Description of Female, Male and Larval Forms of a Tiny
Stalked Barnacle, Ibla idiotica n.sp.

By Elizabeth J. Batham,
University of Otago.

of either of these pedunculates have in no case been found carrying Ibla. But where a number of specimens of S. villosum are growing in a close colony on the dark, wet, underhung surface of a large rock at low tide, I. idiotica not infrequently occurs. It is usually on the villous peduncle of Scalpellum, sometimes on the valves, and occasionally on the sessile barnacle Tetraclita purpurascens Darwin when this latter was attached to S. villosum. To what depth this species of Ibla occurs has not been ascertained. Its upper limit is not much above low tide-mark, continuous moisture externally being probably essential.

Ibla idiotica has been found in the vicinity of Dunedin on the Karitane Peninsula, at Long Beach, and at Bruce's Rocks. Limited examining of S. villosum colonies around Wellington and Stewart Island failed to reveal it. The writer has not searched for it in other parts of New Zealand.

Female.
(Plate 26.)

The female has a thick chitinous covering which lacks calcification. Its colour is a uniform, translucent, dull yellow all over. Live individuals are very active, the muscular peduncle of a young specimen being able to swing the capitulum rapidly through a rightangle. When placed in a dish of sea-water these barnacles will usually extrude and withdraw their cirri at frequent intervals—a contrast to the sluggish behaviour shown by the allied Pollicipes spinosus and Scalpellum villosum. Young females are fairly transparent, and one older one stayed alive for several hours after removal of the body from the mantle. In such specimens, numerous rapid peristaltic waves can be seen passing along the oesophagus. The rectum also frequently contracts, and the minute caudal appendages show considerable movement.

As in other species of Ibla, the only valves present are a pair of scuta and a pair of terga (Figs. 1, 2, 8 and 9). The tips of the terga characteristically are turned in towards the cirri, and under high magnification show a honeycomb-like structure unlike the rest of the valve. But occasionally they are worn down almost to the level of the scuta, giving such individuals an atypical appearance. The peduncle is usually approximately as wide as it is long. But sometimes it may become considerably elongated.

The total length of mature females is from 2 to 3.5 mms. In young specimens the greatest width is at the base of the scutum (Fig. 1). But older ones, with many young in their mantle-cavities, may show a considerably expanded peduncle (Fig. 2). In such specimens the mantle cavity may be very extensive, dipping deeply into the peduncle. In fact, perhaps the most remarkable feature of the females is their astounding fecundity (Fig. 10).

This species, like others of the genus, shows the first pair of cirri placed close to the trophi, their bases being far removed from the remaining legs and their rami short (Fig. 10). The number of segments per ramus for the cirri varies a little among different individuals. An example is as follows: Cirrus, 1; outer ramus, 4;

inner, 4; C.II, 6, 4; C.III, 7, 7; C.IV, 7, 7; CV, 7, 7; C.VI, 8, 8. The caudal appendanges are extremely minute (Fig. 7). Each is unsegmented and carries two setae.

The internal anatomy shows the peculiar feature whereby Ibla stands apart from other adult barnacles—namely, the oesophagus running right round the outside of the adductor muscle anteriorly before joining the stomach (Fig. 6). At the point of junction (and at both sides of the brain) lie a pair of digestive glands. Each is a single hollow bag, lined with large cubical epithelium cells and containing a grey secretion. The ventral nerve-cord is very stout relative to the size of the body, in this respect resembling that of extremely young specimens of other pedunculates. The large sub-oesophageal ganglion is succeeded by 4 others. The adductor muscle is stout, its fibres unstriated. Cirri, trophi and body-wall are supplied with various muscles whose detailed arrangement and nerve-supply were not worked out.

The peduncle is supplied with numerous longitudinal muscle fibres (Fig. 6), which give it externally a faintly lined appearance. The ovaries are relatively compact bodies in its upper half. The cement glands in the connective tissue below them are not clearly defined, but their two ducts, running down the antennules, could readily be seen. In sections stained with Mallory's the light-brown granular cement at the base contrasted sharply with the thick blue-stained chitinous covering of the peduncle.

Contents of Mantle Cavity of Female.

Most of the females carry in their mantle-cavities a diverse assortment of individuals. Not only are males present, of all shapes and appearances. Embryos, nauplii and cypris larvae may also occur in varying numbers.

To obtain a clearer picture of the course of events, 50 females were each measured with an eyepiece micrometer and then opened under a microscope. Their sizes and contents are shown in Table I.

Assuming that the length roughly corresonds to the age of the female, the following conclusions may be drawn:—

The youngest females (less than 1.6 mms. long) carry no males or young.

The slightly larger forms show various stages in the develoment of males.

Females less than 2 mms. long almost invariably lack other young, whereas those over 2.3 mms. almost invariably contain them.

As the females grow in size both the number of males and number of young tend to increase.

The tendency of the crosses in the 6 columns of males to move from the upper left to lower right, when considered together with anatomical evidence, suggests that the course of development of the male is: cypris; pre-male enclosed in cypris carapace; pre-male (i.e. form with cypris antennules still protruding); opaque male; transparent male with well-developed testes; dead male.

The 50 females just considered were all collected from the same locality in February. Other females have been collected in smaller

numbers in the months of January, March, June, August and November. In each instance, the larger females have contained both males and young. Hence, if any cessation occurs in breeding, it must be for a relatively brief period of the year.

A number of other females were dissected without being measured. The maximum number of dwarf males found in a female was six. Amongst fairly small specimens, several were found containing a single cypris, a pre-male in cypris carapace, or a pre-male. Only one, however, was found containing a nauplius but no male. Its length was 2.27 mms. And altogether but a single specimen has been opened with embryos and no male—the specimen in Table I of 1.94 mms. in length. Both of these specimens are above the minimum size at which a fertile male may be present. The significance of these points will be discussed subsequently.

Male.

The fully-developed male is shown in Fig. 25. It is about 0.4 mms. long, transparent, and nearly motionless; muscle fibres being almost absent.

The most conspicuous organs are the two large testes, in which the live sperms may be seen by transparency actively swirling around. Each testis narrows posteriorly into a short duct, the two soon fusing to expand into a median seminal vesicle. This latter varies considerably in size, being sometimes so packed with sperms that it is as large as a testis (Fig. 24). The seminal vesicle opens ventrally near the posterior end by a median longitudinal slit. No penis is present. Slight twitching movements have been observed in this region, which possibly assist in the exit of sperms.

The external appearance of the male is highly bizarre, largely owing to the extreme development of the two spinous horns. These structures are not, as might be expected, embedded in the lining of the female's mantle cavity, the male being quite free. So to what extent they assist in holding it in position is not clear. Posteriorly the body terminates in two protrusions, each carrying one or two setae. Just anteriorly to them lie a couple of protuberances, each with from one to three setae. No trace of mouthparts has been discerned.

Deeply embedded in the body between the testes lie two chitinous structures, which, when dissected out, prove to be typical cypris antennules, complete with setae. They are now empty of muscles, however, and communicate with the exterior only by a minute pore on the median anterior margin.

The alimentary canal is present in a reduced form. An oesophagus is lacking. A small, globular stomach whose lumen contains amorphous contents is placed between the posterior margins of the testes, immediately behind the antennules. From it a slender intestine passes down posteriorly. The presence of a lumen in this was doubtful, and serial sections indicate that it ends blindly with no arms.

A black median eye is a conspicuous feature. From behind it a minute solid “brain” and nerve-cord runs backwards for a short

of showing a few sperms in the middle of a cell mass, now are largely filled with spermatozoa. Simultaneously the horns are elongating and becoming spinous. Thus do we arrive at the fully-developed male, such as was initially described.

Presumably the only supply of food for the male is that already stored in the body. The regular appearance of contents in the stomach suggests that it may have some function in the process of its absorption and utilisation. Finally, however, this supply is exhausted. Then the testes and seminal vesicle become empty of sperms, and the remaining chitinous shell crumples up and dies. That the male has a shorter life than the female is obvious, but just how much shorter it is not known.

Embryos.

Although the adult females are only some 2–3 mms. long, their embryos, of about 1/3 mm. in diameter, are larger than those of most cirripedes (Groom, 1894, p. 130). They are unpigmented, whitish, and are not bound together into the usual pair of ovarian lamellae. There are indications also that different embryos in one female were not all at exactly the same stage of development. Conceivably, size limitations would prevent a female, herself 3 mms. long, from forming in her ovary up to 20 eggs at once.

The embryos are heavily yolked, and by the time of hatching show but little tissue differentiation. The large antennules are directed backwards (Fig. 11). Behind them on the ventral surface is a protruding cell-mass, which subsequently develops into the cypris limbs. Antennae and mandibles are represented, if at all, by slight swellings.

Nauplius and Cypris.

The nauplii, which are retained in the mantle-cavity, are highly modified. The large biramous antennae and mandibles, associated with the first free-swimming phase of most cirripedes, are absent as such. The carapace, instead of being flattened and triangular, is hemispherical. The cypris legs are precociously developed, originating in the late embryo as a solid mass of cells which in the nauplius splits into six pairs of separate biramous limbs (Figs. 11–13, 15).

The nauplii when alive are whitish, opaque, and almost motionless, nothing more than a slight twitching of parts of the ventral surface having been observed. This is explained by an examination of their internal anatomy, for muscle cells do not become fully differentiated until the cypris phase is reached. The main mass of the body is filled by “yolk” in the form of opaque, oily-looking globules. Dorsally and laterally it is bounded for the most part by a single thin layer of ectoderm cells; though a pair of multicellular bands are present which subsequently become muscles running to the antennules. Ventrally, and running into the appendages, is a dense zone of tissue, later to become the muscles and nerve-cord of the cypris.

Embedded in the yolk-mass is a thin-walled globular stomach with amorphous contents. A narrow oesophagus runs down to the

Discussion.

This cirripede shows the various features that characterise the genus Ibla. In the large form the only valves present are paired scuta and terga. The oesophagus runs round anteriorly outside the adductor muscle. The first pair of cirri is reduced and placed far from the second pair. The mandibles show the typical dentition. Chitinous spines occur on the peduncle, into which the mantle cavity extends. Males are present. The nauplii do not swim freely in the sea. In each of these features, I. idiotica agrees with the other members of the genus.

From previously described species, however, several points distinguish it. In the size of the large form and the structure of its male, I. idiotica stands sharply distinct from I. cumingii and the hermaphrodite I. quadrivalvis. It is of much the same dimensions as Ibla segmentata (= I. pygmaea), which has been dredged not far from New Zealand (Broch, 1921, p. 262). This latter species differs from I. idiotica, however, in possessing a penis, in its long, jointed caudal appendages, and in the unequal rami of the first pair of cirri.

The question arises, how is the sex determined in this species? And where does a female's first male come from?

A hypothesis which suggested itself was that the first egg produced by a young female developed parthenogenetically in her mantle-cavity into the first male. This would be able to fertilise subsequent eggs, but any that missed fertilisation would develop into later males. If such a hypothesis were correct, one would expect to find some of the smallest females (about 1.5 to 1.6 mms. long) with single embryos or single nauplii in their mantle-cavities. In actuality, however, whereas a single eypris was found at least five times in a small female, no instance was discovered of embryos or nauplii in females less than 1.94 mms. long in the absence of a male (Table I). So it seems that a more feasible hypothesis is that a cypris from another adult crawls into the mantle cavity of a young female and there metamorphoses into a male. The two specimens found with young but no males at first seem contradictory to this idea. But when one considers the lengths of these two (1.94 and 2.27 mms.) in relation to others in Table I, it looks not improbable that they have already had males which have been lost.

That the male develops from a larva which entered the mantle-cavity from outside was postulated for Ibla cumingii by Darwin (1851, p. 202). It is also in agreement with the conclusions of Callan (1941, p. 258), in considering sex-determination in Scalpellum