
Whatβs the one dietary fact everybody knows about sharks? Correct, they eat human beings β as graphically shown in the creature feature film sensation of 1975, the movie Jaws (and its various good, bad, and indifferent sequelsβ¦). As so-called apex predators, sharks are famously considered to be carnivorous, but, thatβs not necessarily so.
Samantha Leigh et al. have examined the bonnethead shark (Sphyrna tiburo) that eats seagrass.* It needs here to be pointed out that consumption of seagrass β along with cephalopods [animals such as squid and octopus] and crustaceans [animals such as crabs and shrimps] β by this shark species has previously been reported by Dana Bethea et al..
What Leigh et al. have added to that knowledge is evidence that the seagrass is being both digested and assimilated into the sharkβs tissues. This is an important distinction and is a nod in the direction of the discipline of ecological energetics ** and recognises that just because something is taken into the mouth of an organism β i.e. itβs βeatenβ β doesnβt necessarily mean that it is ultimately digested, absorbed and contributes to growth of the consumer. Leigh et alβs study therefore not only adds information that has relevance to a better understanding of the operation of the oceanβs ecosystems, it also serves to underline how complex feeding webs can be β especially in assessing our understanding of the completeness, or otherwise, of the full range of feeding relationships that exist.
What the study doesnβt actually allow us to decide is whether this marine meadow-munching behaviour is a regular part of the sharkβs dietary regime [in which case it may be vegetarian at best, or βmixivorousβ at βworstβ] or whether it represents an attempt at self-medication for an unknown malady that seagrass is a shark-known treatment for. In any event, intake of vegetable matter happens in some sharks.*** Top tip for those considering going into the sea: Donβt wear your seagrass print bathing costume, no matter how sartorially-elegant you think it may be.
* Irritatingly β and another example of zoochauvinism/plant blindness? β Leigh et al. donβt specify the species (or species, plural) of βseagrassβ involved in their article (nor apparently in the Supplementary Material here and here). Given that there are 61 β 72 (approx.) species of seagrass to choose from, weβd like to know what the sharks are eating.
** Although we wonβt here go into the complexities of Consumption, Assimilation, and Production Efficiencies, these are important considerations for any studies related to feeding behaviour of animals.
*** The most notable βvegetarian sharkβ is the whale shark (Rhynchodon typus), which lives on β off? β plankton. But, even so, plankton is a veritable smΓΆrgΓ₯sbord of both plant-like [phytoplankton] and animal [zooplankton] species. So, even this notable vegetarian isnβt exclusively a βplantβ-eaterβ¦
**** This is the second in a 5-item series of ‘plants as food’ articles. The previous article is here. The next item can be found here.
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