The Hitchhikers Guide to the Marine Aquarium

[Jesshika]

Asterina Sea Stars: These are common in most of the live rock brought into your tank.  They can be hard to find because they are beige colored and blend in well with the sand so they can go unnoticed for a long time.  They are also very small, only growing to be a few centimeters.  Unlike regular starfish that have five arms, these sea stars usually have more.  They reproduce by fission or by splitting into another sea star. and can also regenerate any missing arms.  It won't take long for them to reproduce and spread. 

Their main food is algae/bacteria.  Most often you will see them at night coming out to feed on algae on the glass and rocks.  There have been some that feed on SPS corals, but this is uncommon considering how many people have hundreds of these in their tank.  If you have excess nutrients in your tank, you can expect to see many of these stars.  They are usually considered to be beneficial and harmless in tanks.

(pic credits go to saltwaterfish.com)

[Jesshika]

Brittle Stars:  The small white brittle star is another very common hitchhiker star you will find on occasion at night time.  These usually stay around the same size as the asterina stars but they reproduce differently.  Instead of splitting, these brittles are self fertilizing hermaphrodites and they brood their offspring, saving the larvae from the filters and pumps.  This species is also bioluminescent.  When a predator goes after one of them and gets a hold of one of their arms, the brittle will drop the arm and  it will flash light in self defense.  This light will hopefully confuse the predator, giving time for the rest of the brittle to escape to safety.  This star is very beneficial for the tank and is one of the more desirable ones as well.  They eat left over foods and detritus.

(photo by Tizzo of SW.com forums)

[Tropical Dude]

Sorry, I somehow got confused with the bubble algae.

[Jesshika]

Porcelain Crabs:
The porcelain crab is a peaceful hitchhiker and can only get to be about an inch.  It has two claws with a flat round body and is normally colored orange or brown with blue spots.  They are beneficial to have as they are reef safe and will not harm any other inverts in the tank or corals.  They can be scavengers, but they also filter the water for planktonic foods.  Porcelain crabs are a welcomed invertebrate to have in the tank.

(Picture provided by mogurnda of AC forums)

[Jesshika]

Emerald Crab: (Mithrax Crab)
Another beneficial crab, used as part of the clean-up crew that has been known to eat the bubble algae (Velonia sp.) and keep it under control.  These crabs will only get to about 2 inches.  They are generally reef safe but can go after snails, therefore they are considered semi-aggressive when they reach full size and may eat or nip at coral polyps.  Lifespan for an emerald crab is around 2-4 years and the care level is easy.  Another thing to consider about them is that they will sometimes eat the purple coraline algae.  Overall, they are very good scavengers.

(pic credits go to fishlore.com)

[Jesshika]

Cerith Snail Eggs:  If you find a weird white trail on the glass of your tank like I recently discovered, you most likely have Cerith snail eggs.  This long trail consists of thousands of tiny white snail eggs.  This is a sign that the tank is healthy if inverts are reproducing    While most of the time being on the glass, these snails will also lay their eggs on the live rock too.  Most of the time the eggs will be devoured by fish before they hatch.  If they do manage to hatch, they will be planktonic and you wont see them for a few days to a few weeks before the survivors land on the bottom and switch into snails.

[Guppy King]

Tohse crabs are REALLY pretty!