I could be wrong, but it seems to me that among marine creatures 1) the vast majority of species are carnivorous, and 2) those species which are herbivorous tend to be considerably smaller on average than the carnivorous species. This seems to be in contrast to the situation WRT land animals, where ISTM that there are more herbivorous animals and the herbivores tend to be larger than the carnivores on average.
Questions are whether this is correct, and if so, why this came about?
I can think of two possible factors, one for each phenomenon, but neither is particularly satisfying.
[li]Land animals are located on one plane, and this is the same place that is suitable for plants to grow. By contrast, it would seem that the ideal places for aquatic plants to grow would be the ocean floor or the surface, while sea creatures also occupy much of the space between the two.[/li][li]ISTM that a fairly high percentage of fish don’t have much in the way of teeth and swallow their prey whole (by no means all, but it seems far more common than among land carnivores). This means that they need to hunt prey smaller than themselves and can’t rely on ripping out pieces of large sized prey.[/li][/ol]
As above, neither of these is particularly satisfying, and possibly the whole premise is incorrect to begin with.
Your premise is wrong-ish. Marine life doesn’t have the same general plant > herbivore > carnivore sort of food chain that terrestrial life does. For most marine life, the lowest level of the foot chain is plankton. Then you have krill and other tiny-but-not-monocellular organisms, then progressively larger carnivores.
The largest sea animals - baleen whales and filter feeder sharks - are essentially herbivores. They eat krill and other small animals but not by hunting them.
I don’t think it is fundamentally correct. Anywhere from about two-thirds to half of all ocean species are unknown or undescribed, and human efforts are not randomly distributed, but more focused on large, easily-identifiable species. For example, there are about 120 species of marine mammals and possibly as many as 20-40 remain undiscovered and undescribed based on the rate of discovery of new marine mammal species over time (numbers may be slightly off, it’s been four or five years since I read a paper estimating how many undiscovered marine mammal species there are.) On the other hand, there are about 8,000 known species of marine plant and plant-analogues (cyanobacteria, for example, are not technically plants (in the Plantae,) but are plant-analogues) and 70,000 known species of algae, with a large chunk remaining undiscovered, possibly as many as 200,000 for the diatoms alone. (see http://news.discovery.com/animals/whales-dolphins/marine-species-unknown-121115.htm and http://onlinelibrary.wiley.com/doi/10.1111/j.1529-8817.2012.01222.x/abstract for numbers)
The whole premise is incorrect based on our slow progress in thoroughly cataloging the number of species of ocean life, and the ease with which large carnivores are identified as species compared to small algae.
I would have to disagree, whales are carnivores, not herbivores, for example, generally the trophic cascade runs: phytoplankton -> krill -> whales
I would also have to disagree that whales don’t hunt krill. They clearly do, just that they hunt large groups of krill, it being very inefficient to hunt individual krill.
[[Edited to Add: the premise might still be true, but it would be based on the huge number of invertebrate species like small crustaceans outnumbering the huge number of small algal/bacterial species. Fish and other vertebrates would be a rounding error.]]
So you’re confirming the second of my questions; that “those species which are herbivorous tend to be considerably smaller on average than the carnivorous species”, which, again, does not seem to be the case with land animals.
…As long as the sun’s still shining in the big blue sky!
Interestingly, in the ocean, both small algae and various invertebrates tend to move around in ways we don’t think of grass or shrubs moving around on land…
How much vegetation is there in the ocean for large marine herbivores to eat aside from plankton? Just guessing that larger vegetation mostly will be found in shallow water near the shore where manatees and dugong feed.
The thing is that the ecosystem is not at all as on land; by and large, there are no vast expanses of vegetation, or rather algae, to support herbivorous megafauna. And where there is such ecosystem there are mammals taking advantage of it, for example manatees and dugongs feeding on shallow water sea grasses.
I don’t have figures, but I’m certain that vegetable biomass in the seas is hugely dominated by unicellular algae, and that’s just not the sort of thing a mammal can feed on.
Humpback whales often “hunt” small fish by bubble netting. They take turns blowing bubbles in a cylinder shape under a school of fish and the rising bubbles cause the fish to school in a ball. The whales swim up from underneath with their mouths open, swallowing the fish when they breach.
The biomass of terrestrial vegetation is 460 times greater than the biomass of all living marine plants (cite, p124).
The two types of plants found in salty water are seaweeds and phytoplankton. Seaweeds are generally only found in shallow waters and shorelines, where they can get the necessary sunlight for photosynthesis (except for sargasso weed which floats near the surface and can therefore be found in open ocean sea).
So I’m not sure how viable open ocean would be for large herbivores. As noted, whales eat the krill and zooplankton (animal plankton) that are feeding off the phytoplankton (plant plankton). Since all these krill and plankton are likely to be in the same physical space, a filter feeder would be consuming them all together and, I would presume, that the animal-based food content would be more nutritious than the plant plankton. Hence, they are carnivores rather than herbivores.
Which leads to an interesting feature of sea life - turnover of phytoplankton is so rapid that the biomass of consumers at all levels often outweighs the biomass of the primary producers (at least in surface ocean areas where photosynthesis is relevant - not sure if this is known for hydrothermal vents.) This would complicate the development of some new large pelagic herbivore - it would have to become more efficient at catching and eating the phytoplankton than zooplankton, which is very difficult.
On land, the large herbivores have a lot of plant biomass as opportunities to feed from. In the ocean, plant (and plankton) biomass is consumed very rapidly after it’s produced, leaving fewer opportunities for some large herbivore to move in.
However, if that large herbivore decided to eat the zooplankton that had just consumed the phytoplankton instead of trying to beat the zooplankton at their extremely good herbivore game… then you’d have a whale or a whale shark or an anchovy or a siphonophore.
I suspect the high efficiency of marine herbivores is the natural confluence of the advantage of small size due to the surface area/volume ratio and the extreme age of life in the sea relative to life on land - the first is pretty conventional thinking, while the second is a bit of fancy that doesn’t really have any evidence to support it.
This is an interesting paper in PDF relevant to producer/consumer biomass in the oceans.