As in the oceans of a great multitude of worlds their exist barnacles, so do barnacles exist in the depth of wildspace and the phlogiston. Most of these barnacles, with lots of hard work can be removed form ships without harm to ship or crew, but there are exceptions, and three such exceptions are: Jammer Leech, Flow Barnacle, and Infernal Barnacle.

Jammer Leech
    A jammer leech has a hard shell, which grows to be a foot tall. The shell can be almost any color; though it closely matches that of the hull of the ship it is attached to. (This makes spotting the leech very difficult from casual observation alone.) Inside, the leech is reddish-purple in color, with a body much like that of a snail. It has a single, watery eye at one end. It also possesses a sharply spiked tentacle, which is the same color and consistency as its body.

    Combat: In most situations, the jammer leech uses its tentacle for protection. The sharp spike on the end of the foot-long arm causes 1d4 points of damage. That is often enough to discourage most creatures from harassing the parasite.

    Bond (Ex): When a jammer leech spore encounters a ship it bonds to the ship, in the first week the leech is not firmly bonded and only requires a successful strength check (DC 20) to remove. After one week on the ship, the spore develops a hard shell that roughly matches the color of the hull itself. After this time it the task of scrapping it from the hull will require the crew to kill the leech and then scrapping its shell from the hull.

    Consume (Ex): Jammer leech when attaching itself to the hull of a ship, at a point closes to the spelljamming helm, it then draws food and nutrients from the ship's surface at a fate of 10 hit points a month.

    Immunities (Ex): The infernal barnacle is immune to fire and heat damage.

    Magic Reserve (Su): In a situation where a hard swipe from a tentacle doesn’t discourage a predator, the jammer leech uses magic. As it rests upon the hull, close to the spelljammer helm, the leech draws magical energy from the spellcaster powering the ship. For every ten days of jamming, the leech absorbs one spell — of any level — at random from the spellcaster’s mind. On a trip that takes 30 days, for example, the leech would be able to absorb three spells. Luckily, jammer leeches can hold only four spells at a time. However, when more than one leech attaches itself to a ship, they each draw spells from the spelljammer. Spellcasters who are preyed upon by jammer leeches forget the spell absorbed by the parasite and must regain it in the normal manner. However, the spellcaster notices the missing spell only if he attempts to recall it; otherwise, the loss goes undetected.
    In combat, the jammer leech discharges the spells it has stolen at random. The parasite uses all the magic it has stored, one spell per round, to drive away its assailant. The spell is cast at the level, which the victimized spellcaster would cast it. If more than one spellcaster powered the helm during the ten days, the average level is used.
    There are only two effective ways to deal with a magic-laden jammer leech: kill it with a single strike or cast a separate dispel magic spell on each parasite to disarm it before striking. However, the leech has a spell resistance of 17 to magic, so attacking the creature is always a risky business. Once its magic reserve is gone and the parasites hard shell is cracked, it is an easy target.
    It is important to note that leaches will use their spells to ward off any physical attack. They frequently discharge their spells during any battle in which their section of the hull is repeatedly struck. Sometimes this works in favor of the leeches’ host ship, but more often it proves disastrous.

Flow Barnacle
    Flow barnacles look very much like immature krajen, but the two are not related. They also resemble planet bound limpets and barnacles, but are usually at least the size of a human head. These hard-shelled, unintelligent creatures fasten themselves limpet like to the surface of any object they encounter – be it ship, rogue planetoid, or drifting being, slowly eating into it.

    Combat: Flow barnacles will eat metal and organic matter alike – menace to ships, valuables stored in chests, drifting characters encased in softwood, and to a lesser extent all spacefaring beings. They are neither agile nor cunning, but simply drift into characters, or convulse their bodies to expel a jet of captured atmosphere or moister, and ‘spurt’ their way (at the listed movement rate) toward the nearest living thing.

    Attach (Ex): If a flow barnacle successfully hits an opponent, they will adhere with an almost-unbreakable grip and extend three parrot-like beaklets, which bite for 1d2 hit points of damage each.
A flow barnacle can be readily detached when dead. Removing a live barnacle requires a strength check (DC 21) and causes 1d4+1-hit points of damage to the barnacle’s victim as it tears free.

    Blood Drain (Ex): A flow barnacle drains blood, dealing 1d4 points of temporary Constitution damage each round it remains attached. Once it has drained 4 points of constitution, it stops to digest for 1d8 hours while remaining on its host, after digesting it then begins to drain blood again.

    Blood Poisoning (Ex): If a flow barnacle is killed while attached to a victim, that victim has body fluids of the flow barnacle flow into it. The barnacle’s fluid causes blood poison, which causes 1d10 temporary strength drain, make a fortitude save (DC16) to not take damage and again one minute later so as to avoid another 1d10 points of temporary strength drain.

In the Spheres
    Space born barnacles like the jammer leach and flow barnacle drift in space and the flow and at some point come into contact with ships, large objects and even the crew itself. They exist only to eat.

Flow Barnacle, Jammer Leech, SPELLJAMMER, DUNGEONS & DRAGONS, D&D, ADVANCED DUNGEONS & DRAGONS, AD&D, the D&D logo, the AD&D logo, the d20 System logo and d20 are trademarks owned by Wizards of the Coast, Inc., a subsidiary of Hasbro, Inc., and are used with permission. All titles, and all proper nouns, including character names, locations, and named items are considered Product Identity per Section 1 of the Open Game License v1.0a and are exclusively owned by Wizards of the Coast, Inc.
©2002 Wizards of the Coast, Inc.

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Flow Barnacle, Jammer Leech © 1990 Wizards of the Coast, Inc.
d20™ System, Conversion of Flow Barnacle ©2002, Mark Doolan
d20™ System, Conversion of Jammer Leech ©2002, Mark Doolan
Original Sources: Monstrous Compendium #7 [Jammer Leech], Flow Barnacle [Lost Ships]

    Notes on this Conversion: Normally small beasties have high dexterity, low strength, and moderate constitution. With barnacles they just not seem to move fast, so low dexterity score was in order. As they innards are more or less one giant muscle I decided an average strength would do well. The high constitution was a trade off for the low dexterity score. As nacre shell is often very tough, I not see it unreasonable for it to have a natural armor rating of 8. I gave fire immunity to jammer leech as its fluid is fire resistant. And finally gave the Flow barnacle the Blood poisoning if it killed while attached to its victim figuring a ‘back flush’ of fluids when it dies, this would cause a dilemma for many a crew, does one ripe the bugger off or does one kill it and hope the ones fellow crew member not succumb to blood poisoning. I added the blood poisoning, as I not see this nearly mindless barnacle just feeding on blood forever, as it must have a limit (much like the stirge), and would not ‘think’ to run once it found a host, just sit tight till next feeding.