Branchellion parkeri

source: victoria.ac.nz

A small branchiate leech, the largest specimen being 3.0 cm. in length, with the body consisting of an anterior sucker, followed by a neck surrounded posteriorly by a prepuce or collar formed from the anterior segment of the abdomen, which consists throughout the greater part of its length of gill-bearing segments, each third segment bearing a pulsatile vesicle at the base of the gill on either side excepting the last branchiate segment, and tapering to a broad peduncle which terminates in a posterior sucker richly clad with adhesive papillae on the ventral surface.
The anterior sucker is sub-circular, 2.7 mm. long by 2.4 mm. wide, strongly convex, with a smooth margin, bearing two transverse, band-like, black pigment

1. Morphology of anterior abdominal segments, neck, etc.

2. Morphology of posterior body segments.

3. Anterior abdominal segments in dorsal view showing the reduced "gill" evident in the living specimen.

4. Reproductive system from the left aspect.

AN., anus; AT., atrium; BR., gill; BR.L., reduced "gill"; OV, ovary; PR., proboscis; P.V., pulsatile vesicle; S.V., seminal vesicle; TE., testis; V.D., vas deferens.

patches, one on either side of the mid-dorsal line, and several irregular radiating areas marked by black pigment. The mouth is posterior, small, and the proboscis an elongate, conical, tapering structure. The ventral surface of the sucker is covered with small, soft, low papillae.

The neck is relatively short, 8.1 mm. in length and 1.9 mm. in width, compared with the abdomen, which is 21.7 mm. in length (including the sucker) and 3.0 mm. in width (excluding the gills). The annulation of the neck is obscured by many superficial and incomplete furrows, so that determination of the arrangement of this region can only be accomplished by dissection from the ventral surface. Anterior to the segment bearing the male genital pore, there are ten more or less prominent annuli preceded by three very small annuli. The ventral nerve cord commences in the sub-oesophageal ganglion, which is situated posterior to the first three small annuli, and expands into a ganglion in the third of the large annuli. Accordingly, this anterior region appears to consist of a trimeric segment iv, a bimeric segment v, and an undivided segment vi. The segment vii is trimeric; but ganglion viii is placed between the seventh and eighth large annuli, so that segment viii is clearly bimeric. The ganglia of ix, x, and xi, occupy undivided segments, and xii—normally concealed by the prepuce—is also undivided. Segment xiii consists of three annuli, of which a1 is reflected anteriorly, and, with an anterior portion of a2, forms the collar. The a2 of this segment carries the first pair of pulsatile vesicles but no fully developed gills, while the a3 is branchiate. In life, a minute lobe can be seen dorsal to each vesicle on xiii a2. These lobes are static, clearly visible in the living animal, but disappear by contraction on preservation.

Posterior to xiii there are nine segments, each with pulsatile vesicles and gills on a2, but only gills on a1 and a3. The annuli of these segments show many superficial furrows to the sides, and one, or occasionally two, of these furrows, passes across the annulus so that in many segments the dorsal side shows six or more annuli to the segment, but these do not show on the venter, where the anterior margin of each annulus forms a prominent free fold. Segment xxiii lacks pulsatile vesicles, is trimeric, and carries three pairs of gills. Five gill-less annuli follow xxiii. Accordingly, there are 31 pairs of gills and only ten pairs of pulsatile vesicles. This latter point has been confirmed in living specimens.
The terminal abdominal segments are undivided, although on xxiv and xxv furrows are present, but these are irregular and incomplete. The anus is situated between xxv and xxvi. Segment xxvii is the last individual segment, and is undivided. The ganglia in these last segments show an interesting morphological translation and compaction. The ganglion of xxiii is situated in the a2 of its proper segment; ganglion xxiv is in the middle of its segment; but the ganglion of xxv is situated at xxiv/xxv, and xxvi is actually situated in xxv, although its nerves pass posteriorly to the proper segment. Behind the ganglion xxvi the nerve cord is wide, and continues into an elongate ganglionic mass which gives off only five obvious pairs of nerves.

The posterior sucker is wider than the abdomen, moderately convex, and carries some 440 minute pedunculate suckers of equal size, arranged in approximately 20 radial rows, each of 14, extending from near the centre to the rim, and about 20 shorter rows, each of eight or so suckers, which are situated between the main rows, so that the ventral surface is more or less uniformly covered with these structures.

The reproductive system opens by the genital pores on xi and xii. The male system consists of five pairs of testes situated as usual at xviii/xvii, xvii/xvi, xvi/xv, xv/xiv, xiv/xiii. The testes of the one side open each by a short duct into a delicate longitudinal vas deferens, which extends to xiii, where it expands into a seminal vesicle. The vesicle is elongate, extends into xii, and the system continues anteriorly as a convoluted tube, which becomes lateral and then dorsal to the gut before entering the dorsal horn (ductus ejaculatorius?) of the atrium in x. This horn extends into ix, or even viii, and turns ventrally to expand and join with the atrium in ix/x. The common atrium extends beneath the ganglion of xi to reach the male pore situated at the posterior margin of this segment. The penis is large and bluntly conical. The female system commences in a pair of ovaries situated in xiii and above the ganglion of this segment. Each ovary has a short oviduct, the two oviducts joining beneath the nerve cord just anterior to ganglion xii to form a common duct which extends to the anterior margin of xii, where it opens at the female pore.

The alimentary canal is bound in by many strong dorso-ventral muscles, so that the posterior portion has not been clearly seen in dissection.

There is very little in the literature describing live *Branchellion. B. parkeri* was kept alive for a week without difficulty. It is a leech in which both the body and gills are colourless in life. Although many chromatophores are present on the body generally, these are not arranged in any evident pattern, but the transverse bars of black pigment across the posterior portion of the anterior sucker are very evident. These may be light-sensitive patches, but I obtained no evidence of light-sensitivity by rapidly and strongly illuminating these patches.
*B. parkeri* seemed an inactive leech. It did not move freely in the dish, but stayed mostly in the one place. If roughly treated it would move with a peculiar but rapid and erratic movement unlike ordinary free-living leeches, and when in motion is most difficult to catch, for, starting with the posterior sucker attached,the body is straightened and flattened to the surface. The anterior sucker is attached and then the posterior sucker brought up most rapidly into position, not behind, as is usual, but to one side of the anterior sucker, so that the leech is turned somewhat across its first axis. The anterior sucker is then carried out at an angle anywhere up to 90 degrees from the former track, attached, and the posterior sucker again brought up to the side of the anterior sucker, so that, on next extending, the new track will parallel, sometimes in reverse, the original track. The movement is most rapid, erratic, and at first unpredictable. This seems to be a protective movement, for, when moving leisurely, a near-normal looping is used.
The gills are static—at least, they did not show independent movement when the leech was motionless, nor were they markedly moved by the contraction of the pulsatile vesicles. These specimens did not show the common undulating respiratory movement of the body as a whole. The pulsatile vesicles contract uniformly and rhythmically, those of the one side contracting simultaneously and the contraction of the two sides occurring alternately, so that the vesicles appear to be under the control of the lateral longitudinal vessels, which were not visible in the live specimen. Their rhythm is about twice that of the rate of contraction of the dorsal longitudinal vascular sinus. When the body is fully contracted, the pulsatile vesicles are dilated on both sides and are static for as much as a minute. An independent contraction may occur in one vesicle or another, followed by contraction of adjacent vesicles and a contraction of vesicles on the opposite side of the body. No median ventral sinus was detected.

*B. parkeri* is an interesting addition to the genus, for, unlike the other species which are but rarely known excepting as ectoparasites of skates and rays, this species is somewhat common on dogfish, and from the evidence of the one fish where the fin-base showed attachment scars and was eroded, it is clear the relationship can be a semi-permanent one.
The somatic arrangements are interesting. Segment vi is the first free segment described for *B torpedinis, B. orbinensis, B. borealis*, and in these vi, vii, viii, and ix are determined as trimeric, while x, xi, and xii vary according to the species. The region is not well determined in *B. australis*, but in *B. ravenelii*, iv is the first free segment and is bimeric, v and vi are trimeric, vii, viii, ix, and x are bimeric, and xi and xii are trimeric. There is a total of 22 annuli in the neck for *B. ravenelii*, 19 in *B. torpedinis*, 16 in both *B. orbinensis* and *B. borealis*, but only 15 annuli in the neck of *B. parkeri*. This low figure for *B. parkeri* can be correlated with the fact that the majority of the segments of this region are marked with furrows on the dorsal surface, so that from this aspect these segments are subdivided, and a total of 20 annuli can be counted anterior to xi, but these furrows are irregular, commonly fork on anterior somites, few reach from side to side, most are variable from specimen to specimen, and are rarely continuous on to the ventral aspect. Secondary furrows are constant on the dorsal surface of the abdominal annuli, so that the length of the abdomen seems to consist of segments having six annuli, but only the trimeric condition is present on the venter. Accordingly, these furrows have been disregarded in this account.

The arrangement of gills and pulsatile vesicles is noteworthy. The first pulsatile vesicles occur on xii a2 in all species, and are accompanied by gills in *B. torpedinis* and *B. orbinensis*, which each have 33 gills, but in the other species and in *B parkeri* the first gills are situated on the annulus following this vesicle. The presence of a small undivided cylindrical blunt process on xiii a2 dorsal to the vesicle in living *B. parkeri* indicates that the first gill of 33-gilled species is essentially present but incompletely developed in this species.

The unique possession of only ten pairs of pulsatile vesicles was fortunately noted from live material, and. while the last vesicles occur normally on xxiv a2 or xxiii in other species, in *B. parkeri* the last vesicles are definitely on xxii a2 and are followed by four pairs of gills with the last gills occurring on xxiii a3, although this segment lacks any trace externally of a vesicle in life or in death.

*Holotype*: In the Zoology Department Museum, Victoria University College.