The Body Appendages of Balanus decorus.
[Read before the Canterbury Branch, June 4, 1941; received by the Editor, December 10, 1941; issued separately, June, 1942.]
Although as early as 1851 Darwin realised the importance of the body appendages in the classification of the sessile barnacles, little work was done along these lines until 1916, when Pilsbry published his monograph. This masterly treatment of the genus Balanus and its detailed descriptions of the appendages of the various species, has paved the way to a fuller understanding of this difficult problem. Broch (1922, p. 314), examining the material collected by the Dr. Th. Mortensen Pacific Expedition of 1914–16, stated that he was unable to point out the similarity between the body appendages of Balanus campbelli and Balanus decorus as the details of the latter species had not been published. Darwin, who first described this latter species in 1854, was acquainted only with the hard parts. So it is desirable that a description of the body appendages should be made.
Description of the body appendages of Balanus decorus:
Cirrus I. The anterior ramus is longer than the posterior by about four segments. Some of the basal members are slightly protuberant. The spines, which are long and numerous, are arranged more or less in three whorls, those of the middle whorl being longest while the anterior members of each whorl tending to be longer than the posterior. About 22 segments. The posterior ramus is composed of about eighteen segments, with large protuberances. This ramus, unlike the anterior, is similar to the rami of cirrus II. The pedicel is moderately long and broad and clothed on its posterior edge with dense spines. There are a few shorter and stouter spines on the anterior edge.
Cirrus II. The anterior ramus is longer than the posterior by about four segments, otherwise the two rami are similar. The segments are all very protuberant, this structure bearing upwards of 25 long spines. A row of long spines occupies the anterior half of the distal border of each segment. Postero-distally there is a tuft of about twenty spines of medium length. Between these two sets of spines the distal border is completed with a row of short erect spinules. The pedicel is short and broad, bearing tufts of spines in its anterior and posterior distal angles.
Cirrus III. The anterior ramus is longer than the posterior by about four segments. Both rami are only from one quarter to one third longer than the corresponding rami of cirrus II. The segments of the anterior ramus are somewhat protuberant and three times as
broad as they are long. Anteriorly there are six long pairs of spines. Arranged posteriorly along the distal border is a short row of small spines. Completing the distal border is a row of small erect spinules. The posterior ramus is composed of about fifteen segments which are similar to those of the anterior ramus except that the proximal pair of spines is usually short and downwardly directed as in cirri IV, V, and VI. The pedicel is relatively long and broad and clothed in spines.
Cirrus IV. The anterior and posterior rami are equal in length and composed of about thirty-four segments, each bearing anteriorly three pairs of long spines with a fourth (proximal) pair of short downwardly-directed spines. There is a group of eight small spines on the posterior distal angle. The distal border bears a row of small erect spinules. The pedicel is long and narrower than the third. Its anterior edge is spinose and a small patch of spines occurs on the posterior distal angle.
Cirrus V and Cirrus VI. The rami are nearly equal in length and composed of about forty segments, each bearing anteriorly three pairs of long and a fourth (proximal) pair of short downwardly-directed spines, as in cirrus IV. There are usually only four posterior spines. The distal border bears a row of short erect spinules. Occasionally the segments are somewhat protuberant and sometimes short tufts of spines are apparent between the pairs of spines of the sixth cirrus as occurs in Balanus tintinnabulum. The pedicels resemble the fourth except that the posterior distal tuft of spines is usually absent.
Penis. The penis, which is about one and a half times as long as cirrus VI, is distinctly annulated, bearing a few fine hairs.
Labrum. The notch is deep and narrow, the edges straight and bordered with fine hairs which continue right into the notch. The inner lateral surfaces of the labrum are covered with fine hairs.
Mandible. There are two large upper teeth, followed by a third, less distinct, and a fourth nearly fused with the basal angle. The lower angle bears a number of minute denticles. A belt of fine hairs lies parallel to the cutting edge.
First Maxilla. This is somewhat variable, but in its simplest form the cutting edge is composed of a long straight “step” followed by a large prominence. The first portion bears one large and three or four smaller pairs of spines, while the prominence bears three to six long single spines. A tuft of finer spines or hairs occurs on the basal angle, while a belt of the same material lies parallel to the cutting edge.
Second Maxilla. Long and sub-triangular in section. The two meet above the labrum and at the point of apposition there is on each maxilla a small depression beset with fine hairs. The two upper surfaces are clothed with long hairs.
Palpus. Relatively large, globose, and densely covered with hairs.
Distribution. Balanus decorus is a common New Zealand barnacle occurring from low water mark to a depth of several fathoms. Two specimens were taken by the New Zealand Government Trawling
Expedition of 1907 from a depth of between thirty and forty fathoms. This is essentially a marine animal, but can tolerate slightly brackish water, as evidenced by a fairly large colony in the estuary of the Avon and Heathcote Rivers near Christchurch, where the salinity varies between 28·7 and 33·3 parts per thousand. Chilton (1911, p. 317) observes Balanus decorus to occur on the carapace of the crab Paramithrax longicornis Thompson and states that it seems to be particularly associated with this animal. The writer has identified five young specimens from the common New Zealand crayfish, Jasus edwardsii Hutton, but as yet no larger specimens have been taken from this source. Balanus decorus is characteristic of and confined to the New Zealand Region, being common in New Zealand waters and extending west to South Australia (Hutton, 1879, p. 328) to the Chatham Islands in the east and southwards to the Auckland Islands in the Subantarctic group. The writer has identified this species from material from the Kermadec Islands. Withers (1924, p. 26) gives the range in time of this species as Hutchinsonian (Miocene) to the present day in New Zealand. He also reports its presence in the Australian Miocene. This species is often very abundant in more recent New Zealand deposits, and the writer has observed in a pebbly limestone (Pliocene) of Wakarara, Hawke's Bay, thick bands composed almost exclusively of this species and its close ally Balanus tubulatus Withers.
Subgenus Megabalanus Hoek.
Balani having parities, basis and radii permeated by pores.
A description of the following species, containing an account of the body appendages has already been published: Balanus tintinnabulum (Linnaeus) and Balanus agricola Pilsbry by Pilsbry (1916, pp. 56 and 73) and Balanus campbelli Fihol by Broch (1922, p. 313). A comparison of Balanus decorus with the type Balanus tintinnabulum is set out in tabular form below (Table I). Of the short rami of cirrus III, Pilsbry (1916, p. 52) says: “In B. tintinnabulum (various varieties) and B. agricola the cirri are rather characteristic. The rami of the third pair are exceptionally short, like those of the second pair, the pedicel rather long and the first segment extremely broad. Whether this particularly characterises the other species of the Megabalanus I do not know.” Since then Broch (1922, p. 313) has reported this feature to occur in B. campbelli. He has also pointed out a close relationship between this species and B. decorus, but states that a comparison of the body is not yet possible as details of B. decorus are missing.
Comparison of B. decorus, B. campbelli and B. tintinnabulum.
The Cirri. In general form the cirri of all three species are similar, but the relative lengths of the rami show a significant variation. Although cirrus III in all cases shows a greater similarity to cirrus II than cirrus IV, here also differences in relative size are apparent. The peni likewise show variation. These differences are summarised below in tabular form (Table II). Unfortunately a specimen of B. campbelli was not available, and the lengths given
for this species are only approximate, being based on Broch's description. Differences in structure occur in segments of cirrus VI. In B. campbelli the fourth (proximal) pair of anterior spines is longer than in the other two species. There is a variation in the arrangement of the spinules edging the distal borders. In B. campbelli the row is arched; in the lower segments, double; or the spinules may be arranged in groups near the bases of the upper spines. In B. decorus the row is straight and rarely double; no case was observed where the spinules were crowded together. In B. tintinnabulum the row is straight and apparently always single. In this species there is always a tuft of hair between the paired spines of the anterior edge Although this has been observed in a few specimens of B. decorus, it is not reported in the case of B. campbelli.
Labrum. The notch is deep in all three species, and the edge, which is fairly straight throughout, is most rounded in B. campbelli and straightest in B. tintinnabulum, with B. decorus intermediate between these forms. In the two latter species the edge bears fine hairs. In B. campbelli, Broch (p. 312) reports: “Close by the notch the margin has on each side three low transverse ridges, each with a rudimentary denticle at its top.” Pilsbry observes small denticles sometimes to occur in B. tintinnabulum, but no trace of this structure was observed in any of the B. decorus material examined.
Mandible. The mandibles of B. decorus and B. campbelli are very similar, differing from B. tintinnabulum by the possession of only two strong distinct teeth instead of three, by having a belt of hairs lying parallel to the cutting edge, and by having a denticulate basal angle which seems to be entirely absent in the latter species.
Maxilla. In B. tintinnabulum the edge is straight, while in the other two species there is a large basal prominence. This bears large spines which in the case of B. campbelli are as large as the first pair. A belt of fine hairs lies parallel to the spine-bearing edge. The chief difference between these two species seems to be only the larger spines on B. campbelli.
It is apparent from the above that the cirri of the three species are very similar, with B. decorus intermediate between the other two species, but more closely resembling B. tintinnabulum. The mouth appendages of B. decorus and B. campbelli are very similar, the mandible and maxilla showing no great affinity to B. tintinnabulum.
Note. Material was collected from many parts of New Zealand, but the chief source of supply was a large colony in the estuary of the Avon and Heathcote Rivers near Christchurch. Material was examined from Lyttelton Harbour; Akaroa Harbour; washed up on wood from Leithfield Beach, North Canterbury; Kaikoura; St. Clair, Dunedin; Barrytown, Westland; Point Elizabeth, Westland; Raglan, Waikato; Auckland Harbour; also some specimens presumed to have come from the Chatham Islands, but their origin was not definite. The largest specimen was 42 mm. basal diameter and 36 mm. high. The average size was about 20 mm. basal diameter. A set of typical appendages were mounted in balsam and placed in the Canterbury Museum.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Balanus decorus.||Balanus tintinnabulum.|
|Anterior Ramus.||Segments not protuberant.||Segments not protuberant.|
|Posterior Ramus||Segments protuberant.||Segments protuberant.|
|Anterior Ramus and Posterior Ramus||Both rami shorter than corresponding rami in cirrus I. All segments protuberant.||Both rami shorter than corresponding rami in cirrus I. All segments protuberant.|
|Anterior Ramus and Posterior Ramus||Both rami only slightly longer than corresponding rami of cirrus II.||Both rami about one-third longer than corresponding rami of cirrus II.|
|Segments slightly protuberant; bordered distally with a row of small erect spinules.||Segments slightly protuberant; bordered distally with a row of small erect spinules.|
|Cirrus IV and Cirrus V and Cirrus VI.||Cirri nearly similar composed of a large number of short segments each bearing three long pairs of spines and a smaller pair. A regular row of erect spinules borders each segment distally. There are usually no tufts of small spines on the anterior edge between the spines of the pairs.||Cirri nearly similar composed of a large number of short segments each bearing three pairs of long spines and a smaller pair. A regular row of small erect spinules borders each segment distally. There are tufts of small spines on the anterior edge between the spines of the pairs.|
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Ratio of Anterior to Posterior Ramus.||B. decorus.||B. campbelli.||B. tintinnabulum.|
|Cirrus I||1.3 : 1||2 : 1||Nearly 1 : 1|
|Cirrus II||1 : 1.2||Nearly 1 : 1||1 : 1.2|
|Cirrus III||1 : 1.2||Nearly 1 : 1||1 : 1.2|
|Cirrus V||1 : 1||1 : 1||1 : 1|
|Cirrus II/III||1.2 : 1||Less than 1 : 1||1.3 : 1|
|Penis/Cirrus VI||1.5 : 1||1 : 2||2 : 1|
Darwin, C., 1854. Monograph of the Cirripedia. The Balanidae and Verrucidae, Ray Society.
Hutton, F. W., 1879. A List of the Cirripedes in the Otago Museum, Transactions of the New Zealand Institute, vol. 11, p. 328.
Chilton, C., 1909. The Crustacea, The Subantarctic Islands of New Zealand.
—– 1911. Scientific Results of the New Zealand Government Trawling Expedition, 1907, Crustacea, Records of the Canterbury Museum, vol. 50.
Gruvel, A., 1912. Monographie des Cirrhipèdes. Paris.
Chapman, 1914. Proceedings of the Royal Society of Victoria, vol. 27.
Pilsbry, H., 1916. The Sessile Barnacles (Cirripedia) contained in the Collections of the U.S. National Museum; including a Monograph of the American Species, Smithsonian Institution, U.S. National Museum Bulletin 93.
Jennings, L., 1917. Revision of the Cirripedia of New Zealand, Transactions of the New Zealand Institute, vol. 50.
Broch, H., 1922. Studies on Pacific Cirripedes, Papers from the Dr. Th. Mortensen Pacific Expedition, 1914–16.
Withers, T., 1924. The Fossil Cirripedes of New Zealand, N.Z. Geological Survey, Palaeontological Bulletin 10.