The Bountiful Burrowing Barnacle

Barnacle: master of us all?First of all, thank you Brody for that insightful post. Really exemplary work. You have not changed a bit after all these years.

All adulation aside, Brody’s post did remind me of one extraordinary specimen in the grand menagerie of animal penises: the barnacle’s penis.

Barnacles lead a stoic existence, confined to a rock or a hard place for the entirety of their life. Despite their humble appearance, some species of barnacle wield what may be the longest penis of any animal at up to eight times its body length; see Table 1 below for some comparisons.

Table 1: Extended Penis Length in Animals
Common Name Scientific Name Penis Length Relative to Body Length
burrowing barnacle Cryptophialus minutus 8.0
Japanese acorn barnacle Tetraclita japonica 3.9
Pacific acorn barnacle Balanus glandula 3.6
rove beetle Aleochara tristis 2.0
Argentine lake duck Oxyura vittata 1.0
sand flea Tunga penetrans 1.0
spider Tidarren spp. 0.5
human being Homo sapiens 0.0837*

Source: Neufeld & Palmer (2008).  *Self-reported, as of 1/21/12 in the AM. May vary by individual.

However gratuitous this may seem, this kind of length is necessary for successful reproduction. Unlike other non-motile creatures most barnacles reproduce sexually (though it’s theoretically possible for them to reproduce asexually, its occurrence is rare in nature).

And who can blame them. Imagine the profound loneliness of the barnacle, spending the majority of their adult life attached to something, absolutely dedicated to–you might even say obsessed with–their little personal area, making interaction with other barnacles impossible. Except, that is, with their penis which can make that connection with others, however fleeting and revolting it may be. At least it is real and tangible. And who can blame them if they are willing to pay–you know, in caloric terms–for such connections. Who, I ask, can blame them?

Desperate Barnacle Sex. Video by Stefan Siebert, via CreatureCast

So it may seem having a superbly long penis should be the only thing that matters to the barnacle. But in fact, this is not always the case. As barnacle researchers Neufeld and Palmer point out, there is a trade-off:

Although longer penises greatly increase the number of potential mates—because the searchable area expands as the square of penis length—the benefits of larger penises may be outweighed by increased drag as water turbulence intensifies, particularly in species that live on wave-exposed shores (Neufeld and Palmer 2008)

In other words, it becomes difficult to mate when your penis is so long it flails around like a cartoon fire hose (food for thought, Brody). But fret not because barnacle penises display a remarkable “phenotypic plasticity”, that is, many barnacles appear to be able to change the size and shape of their penises depending on the conditions of their local environment, such as water turbulence and proximity to other barnacles (Neufeld and Palmer 2008, Hoch 2008).

Relaxed penis form of similar-sized barnacles

Relaxed penis form of similar-sized barnacles (Balanus glandula).Left, barnacle from protected shore; right, barnacle from wave-exposed coast.

Alas, this type of genitalic plasticity seems to be absent in humans, as I know many people who are distant from other mates and live quiet, “still water” lives, so to speak, yet it never seems to affect their penis size, despite frequent measurements. So all we can do is tip our hats to the barnacle and admire, with strict scientific detachment, their extraordinary penises.


Hoch, J. M. 2008. Variation in penis morphology and mating ability in the acorn barnacle, Semibalanus balanoides. J. Exp. Mar. Biol. Ecol., 359 (2): 126-130.

Neufeld, C. J.  and Palmer, R. A. 2008. Precisely proportioned: intertidal barnacles alter penis form to suit coastal wave action. Proc. R. Soc., 275 (1638): 1081-1087.


2 Comments on “The Bountiful Burrowing Barnacle”

  1. veryinterestedindeed says:

    Riveting work Doctors! You have clearly been doing your research. We now know the longest penis in relation to body size, what specimen pray tell is the girthiest?

  2. Dr. Richard Cox, PhD. says:

    Thank you, veryinterestedindeed, I am flattered and I’m sure Mr. Dickworth is as well.

    Girth measurement poses significant problems for the animal penis specialist. To determine a relative measure of girth one must determine the girth of the animal as a whole, and therein lies the trouble. It is easy to see how girth should be measured on the penis because it typically has a natural directionality which determines the so-called “axis of girth”, but the body of the animal may not have such a well-defined axis. Indeed, the girth measurement dilemma is widely regarded as one of the most important open problems in the field. When time allows we will have to give the topic the attention it deserves in a full posting.

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