Feeding Behavior of Balanus nubilus

Balanus nubilus (Arthropoda, Crustacea, Thecostraca, Cirripedia, Thoracica, Sessilia)

The giant barnacle, Balanus nubilus, is a sessile organism that attaches itself on a chosen substrate and remains there for the duration of the organism’s lifespan (Figure 1). They do not leave their shell, yet open it up for life functions such as: feeding, spawning, and respiration. They are suspension feeding animals that use their feet to extract food from the water column. My observations are based upon organisms with apertures or openings that are around 4cm in length. The giant barnacle has long and short cirripedia showing a clear distinction between the two (Figure 2). Under the short cirri are feeding appendages composed of maxillae and a mandible, which looks very sturdy in comparison to the cirri (Figure 3).

Figure 1. Balanus nubilus with open scutum opercular plates.

Figure 2. View of cirripedia, feeding appendages of Balanus nublius.

Figure 3. Dissected Balanus nubilus‘ feeding appendages, a view into the mouth.

When feeding, the giant barnacle first opens its operculum and then extends and uncurls its cirripedia into the water column (Figure 4). The cirri still retain a slight curve when fully extended into the water column. I found when orienting the water flow from the sea table in the test barnacles direction, verses turing the water flow off, the feeding rate changed. The giant barnacle extends its cirri more frequently when there is less water movement. This appears to increase the water flow around the cirri and helps the barnacle to feed more efficiently. When there is ample water flow, barnacles seem to keep their cirri extended into the water column for longer periods, therefore beating them less frequently. The cirripedia can be swiveled to either side once extended (Figure 5). This action seems to better orient the cirri and face into the current’s flow. Smaller barnacles species feed more rapidly than larger barnacle species. They frequently extend their cirri at a rate of more than once per second whereas larger barnacles beat their cirri at a slower rate.

Figure 4. Ventral view of Balanus nubilus feeding with cirripedia.

Figure 5. Ventral view of Balanus nubilus orienting cirri in the water column

Barnacles are likely to stop feeding and narrow the operculum if they sense a predator above them. They seem to most likely sense predators by an interruption of light and shadows being cast over them. An agitated barnacle is timid to readily feed again and may cautiously take many minutes to resume feeding activity.

If a barnacle is poked along the soft tissue under the edge of the operculum, it swivels its operculum around the aperture in order to hide the soft and vulnerable tissue from a predator (Figure 6). On large barnacles this sometimes means they expose a vulnerable area of soft tissue on the other side of their opercula. The swiveling movement seems very quick for such a large barnacle.

Figure 6. The soft tissue of Balanus nubilus exposed.

Bailey Navratil
University of Washington

Introvert Behavior in Phascolosoma agassizii (Sipuncula)

Figure 1: Phascolosoma agassizii with its introvert fully evaginated from the body cavity (coelom). At the most anterior end of Phascolosoma spp., a notch in the crown of red tentacles indicates the dorsal orientation (Kozloff, 1990).

One of the major features of the phylum Sipuncula, or the Peanut Worms, is the presence of an introvert extending from a round, vermiform trunk.  The introvert has two pairs of retractor muscles that connect the apical portion of the introvert (which bears the tentacles) to the inner trunk body wall. This allows the animal to retreat into the coelomic cavity or extend its introvert and nearly double its length. To extend its introvert, the retractor muscles relax; this allows the internal pressure within the coelom to force the introvert outwards. (Kozloff, 1990)

Figure 2: Diagram shows a pictorial representation of the internal anatomy of P. agassizii. Two pairs of large retractor muscles attach low within the trunk body wall and allow for retraction of the introvert. The dissection this image reflects is the result of cutting along the dorsal body wall.

The specimen I observed was collected July 9^th, 2013 at Argyle Beach on San Juan Island, WA. My specimen was found under a rock in a cobble slew connecting the Argyle Bay to the Argyle Lagoon. I observed this specimen in both its habitat and in a water table. My observations focused on the morphology and behavior of the introvert.

I have found there are three major methods in distinguishing trunk from introvert. The first and most obvious method is to observe the extent of invagination by the introvert. The second method in distinguishing the two features is by an abrupt narrowing of the body from trunk to body. Unfortunately, when the animal is relaxed, this difference in body width can be difficult observe. In the specimen I observed, Phasolosoma agassizii (Kozloff, 1974), the introvert could be distinguished from the trunk by the stripe pattern emerging and becoming prevalent as you moved anteriorly. A striping pattern was readily visible on the introvert, whereas the trunk was more or less spotted.

The behavior of the introvert is variable depending on the environment. When the animal is placed on a flat surface in a saltwater tank, it will evert and withdraw its introvert frequently in what superficially looks like a blind animal’s “sniffing” behavior. If given enough time, P. agassizii will move, using a series of rolls and tumbles assisted by the introvert, to a rock or sediment. It will then use its introvert to anchor itself underneath the rock, or in the sediment, and bury itself (anterior positioned down in the case of sediment burrowing). Often, I would find my specimen hiding underneath a rock. Similarly, P. agassizii in the field can be found under rocks anchored to the underlying rocks and sediments.

Figure 3: Image shows the discrepancy in length of the same specimen of P. agassizii when its introvert is near fully invaginated (left, 6 cm) and fully evaginated (right, 11 cm).

When the animal is underneath an object, it will remain there with no obvious effort to escape. Rather, it will evert its introvert completely. Moreover, despite it appearing to be in a rather uncomfortable situation, if one were to remove P. agassizii from its “pinned” state, it will ultimately roll, tumble, and pull its way back and resume being pinned with its introvert fully extended. Not only do these behaviors show the versatility of the introvert, they are also indicative of the durability of the body wall as the animal positions itself tightly under large stones.

Introverts are expressed in other animal phyla, so it appears to be a rather efficient organ. Morphologically, the introvert allows the animal to keep its trunk, which is full of essential viscera, stationary while the introvert may be extended and retracted as it searches for food in sediments. It also allows for rather unique forms of locomotion and a method to hide a large portion of its body from disturbance. Despite the basic morphology of the introvert, the behaviors are diverse and contribute to a very unique organism.

Michael Tassia

University of Washington

Citations:

Kozloff, Eugene N. “Phylum Sipuncula: Peanut Worms.” Invertebrates. Philadelphia: Saunders College Pub., 1990. 345-347. Print.

Kozloff, Eugene N., Linda H. Price, and Eugene N. Kozloff. “Phylum Sipuncula.” Marine Invertebrates of the Pacific Northwest. Seattle: University of Washington, 1987. 181. Print.