That Fish Blog – Aquarium Advice and Information

Hello, Frank Indiviglio here. The ways of crabs never cease to amaze me, and we are fortunate indeed that so many interesting species do exceedingly well in marine aquariums. Today I’d like to draw your attention to certain hermit, arrow, boxing and spider crabs that increase their survival odds by carrying food or weapons wherever they go.

A Mutually Beneficial Partnership

After locating a suitably-sized sea anemone, the anemone hermit crab (Pagurus prideauxi) places the stinging invertebrate on its shell as a deterrent to predators. The anemone attaches itself to the new home, and may benefit by gaining access to leftovers from the crab’s meals. Anemone hermits that I’ve kept have invariably relocating their protectors when switching their own living quarters from one shell to another.

Anemones as Boxing Gloves

Another weapon-bearing crustacean, the boxing crab (Lybia tessellata), goes through even greater lengths to arm itself with stinging anemones. This tiny (2.1 inch) fellow has the fascinating habit of carrying small anemones about in its claws. When threatened, it will rear up on its hind legs and wave the “weapons” at the interloper!

Lunch to Go

The ever-popular arrow crab (Sterorhynchus seticornis) is more concerned about food supply than weaponry, and impales food upon its pointy carapace. This is quite comical to see, because, due to the location of the spine, the crab seems to be carrying food about on the tip of its nose! The stored food is consumed when the crab is safe within a retreat, or in times of need.

A Surprising Discovery

The Atlantic spider crab (Libinia emarginata), a temperate relative of the arrow crab, can easily be collected along the eastern coast of the USA. They are inoffensive towards one another and most tank mates, and make interesting aquarium pets.

I still remember my shock upon seeing one wedging kale (provided as food) into the many crevices of its carapace. Eventually, the small crab looked like a walking patch of marine algae…when it ceased moving, its camouflage was perfect (well, nearly perfect…kale doesn’t grow in the sea!).

I later observed spider crabs to pick at and consume bits of their movable garden, so the vegetation serves two purposes (I have not determined if spider crabs will use inedible materials for camouflage, as do decorator crabs and some others). The spider crabs that I have kept gave up this habit upon reaching a carapace size of approximately 4 inches.

The aforementioned crabs are readily available and adjust quite well to aquarium life. I’ll cover their care in detail in future articles. Until then, please write in with your questions and comments. Thanks, Frank Indiviglio.

Further Reading

The diversity of crab species and lifestyles is astounding. To learn about current research in East Asia (where over 1,000 species have been described) and view some remarkable photos, please visit http://www.dbs.nus.edu.sg/biodiversitii/bio/m_crab.html#major.

I have written about the natural history and captive care of crabs and other crustaceans in The Everything Aquarium Book
.

Knowing where to start and stop when it comes to writing about invertebrates is a real problem – any single group could keep one occupied for a lifetime. Today I’d like to highlight some interesting facts concerning a few commonly kept types and their relatives. I’ll add to this from time to time.

General
Invertebrates (animals without backbones) account for approximately 97% of the world’s animal species, yet we have no idea of their total numbers. The smallest are invisible to the naked eye while the largest, giant squid, may exceed 60 feet in length.

Nearly every injectable drug manufactured in the USA is tested for bacterial contamination with a chemical extracted from horseshoe crab blood (synthetics do not work as well). Several states are restricting the collection of these animals (often used as fertilizer!) and requiring that they be released after blood specimens are taken.

Invertebrates have colonized every habitat imaginable, from freezing Antarctic seas to the boiling hot water of geysers. The sponges, crabs, tubeworms and others living near deep sea vents form the only animal community that does not rely upon photosynthesis as the basis of the food chain (bacteria that consume methane function as “plants”).

Despite being a creature of legend for centuries, the giant squid, Architeuthis sp., was not captured on film until 2004. Two years later, the same Japanese scientists that filmed the animal caught a specimen on a fishing line, thus giving the world its first view of a living giant squid.

Eating and Being Eaten
Despite radically different appearances, jellyfishes, sea anemones and corals are closely related (Phylum Cnideria). All gather food and excrete wastes through a common opening, and overcome their prey with stinging cells.

Jellyfish, although comprised largely (95%) of water, are able to snare prey as large as small fishes. Surprisingly, they form the bulk of the diet of many huge sea creatures, including the world’s biggest turtle, the leatherback.

The dried krill (shrimp-like creatures of the Class Brachiopoda) that you may use as fish food form the basis of the food chain in most of the world’s oceans. Also, a number of surprisingly large creatures, including whales, manta rays and basking sharks, rely upon krill as their primary diet, consuming billions each day.

Although viewed by most as sluggish creatures, many of the world’s 70,000+ species of snails and slugs (Phylum Mollusca) are quite effective predators. Various types pry open or drill through clam shells and cone snails impale fish by shooting out barbed tongues. Certain sea slugs consume anemones and incorporate the stinging cells into their own gill tufts.

Reproduction
Australia’s Great Barrier Reef, the world’s largest, is 1,250 miles long. Somehow, its untold billions of individual coral animals synchronize reproduction so that the sperm and eggs of all are released into the sea at the same time.

Banded coral shrimps, Stenopus hispidus, form long-term pair bonds, and males have been observed to share food with gravid (pregnant) females. The eggs, which are glued to the females’ swimmerets (feathery structures below her abdomen), are aerated and protected by her. Upon hatching, however, the young may be consumed by both parents!

Surviving
Sea cucumbers make interesting if occasionally unsettling aquarium inhabitants – when disturbed, they discharge their stomachs through the anus! Amazingly, these sea star relatives can regenerate the discarded stomach.

The unique tube feet of sea stars (Phylum Echinodermata) function in locomotion, respiration and as sensory organs. Water-filled canals linking the feet can, via a series of valves, build up enough pressure to enable sea stars to pry open clam shells (try that with your hands!).

The anemone hermit crab, Parurus prideauxi, places a stinging sea anemone on its shell as protection and camouflage and re-locates it when changing shells. The anemone, in turn, gets a safe anchoring place and, perhaps, access to leftovers from the crab’s meals.

Using Invertebrates – Now and Then
Over 10,000 species of sponges (Phylum Porifora) inhabit both fresh and salt water. Several types have been collected from the Mediterranean Sea since ancient times. After drying in the sun, their fibrous structural tissue (spongin) made an excellent bath sponge.

Horseshoe crabs (Phylum Chelicerata) are among the world’s most ancient creatures and have remained relatively unchanged for over 300 million years. Closely related to spiders and not crabs at all, small specimens make interesting additions to a marine aquarium.

And, Finally…An Odd Personal Tale
Octopuses (Class Cephalopoda) are the most intelligent of the invertebrates and make fascinating aquarium subjects. They are also quite well-sighted – one I kept would, according to my grandmother, “stare” at her while she worked in the kitchen. Not wishing to upset my beloved pet, she covered its tank when preparing octopus for dinner!

Well, only a few billion more facts to go! I’ll continue next week, and periodically after that. As there are so many possibilities, I would greatly appreciate your suggestions concerning invertebrate-oriented subjects that you may wish to learn more about. Thanks, until next time, Frank.

You can learn a great deal about invertebrate biology at the web site of the Australian Museum:
http://www.amonline.net.au/invertebrates/ara/index.htm

Today I’d like to introduce you to the maintenance of cold water aquariums, a fascinating but often overlooked branch of aquarium keeping focusing on marine and freshwater fishes and invertebrates from temperate regions.

Exotics Close to Home
Please don’t be misled into equating exotic and interesting creatures with “far-off, tropical places”. Shorelines, ponds, tidal pools and rivers throughout the world’s temperate regions (with the exception of extreme s. Florida, the entire USA is in the temperate zone) yield animals of unimaginable variety and interest. While it is true that many of the most spectacularly colored fishes are found in the warmer parts of the world (their bright colors likely help males and females of the same species recognize each other among the great diversity of similar species in the tropics), breeding males of many temperate fish species, i.e. the sunfishes, do rival those of their tropical counterparts.

Why keep Temperate Fishes and Invertebrates?
Animals from temperate areas offer many advantages to aquarists who live in the USA. Often, our normal seasonal rhythms of temperature and light fluctuation are sufficient to encourage such creatures to exhibit natural behaviors, and even to breed. Also, it is often easier for us to provide a wider range of foods and a more natural environment for them than we can for creatures from faraway places. In some cases, where legal, we may even be able to collect and keep native fishes and invertebrates.

Creating Natural Habitats
If you live near a body of water, try to observe firsthand the environments and habits of a variety of aquatic creatures. You might consider modeling your aquarium after a particular habitat — a tide pool, a weedy, fresh-water pond, or a river bed, for example. Take note of the local substrates, rocks and other natural items and purchase similar ones when setting up your tank prod (collecting natural substrates is risky, due to the possibility of mineral, pesticide or other chemical leaching).

Heat
Cold water aquariums are maintained in a similar fashion to tropical aquariums, but we must consider the effects of heat. Many animals from temperate regions are very sensitive to rising water temperatures and to the lower oxygen levels that accompany them. You may, therefore, need to install a chiller to moderate temperatures during the summertime.

Species to Consider
Some of the animals that I have maintained with success in cold water aquariums, and which I plan to cover in future additions to this article, include American eels, fifteen-spined sticklebacks, pygmy and other sunfishes, mantis shrimp, sea stars, Chinese sailfin suckers, weather fish, horseshoe crabs, blue claw crabs, bullheads and madtoms, white clouds, various snails, crayfish and shrimp, giant water bugs and other aquatic insects and many others.

I’ll now give a quick overview of two unusual animals – the oyster toadfish and the spider crab – which are easily maintained in unheated marine aquariums. I’ll cover their husbandry in detail in future articles.

Oyster Toadfish, Opansus tau
This unusual marine fish, ranging down the Eastern Seaboard from Maine to Cuba, seems to cross the line between fish and amphibian – its face even resembles that of a toad. In addition, toadfishes produce sounds audible above the water and certain Asian species can travel overland for considerable distances.

Oyster toadfishes will become quite tame in captivity, and, given a tank of 55 gallons or so, may well breed. Males are ferocious guardians of their eggs, and have been known to stay with nests that are exposed at low tide.

Oyster toadfishes have survived for 15 years in captivity, and will accept nearly any meat-based frozen or pelleted food as well as live shrimp, small fishes and worms. They learn to associate their owners with food, but will bite when handled, and the spines that can inflict painful wounds.

Atlantic Spider Crab, Libinia emarginata
Spider crabs, common yet fascinating if you take the time to know them, are members of a family which includes the Japanese spider crab. With legs spanning 8 feet, these giants awed me when first I observed a large group in a pubic aquarium in Osaka, Japan. American aquariums now exhibit them as well – trust me, they are well worth the trip.

The Atlantic spider crab is a valuable aquarium scavenger, and, using its tiny pointed claws to probe into nooks and crannies, misses little. Younger animals have the endearing habit of jamming algae and vegetation into the crevices of their shells, taking on the appearance of a “walking plant”. They will also nibble at this portable garden from time to time. I have found that they forgo this habit when they reach a carapace size of about 3 inches, although I have yet to discover why.

Spider crabs rarely bother tank-mates and readily consume fish flakes, pellets, blackworms, frozen food fish foods, algae and green leafy vegetables.

In the next installment of this article I’ll write about temperate, fresh water animals that are readily available in the pet trade. Until then, please write in with your questions and comments. Thanks. Until next time, Frank.

Information on the natural history of the oyster toadfish, along with a photo, is available at:
http://fishbase.mnhn.fr/Summary/SpeciesSummary.php?id=3069&lang=Portuguese