Nutrition
 
  Nutrition
 
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Carnivory hot buttons for carnivory part of Biology of Caribbean Coral Reefs website hot button for zooplanktivore part of BCCR hot button for spongivore part of BCCR hot button for corallivores & other cnidivores part of BCCR hot button for gorgonivores part of BCCR hot button for benthic invertebrate-eaters part of BCCR hot button for piscivores part of BCCR

This part of nutrition deals with carnivory, that is, with coral-reef organisms that eat animals. Other topics relating to nutrition of coral reefs can be accessed via the icons.

Major predators of sponges are vertebrates, including fishes and turtles, but a few invertebrates, includings sea stars and nudibranchs, also eat sponges. Turtles will be considered here, while FISHES AND SEA STARS/NUDIBRANCHS/SNAILS are dealt with in their own sections.

 
 

Carnivory: spongivores: turtles that eat sponges

 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of turtle-chewed sponge Geodia

"Here's another preyed-upon sponge. This time a leathery barrel sponge, Geodia. And look here, the bite marks are fresh. Let's find the perp. Ah, hah...here he is, a hawksbill turtle, intently eating and quite oblivious to our presence." - Turneffe Island, Belize. Video courtesy Andy Stockbridge, Belize.

NOTE Geodia neptuni

 
 

Sponges comprise the bulk of the diet of hawksbill turtles. Studies on Caribbean hawksbills show that of the many sponge species potentially available as food, 10 are favoured. The 2 sponges shown here rank high in terms of turtle-preference, chicken-liver sponges being the No. photograph of hawksbill turtle Eretmochelys imbricata eating a sponge Geodia neptunifavoured food of hawksbill turtles, while the photograph of chicken-liver sponge Chondrilla nucula leathery sponge Geodia neptuni being No. 3. This latter species is one of only a few dominant reef sponges that is known not to contain deterrent chemicals. Meylan 1988 Science 239 (4838): 393; Dunlap & Pawlik1996 Mar Biol 126: 117.

Chicken-liver sponge
Chondrilla nucula
0.8X

 

Hawksbill turtle Eretmochelys imbricata eating Geodia
neptuni
0.33X.

 
 

SCUBA-divers commonly encounter hawksbill turtles Eretmochelys imbricata during dives. Since an important function of diving by a turtle is to feed, we can ask whether diving behaviours differ during day and night, in part relating to the need for the turtles to see their sponge foods. Time-depth recorders attached to immature hawksbill turtles in Puerto Rico provide information on some of their diving behaviours. Van Dam & Diez 1997 Coral Reefs 16: 133.

NOTE is smell involved in a turtle's identification of its prey?

Some additional data on diving behaviour of hawksbill turtles in Bloody Bay, Little Cayman Island are included as graphs beside the cartoons for turtles in Puerto Rico. In the Little Cayman study, 21 juvenile/adult specimens are captured by hand, equipped with time/depth recorders, and monitored at various times over an 8d period in autumn. Following this, the tagged individuals are recaptured, the devices removed, and the animals released. Major points of difference in the 2 sets of results are given. Blumenthal et al. 2009 Coral Reefs 28: 55.

 

drawing comparing time spent underwater by hawksbill turtles during daytime and nighttime
Total time spent underwater does not differ significantly during daytime
and nighttime
 
 
drawing comparing duration of dives by hawksbill turtles in daytime and nighttime Dives are almost twice as long at nighttime than in the
daytime in Puerto Rico. Van Dam & Diez 1997
   
graph comparing dive durations between night and daytime for hawksbill turtles in Little Cayman Island
Dives in Little Cayman Island are similarly longer during nighttime than daytime. Blumenthal et al. 2009
 

Note in the above graphs that dive duration at night correlates better with body size than during the day. Larger individuals can stay down longer, as would be predicted from their relative metabolic expenditures. This would be shown for daytime divers, as well, were it not for the more variable metabolic activities engaged in during daytime, such as searching for food, feeding, avoiding predators (sharks, goliath groupers), and inter-individual interaction.

Dive depth is regulated in a turtle by extent of inflation of the lungs. Therefore, depending upon depth, a turtle may have partially inflated lungs. A likely scenario is that a full breath is taken at the surface, then the turtle powers down to, say, feeding depth, and exhales enough air to produce neutral buoyancy. This saves energy. However, while neutral buoyancy can be maintained at any shallower depth than this just by exhaling, the turtle can't go any deeper and still maintain neutral buoyancy owing to compression of the lungs. A SCUBA-diver accommodates this by adding air either to lungs or buoyancy device from a "self-contained" supply. Might a turtle "know" the depth to which it plans to go and charge up its lungs at the surface accordingly? Yes, the turtle could well know the depth to which it is going (mainly because it has just come back from there), but why wouldn't it "hedge its bets" by taking a full lung-fuil of air, then swimming down and exhaling for optimum buoyancy?

During nighttime, hawksbill turtles often are seen wedged into crevices, thus enabling them to take in more air at the surface and to stay down longer more safely (away from wave turbulence and protected from predators) with more energetic efficiency than otherwise. This is termed "assisted resting". Cool, huh?

 
Resting interval at the surface is less than
a minute whether daytime or nighttime

 

Although the authors term this surface interval "resting", it is anything but. This is the time when the turtle exhales whatever air remains in its lungs and inhales in new, fresh volume for the next dive, and does it as quickly as possible.

 

drawing comparing surface interval of hawksbill turtles in daytime and nighttime
   
 
drawing comparing diving depths of hawksbill turtles in daytime and nighttime

Dive-depths of juvenile turtles do not differ significantly
between daytime and nighttime in Puerto Rico. Van Dam & Diez 1997

   
graph comparing nighttime and daytime dive depths of hawksbill turtles Eretmochelys imbricata in Little Cayman Island
Juveniles in Little Cayman Island dive deeper during daytime. Blumenthal et al. 2009
 

A complicating factor that is present in any study of animal behaviour, and one that is not easy to account for, is individual variation. The set of data on the Right compares dive behaviour in one individual, a "shallow" diver, with another individual, a "deep" diver. Note that on each day out of the week's assessment, the 2 turtles show quite consistent behaviours-throughout several dives a day. Blumenthal et al. 2009

Other than to point out the individual variability just mentioned, the authors also speculate on what a turtle finds so attractive in deep water over shallow. Do we assume that a "shallow" diver is just hitting bottom (and thus finding food) at a shallower depth than a "deep" photograph of reef drop-off in Little Cayman Island with hawksbill turtle Eretmochelys imbricatadiver? In fact, Little Cayman has a shallow (6-12m depth) shelf extending some 400m from shore, beyond which the bottom drops away precipitously to abyssal depths (see photograph). The deepest divers may be accessing what is termed the "sponge belt", an area of sponge proliferation at 80-120m depth that exists in this area of the Cayman Islands.Blumenthal et al. 2009 Coral Reefs 28: 55.

Down we go.....!

 

graph comparing daily dive behaviour in 2 different individual hawksbill turtles Eretmochelys imbricata in Little Cayman Island
 
 

So, from these data we conclude that hawksbill turtles spend most of their time underwater, and that dive durations are longer at night than in the day. In the absence of hawksbill turtle Eretmochelys imbricata feeds on a Geodia neptuni sponge while a French angelfish Pomacanthus paru watchesdata on what they are doing underwater at night, we can only presume that they are resting, but what advantage would there be in resting at depth rather than at the surface (avoidance of waves, boats, surface-water currents)?

In the daytime, we know that angelfishes sometimes join hawksbill turtles in feeding on sponges. Since the outer covering ot the sponge is tougher than the inner cortex, it is probably hawksbill turtle Eretmochelys imbricata and a French angelfish Pomacanthus paru head off tegether after a meal of Geodia neptunieasier for the angelfish to eat once the turtle has done the initial work. As for the angelfish, it's handy that the turtle has to return to the surface for periodic breathing.

Hawksbill turtle Eretmochelys
imbricata
feeds on a Geodia
neptuni
sponge under the
attentive eye of a
French
angelfish Pomacanthus paru


 

Le bon emploi, eh, Pierre?

 
 
Let's hear what the turtle thinks of it all:
  cartoon 1 in a series showing SCUBA-diver interviewing a hawksbill turtle cartoon 2 in a series showing SCUBA-diver interviewing a hawksbill turtle
  cartoon 3 in a series showing SCUBA-diver interviewing a hawksbill turtle cartoon 4 in a series showing SCUBA-diver interviewing a hawksbill turtle
 
 

Sea turtles have greatly diversified feeding habits, with HAWKSBILLS Eretmochelys imbricata eating sponges, LOGGERHEADS Caretta carettaeating molluscs & crustaceans, GREENS Chelonia mydas eating seagrasses, and LEATHERBACKS Dermochelys coriacea eating jellyfishes. Let's compare one aspect of the functional morphology of their digestive systems, photograph showing carapace length of a hawksbill turtlenamely, relative gut length, in relation to what each turtle eats. Bjorndal 1985 Copeia 3: 736.

NOTE how well the overall shape of an organism or the shape of a certain body part functions to benefit an organism

NOTE relative gut length refers to total length of gut divided by carapace length. Thus, a hawksbill turtle of carapace length 0.5m and total gut length of 4m would have a relative gut lenth equal to 8; that is, the gut is 8 times longer than the carapace

  photographs of 4 types of turtles, hawksbill, green, loggerhead, and leatherbackNow for some (simple) questions:
  1. Which species has the longest relative gut length? You are correct if you thought immediately of a green turtle. Its gut is about 13 times longer than its carapace. Herbivores of any type tend to have longer relative gut-lengths than carnivores. This permits longer digestive action on the tough fibrous components of plants.
   
  2. Which species has the shortest relative gut length? A choice of any one of the carnivorous species is good, as there is not much difference between them: hawksbill = 9 (known only for hatchlings); loggerhead = 9.5, and leatherback = 8.6. Of the 3 types of diets, the loggerhead's jellyfish diet may be most rich in proteins.
 
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