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Bio-bullets

Why do mangroves lack fruity succulence?

Intertidal trees are not dispersed by animals because palatable fruit-pulp depends on potassium, a nutrient outweighed by sodium at the tideline.

professor caricatureProf. Mumblebard claims: "Mangroves are dispersed by flotation because they need to be sown in the intertidal zone, an unlikely destination for seeds carried in the guts of fruit-eating animals. These trees lack{njaccess 2, 3, 4, 5, 6, 7, 8} succulent fruit-pulp palatable to animals because the families are specialised to have buoyant diaspores with, in the case of several genera, precocious germination which is incompatible with surviving the digestion of the dispersal agent." {!njaccess}… See the hidden half of Prof. Mumblebard’s claim by subscribing here{/njaccess}

logoRobin and the Honey Badger respond: "Most families of mangroves are unspecialised enough to contain non-mangrove genera producing fleshy, animal-dispersed diaspores in other vegetation types. So the evolution of dispersal by animals is theoretically possible among mangroves. And flotation of fruits need not exclude either precocious germination or dispersal and sowing by fruit-eating animals. For example, many species of trees in Amazonian flooded forest have palatable diaspores similar to the bird-dispersed fruits typical of tropical rainforests, and are dispersed and sown by fishes instead of birds. The real reason lies in the nutritional imbalance typical of intertidal forests. Fruit-pulp which ripens to a succulent texture is typically rich in potassium while being poor in all other nutrients. Because the potassium is necessarily lost to the plant during dispersal, it follows that plants tend to evolve for dispersal in the guts of animals in environments where potassium is particularly freely available relative to other nutrients. In those environments{njaccess 2, 3, 4, 5, 6, 7, 8} where the supply of potassium remains in equilibrium or in short supply, it tends to be allocated to essential functions such as the growth of foliage and flowers and seeds. Sea water and mangrove muds are rich in potassium but tend to be disproportionately rich in other elements including sodium, which is not only toxic for plants but tends to be antagonistic to potassium owing to their chemical similarity. Although halophytic plants achieve succulence, no halophyte has succulent fruits in which sodium substitutes for potassium in maintaining turgor or nourishing fruit-eaters. Furthermore, the absorption of potassium is not passive but depends on metabolic activity in the roots, and this is constrained in the anoxic, waterlogged muds inhabited by mangroves. As a result, intertidal plants are frugal with potassium, avoiding the use of ripe fruits attractive to fruit-eating animals, and instead tending to rely on flotation exclusively." {!njaccess}… Reveal the hidden half of this response by Robin and the Honey Badger by subscribing here{/njaccess}

 

Please join us here at the Bio-edge with your own comments. In the discussion below we encourage links to any evidence supporting either Prof. Mumblebard or Robin and the Honey Badger. Illustrations are welcome but please cite all sources or we may be forced under copyright to delete your comment.

Why do mangroves lack fruity succulence?

Last modified on 24 June 2015

Comments   

0 #1 Greenbrain 2014-02-18 14:17
Now that Robin & HB mention it it is odd that the mature fruits of the grey mangrove Avicennia marina, one of the commonest mangroves in Africa/Arabia/A sia/Australia/N ew Zealand, look as if they are just one step away from being edible. These fruits are ‘ripe’ but they are far from edible, so appearances are deceptive. The seeds germinate inside the fruits before they fall into the water where they float off. Maybe big fishes might swallow them by mistake?


Photo by Peripitus under CC license 3.0
0 #2 Phytoforce 2014-02-18 14:48
I think what a lot of people would assume when thinking about dispersal biology of mangroves is that these trees have to make their diaspores float so that rules out conventional edible succulent fruits. But thats not really logical because there are lots of edible succulent fruits that float fine. I’ll give you one example Syzygium samarangense, see the pictures here. This tree grows at the coast quite close to the mangroves but not in the salty zone. Its fruits are edible to humans and ripen as expected. This genus combines ‘zoochory’ and also flotation-dispe rsal. So it is interesting to try and explain why mangroves don’t have fruits like this. Actually its interesting to think why Syzygiums didn’t evolve into mangroves because the species in this genus are so widespread around the world and like seasonally wet places. Does anyone know if syzygium seeds germinate when they’re still inside the fruits?
0 #3 Phytoforce 2014-02-18 14:51

Syzygium samarangense [photo by Allen Timothy Chang under CC license 3.0]
0 #4 Phytoforce 2014-02-19 12:01
Here’s what the inside of the floating fruit of Syzygium samarangense looks like. The ripe edible fruit floats well because the fruit-pulp at the centre of the fruit is naturally full of tiny air pockets.

[photo by M0rphzone under CC license 3.0]
0 #5 Greenbrain 2014-03-03 14:15
You don’t have to go to faraway lands to find fruits that combine being flotation dispersed with being animal dispersed. The domestic apple is an example. Apples float. That’s because they have lots of tiny air spaces built into the fruit-pulp in a way that their close relatives, pears, don’t. I’ve never seen this written anywhere, but I think this is part of the natural dispersal of the ancestral crab-apples. Apples ripen in autumn and there are too many to get eaten before winter falls. Apples can keep well in the snow all winter long and come to think of it maybe that’s helped by their air content. Then in spring, they stand a chance of being dispersed some way by meltwater before they are eaten by an animal that poops out the seeds some distance away again. So R & HB have a point. For a mangrove in theory there doesn’t seem to be anything mutually exclusive between hydrochory and zoochory. It’s a fair question why mangroves arent zoochorous as well as being hydrochorous.
+1 #6 Fanning 2014-03-11 09:27
If the idea is that potassium-rich habitats tend to have plants with succulent fruits, then this can be tested by looking at the vegetation on ultramafic rocks and serpentine soils. Ultramafics have very little K and excess magnesium. Serpentine vegetation should theoretically have little endozoochory.
0 #7 Julien Peter Benney 2014-05-31 02:56
Quoting Fanning:
If the idea is that potassium-rich habitats tend to have plants with succulent fruits, then this can be tested by looking at the vegetation on ultramafic rocks and serpentine soils. Ultramafics have very little K and excess magnesium. Serpentine vegetation should theoretically have little endozoochory.
That is striking true of the heavily ophiolitic regions of New Caledonia and northwestern California – both regions lack fruiting trees even in hot or mild climates, instead having conifers like Araucaria and Pseudotsuga, which with their hard seeds and lacking flowers can conserve potassium much more effectively than angiosperms.

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