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Common
in pet fish and souvenir trade alike, the Chocolate chip star is quite well
known to aquarists as well as to people not associated with the hobby.
Nonetheless, much misinformation or misunderstanding exists with regard to this
interesting and decorative (at least when alive) species. Its biology and
successful aquarium care based on observations in nature and in captivity are
summarized here.
Natural occurrence
Chocolate chip sea stars inhabit mostly sandy to muddy lagoons and seagrass
beds, they are more rare in reefs themselves and seem to prefer the back reef
area. Typically they are found in rather shallow depths of 1-30 metres (3 to 100
feet), juveniles are most common in depths smaller than 2 m (7 feet). About 30
specimens are generally found in an area of 100 m? (1100 square feet) in their
sea grass biotopes.
This
species occurs in many parts of the tropical IndoPacific: Eastern Africa
including Madagascar, Sri Lanka, Indonesia, North to the Southern parts of Japan
and South to the tropical Northern coast of Australia as well as some island
groups of the Southern Pacific. Regionally it can be a very common sea star and
is often collected, dried and sold as a souvenir.
The
Chocolate chips of Protoreaster nodosus can be rounded or pointed. Systematics and physiology
The
Chocolate sea star was among the first animals to achieve a scientific name. It
was named by Linne himself in 1758. An alternative common name often found in
literature is ?horned sea star?. The chocolate chip star is part of the star
fish family Oreasteridae. This family consists of rather stout species with
thick, but short arms. Other members of the family like the sometimes traded
Protoreaster linkii and Pentaceraster spp. have needs quite similar
to the Chocolate chip star, their aquarium care is comparable. The
eponymous chocolate chips or thorns found on the aboral (=not the side with the
mouth) surface of Protoreaster nodosus are thought to deter at least some
star fish predators from eating this species. Sadly, it is quite useless with
regard to humans collecting and buying this species as a dried souvenir.
Other
sea stars from the family Oreastidae require similar care as the Chocolate
chip star.
Following literature, the central disc can reach a diameter of up to 12 cm (5
inches) and the arms a length of about 14 cm
(5.5 inches). Consequently the entire star can reach a diameter of up to
40 cm (16 inches). These maximum sizes are rarely reached in nature as well as
captivity. Most Protoreaster nodosus do not grow larger than 20-25 cm
(8-10 inches). Even specimens larger than 14 cm (5.5 inches) appear to be rare
at the Philippines (perhaps due to collecting). In this area the mean diameter
of adult specimens is 10 cm (4 inches), the mean weight is about 250 g (1/2
pound). The stars are a little larger at Palau with mean diameters of 14-16 cm
(5.5-6.5 inches).
Chocolate chip stars reach their adult stage (with working gonads) with a
diameter of about 8 cm (3 inches). At this point they are about 2-3 years old.
Typically sold specimens of about 10 cm (4 inches) are about 5 or 6 years old
and specimens of 14 cm have an estimated age of 17 years. It appears to be
unclear if the large specimens (40 cm or 16 inches) found in older literature
are erroneous records, other species wrongly identified or simply very old stars
with a good genetic basis, which grew up in biotopes with ideal food supply. The
colours of the Chocolate chip star are quite variable, the background colour can
be white, yellow, brown, red and even bluish. The ?Chocolate chips? are mostly
grey to dark brown. The
physiology of Protoreaster nodosus is in line with other well known sea
stars. Food and waste enter and exit the same opening, the ?mouth?. Water is
pumped into the vascular system by the madreporite, a small off-centred, wart
like structure on the surface of the sea star. The madreporite supplies various
canal systems in the star and is inevitable for locomotion as well as the
functionality of the mouth. Air trapped inside this system can result in a sea
star unable to move and consequently dying. Therefore, the stars should not be
taken out of the water too suddenly, they need time to close the madreporite. If
they leave the water by themselves, this is not a problem, because the vascular
system will likely be closed already. In nature Chocolate chip stars can be
found in the intertidal areas exposed to air and sunlight at low tide without
being harmed.
The
madreporite, the sea star organ used for water exchange of the vascular
system ist visible as a dark grey, oval spot. The
vascular system of sea stars is filled with salt water. Due to the osmotic
pressure, because of different salinities and temperatures at the pet shop and
the home aquarium it is advised to acclimate sea stars slowly. The larger the
difference in salinity and temperature, the slower the acclimation process
should be. Half an hour to one hour are sufficient using drip acclimation in
most cases. Most
interestingly these stars seem to possess some kind of memory and apparently are
able to be trained to be fed at a specific point at a specific time. Given their
simple nervous system lacking a brain per se this is a tremendous effort. In the
1980s Chocolate chip stars were in the scope of the biochemical industry.
Several interesting molecules like saponins, sterols and steroids have been
found in them. This interest seems to have been discontinued in the last
decades.
Spawning takes place from March to May, the more shallow depths of their
habitats are avoided during this time. Males are about as common as females and
cannot be sexed externally. Synchronous spawning of the entire population
happens at full moon and coincides with increasing water temperature and a
decreasing salinity. It has happened in the aquarium, too. The eggs are about
200 um small. The larvae are planktotrophic (feeding in the plankton) and can be
expected to be notoriously difficult to raise by the hobbyist. The floating
larvae are found more close to the bottom than to the surface water, which is a
quite unusual behaviour. They stay planktonic for 10-14 days until they settle
to the ground and start a benthic life. Protoreaster nodosus larvae are
spread in much smaller areas than other planktonic larvae. No report exists of
this species with regard to reproduction by splitting as it is known from many
other sea stars. Easy or difficult? ? Nutrition gives the answer
Quite a
number of hobbyists consider this sea star as hardy and easy to feed, although
it is equally often stated that the success rates with this species are rather
poor (e.g. Goemans, 2007). I believe this is highly dependant of the aquarium
system and available foods.
This
species cannot squeeze into narrow gaps like Brittle stars, food items sinking
into such crevices are lost for this species. So, it mostly depends on the
rockwork of the aquarium if the sea star is useful as a cleaner organism at all.
In addition, it does not eat most types of nuisance algae and undesired
bacterial films (Cyanobacteria).
Chocolate chip stars eat by expelling their orange coloured stomach (to be more
exact this is one of their two
stomachs) and digesting their prey. Thus, it is quite easy to observe what they
eat. Captive Chocolate chip stars are reported to consume sponges as well as
other benthic sessile and vagile invertebrates such as soft corals, bivalves,
slow snails, bryozoans, feather dusters, anemones, slow urchins, sea stars and
even small fishes (if they can get them) as well as fish feces, carrion and sea
grass. Interestingly, their natural diet was documented by more recent studies
to consist mostly of meiobenthos (diversity of small benthic animals between
0.063 mm und 1 mm) and microbial/microalgal films (Scheibling & Metaxas, 2008).
Consequently, it is well possible that the macroscopic foods these stars
generally consume in captivity are ? at least in part ? just surrogate foods.
Consequently, Chocolate sea stars do best in systems with high water quality,
but enough detritus and small benthos available. Tanks without skimmers or
mechanical filters seem to be ideal, but are more difficult to keep in top
condition with regard to water quality and can only be recommended to the
advanced hobbyist. Deep sand beds commonly result in abundant benthic
invertebrate life and, if well populated, will be a tremendous help with the
nutrition of a Protoreaster.
Over-cleaned stony coral reef systems do not resemble their natural environment
and generally have too little adequate food to offer to a Protoreaster.
Equally inadequate are fish only tanks with little surface to graze on, no
benthic life and high nitrates. It?s no real surprise that Chocolate Chip stars
do best if their tank is similar to their natural environment. FOWLR systems
with some macroalgae appear to be the best systems for Chocolate chip stars. As a
minimum tank size for a 9 inch or smaller Chocolate chip star 50 gallons are
recommended if pristine water can be guaranteed. For specimens growing beyond 10
and up to 16 inches larger tanks of at least 100 gallons, preferably more are
needed. General
water parameters recommended are a specific gravity of 1.022 ? 1.027 , a
temperature of 22-27?C (72-81?F), a pH of 8.0-8.4 and nitrates << 20 ppm.
My own
approach to this species is a tank with live rock, a 4.5? well populated DSB,
only a few medium sized fishes specialized on larger pieces of food (no grazers
or plankton eaters) and various green and red macro algae instead of corals. The
tank has no skimmer or mechanical filtration. The Protoreaster feeds
mainly on biofilms in the tank (also at the water surface), small benthic
invertebrates like tiny snails, worms, arthropods and leftovers from feeding the
fishes (varied diet of various types of shrimps, pieces of various mussels,
small pieces of fish filet). While this star has consumed soft and stony corals
in the past, it did not eat several types of sponges and only partially digested
a hydroid colony. It did not eat Aiptasia anemones, but consumed a
Thalassianthus, which has a potent sting. The speed with which a wound from
an tumbling rock accident in its former tank healed indicates that this systems
seems to be sufficient. In addition, the star shows slow growth similar to what
is found in nature.
In
tanks with mechanical filtration, skimming and more fishes the Chocolate chip
star may need to be fed specifically (can be done by carefully placing the star
onto a piece of food), because it may not find enough food or have to much
competition by the fishes. This may work, but likely is less ideal, because the
star will not be able to choose its diet from what is offered by the
environment. Disease
Protoreaster spp.
are able to recover from severe wounds given chemically and biologically
pristine water quality as well as appropriate food, but they can fall prey to
heterotrophic bacteria infecting small scratches in dirty water with high
nitrate concentrations and bacteria counts. Enemies
Sea
stars can be severely hurt by triggers, puffers, parrot and box fishes. Not
every specimen of these groups is a sea star predator, this mostly depends on
the experiences and character of the fish specimen in question. Other sea star
predators include shrimps from the family Hymenoceridae like the often traded
Harlequin shrimp (Hymenocera picta and H. elegans). Last but not
least it has to be noted that larger sea stars can be cannibalistic.
Protoreaster nodosus
can consume sea stars of the same or other species. Here the prey is a small
Asterina sp. star. Tank mates
In
nature Protoreaster nodosus are often found accompanied by the glass
shrimp Periclimenes soror, which live on the sea star and adapt to its
colouration. Although this co-existence or even possible symbiosis might be a
highly interesting subject to study in the adequate home aquarium, the stars are
practically always imported without the Periclimenes, which are rarely
found in trade, due to the collection process and because they would be eaten by
many reef tank fishes. Other naturally commensal animals include Polychaete
worms (?bristle worms?) living on the star without hurting it, but given common
territoriality shown by aquarists towards bristle worms, I doubt they will have
great survival rates in captivity. What
about keeping Chocolate chip stars as a group? Protoreaster nodosus seem
to restore their density in nature following experiments by placing more
individuals into a given area. Therefore, it may not be wise to permanently
overstock them the aquarium, because this will create a situation where the
animals will be uncomfortable and always try to get a larger distance to each
other. A Protoreaster nodosus will occupy an area of roughly 3 m? (33
square feet) in nature, which is much larger than what we can generally offer in
the average home aquarium. Only a tank with a length of more than 3 metres (10
feet) and a width of more than 1 metre (3 feet) will be sufficient if one wants
to duplicate the natural habitat with more than one star. Chocolate chip stars do best in FOWLR systems with macro algae or sea grass
Closing words In an
adequate tank the Chocolate chip star can be a hardy as well as decorative pet,
a much more delightful sight compared to dried conspecifics on the bookshelf. Literature
Erhardt, H. & Moosleitner, H. (1997):
Meerwasser Atlas, volume 3, 1328 p. (in German).
Goemans, B. (2007): Marine invert of the month ? Protoreaster nodosus (Linnaeus,
1758).- TFH, 56(1) September 2007, p. 76. Bos,
A.R.; Gumanao, G.S.; Alipoyo, J.C.E.; Cardona, L.T. (2008): Population dynamics,
reproduction and growth of the Indo-Pacific horned sea star, Protoreaster
nodosus (Echinodermata; Asteroidea).- Marine Biology, 156(1), p. 55-63.
Scheibling, R.E. & Metaxas, A. (2008): Abundance, spatial distribution, and size
structure of the sea star Protoreaster nodosus in Palau, with notes on feeding
and reproduction.- Bulletin of Marine Science, 82(2), 221-235. Riccio, R.;
Zollo, F.; Finamore, E.; Minale, L.; Laurent, D.; Bargibant, G.; Pusset,
J.(1985): Starfish saponins 19.
A novel steroidal glycoside
sulfate from the starfishes Protoreaster nodosus and Pentaceraster alveolatus.-
Journal of Natural Products (Lloydia), 48(2), p. 266-272.
Crandall, E.D.; Jones, M.E.; Munoz, M.M.; Akinronbi, B.; Erdmann, M.V.; Barber,
P.H. (2008): Comparative phylogeography of two seastars and their ectosymbionts
within the Coral Triangle.- Molecular Ecology. 17(24), p. 5276-5290. Hartmann-Schr?er, G. (1984): 2 new commensal polychaeta of the genus Hololepidella polynoidae from the Philippines.- Mitteilungen aus dem Hamburgischen Zoologischen Museum und Institut, 81, p. 63-70. |
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