Predators & defenses
 

During their time in the plankton the eggs, early developmental stages, and pluteus larvae of sand dollars Dendraster excentricus are subject to intense predation by a wide variety of filter-feeding invertebrates and fishes.  Even a few larval invertebrates, most notably zoea larvae of crabs, get in on the act.  Adults are eaten by birds, crabs, sea stars, and fishes.  In this section, the topic of PREDATION ON DEVELOPMENTAL STAGES is considered first, followed by PREDATION ON ADULTS.

NOTE  developmental stages of crabs are considered in detail elsewhere in the ODYSSEY: LEARN ABOUT CRABS: REPRODUCTION

  black dot
  Predation on developmental stages
  black dot
Research study 1
 

view of sand-flat habitat of sand dollars Preferential settlement of larvae of Dendraster excentricus into established beds of adults, leading to aggregations, may have immediate survival benefit.  A study at Friday Harbor Laboratories, Washington predicts higher survival of larvae because the bioturbation activities of adult Dendraster tend to exclude a predatory tanaid Leptochelia dubia. The tanaid is a tube-dwelling form, is extremely abundant in these areas, and is known to prey on settling and post-metamorphic Dendraster. The author's counts of the predator show 1-3 orders of magnitude fewer individuals within sand-dollar beds as compared with nearby non-bed areas. Highsmith 1982 generalised form of a tanaidaceanEcology 63: 329.

Right: "bioturbating" track of a single adult sand dollar Dendraster excentricus


Left: generalised form of a predatory tanaidacean 3X

  black dot
Research study 2
 

photograph of arm rod of a sand dollar Dendraster excentricusThe endoskeleton of the pluteus larva of echinoids consists of either simple cylindrical rods or more complex fenestrated rods made up of calcium crystals. Sand dollars Dendraster excentricus have the fenestrated type and bending stiffness of these is 3-5 times that of the simple ones. It is not known why the arm spicules are fenestrated in some species and simple in others. Clearly, the arm spicules are much stronger than required to resist bending from forces generated by ciliary locomotion or to support the arms, and it is thought that they may also play a defensive role.  Perhaps it makes them too crunchy, hard to manipulate, or reduces the organic yield per mouthful for a predator.  Emlet 1983 Biol Bull 164: 433.

 

 

The fenestrations, or holes, can be seen in one
of the arm rods of D. excentricus under SEM

  black dot
Research study 3
 

Do the developmental stages of sand dollars have any escape defenses against the highly predatory planktonic zoea larvae of crabs?  For eggs that are non-motile, and gastrulae and early prism stages, which can only swim in a forward direction, the answer is ‘no’.  The schematic shows what happens to Dendraster excentricus gastrulae when dropped into the immediate vicinity of a tethered zoea larva of a red rock crab Cancer productus.  Most (56%) are swept into the feeding currents of the zoea and eaten (see zoea on Left).  However, when 4-8-armed pluteus larvae, which can swim both forwards and backwards, are similarly dropped near to the predator, their agile evasive behaviour leads to only 12% being caught and eaten (see zoea on Right).  Rumrill et al. 1985 J Exp Mar Biol Ecol 90:193.

NOTE  the larvae are tethered by pressing their long spines into glass tubes filled with plasticine.  In such positions they feed and survive well

NOTE  the authors offer 25 of each prey type to the zoeae and provide 5 representative pathways as examples. Thus, in the accompanying schematic, 5 representative gastrulae are offered to a tethered zoea and 3 are eaten. In contrast, of 5 representative plutei offered to a zoea, only one falls prey. The developmental stages are not drawn to scale

 

  black dot
Research study 4
 

Size and motility of a predator in comparison with size and motility of the embryonic and larval stages of Dendraster excentricus may ultimately dictate whether a predator will be successful.  Shown here are line drawings of several invertebrate predators drawn more or less in scale with the developmental stages of Dendraster shown in the centre circle.  The invertebrate predators are quite successful in catching and eating the embryonic stages, but not the larval stages, while certain vertebrate predators are quite successful in catching and eating both embryonic and larval stages.  Pennington et al. 1986 Bull Mar Sci 39: 234.

NOTE the vertebrate predators tested in the study are juvenile pink salmon and sticklebacks

 

  black dot
Research study 5
 

histogram comparing predation effects on sand-dollar egg-mimics in the plankton and on the benthosdiagram of tethering set-up for use in sand-dollar egg-predation studyAn interesting research question posed by researchers at Friday Harbor Laboratories, Washington concerns the relative importance of predation on invertebrate eggs and larvae by planktonic versus benthic predators.  The study, done entirely in the field, uses tethered agarose baits flavoured with homogenates of eggs of sand dollars Dendraster excentricus.  The plankton baits are tethered at 3m depth, while the benthic baits are tethered at 10m depth, mostly resting on the sea bottom (see diagram).  Baits are deployed for periods of 1.5-3h during daylight hours, then assessed for predation effects. Loss in mass of the baits, expressed in percentage, is taken to represent consumption by predators.  Control (unflavoured) baits are also deployed, with their results included in the multiple-regression analyses to account for possible differential seawater dissolution at the 2 depths. Results show that loss rates to benthic predators are almost twice that to planktonic predators.  The authors note, importantly, that the study site used is located close enough to known populations of adult sand dollars that their reproductive products are likely in the water column over the duration of the study period.  They also discuss possible limitations in the study, most relating to use of tethering methodology.  Allen & McAlister 2007 J Exp Mar Biol Ecol 347: 77.

NOTE  in addition, the researchers test tethered megalopae of Dungeness crabs Cancer magister, data not shown here

  black dot
Research study 6
 

photograph of pluteus larva of sand dollar Dendraster excentricus forming a bud in response to the presence of fish-mucus stimulusLarval cloning in response to certain conditions of nutrition and temperature is a phenomenon known for many echinoderms, but researchers at Friday Harbor Laboratories, Washington show for the first time that cloning may be a strategy employed in the presence of potential predators.  Laboratory experiments show that 4d-old plutei of sand dollars Dendraster excentricus respond to presence of fish mucus by forming buds or dividing in two within 24h.  The buds later detach as ciliated gastrulae and develop into larvae, while the fission halves replace their missing parts and swim off.  All clones, whether by budding or fission, are thus smaller than uncloned larvae.  The authors speculate that the smaller sizes of the clones may decrease their detection by visual predators such as fishes.  The article is interesting and generates many questions. One is whether the inducing chemical cue is specific to mucus of the fish species noted, or whether it may be a common component of other types of mucus, perhaps even from invertebrates or non-predaceous fishes. Vaughn & Strathmann 2008 Science 319: 1503.

NOTE  fishes used as mucus-sources are dover sole, a bottom predator.  Earlier observations show also that water from aquaria housing sticklebacks stimulates sand-dollar larvae to clone 

  black dot
  Predation on adults
  black dot
Research study 1
 

Adult sand dollars Dendraster excentricuas are preyed upon by sea gulls, which eat exposed individuals when the tide is out, and by a variety of invertebrate predators including crabs Cancer magister and sea stars Mediaster aequalis, Pycnopodia helianthoides, and Pisaster brevispinus when the tide is in.

 

 

 

 

Tests of dead sand dollars Dendraster excentricus exposed by
the tide. One individual at 6 o'clock shows typical signs of bird
chipping, as does the single living individual at the top of the photo
which displays edge damage also suggestive of bird predation 0.5X

  black dot
Research study 2
 

Movement of a sea star Pisaster brevispinus through a bed of sand-dollars Dendraster excentricus will cause its members to burrow to some distance on either side of the sea star even without being touched.  Feder 1963 Ecology 44: 505.

  black dot
  RETURN TO TOP