelagicus). All species are accomplished long-distance foragers, and many undertake long trans-equatorial migrations. They have elongated tubes or nostrils on their thick upper beaks that probably contribute to their enhanced ability to smell prey in the open ocean (see insert). But more importantly perhaps is that these birds have olfactory tissues in the brain that take up about 37% of total brain volume (as compared with the 3 % of songbirds).
Until Gabrielle Nevitt from the University of California, Davis, came along, no one had fully explored how seabirds use their elaborate nose gear to track their prey over vast ocean expanses. How do they do it? One chemical in particular called dimethyl sulfic (DMS) seems to play a crucial role. DMS is generated when zooplankton (krill) devours phytoplankton. For us, DMS may smell like oysters on a half shell , or the seashore. DMS forms odor plumes, like puffs of cigarette smoke, that drift over the ocean surface and are picked up by the birds, zigzagging and continuously sniffing until zeroing in on their prey. Storm petrels are especially good at detecting the chemicals at a great distance. In addition, albatrosses may cover thousands of kilometers in a single foraging trip. Their survival thus depends on finding the proverbial needle in the haystack on a daily basis, often in dark and cloudy conditions within very limited visibility.
Nevitt, C.A. (2008). Sensory ecology on the high seas. The odor world of the procellariiform seabird. J. Exp Biology, 211, (2008).1706-1713.
Ackerman, J., The Bird way, Chapter four. The scent of sustenance. Penguin Books. 2021.