Tanystylum bealensis male, ventral view, showing eggs and instar 1 (protonymphon) on ovigerous legs. in. 1, instar 1 (protonymphon); pa, palp; pr, proboscis; 1, first walking leg; 2, second walking leg; 3, third walking leg; 4, fourth walking leg. 

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Context 1

... sexual dimorphism ( Fig. 2I,J). The female abdomen (Fig. 2I) extends to the base of the second coxa of the fourth leg, while in most males the abdomen (Fig. 2J) is typically shorter, extending only over the first coxa of the fourth legs. In a few males the abdomen is longer than the female abdomen. In both sexes the adult abdomen bears an increased number of spines. The abdomen tip now has five or six spines, instead of two, as in the previous instar, and the base of the abdomen, where it connects with the trunk, has five or six spines. The proboscis (Fig. 2I,J) has increased in size to 3/4 the trunk length and the lateral processes (located at the base of each walking leg), previously spineless, have added dorsodistal spines (Fig. 2I,J). The first three pairs of lateral processes have two or three spines and the fourth pair has one to three spines with one spine the most common state. The female gonopores are located on the ventral side of the second coxae of all four pairs of walking legs. The male gonopores are in the same location, but are only found on the last three pairs of walking legs. The cement gland (male only) is a single dorsodistal pore located directly behind the group of dorsal spines at the distal end of the femur on all four pairs of walking legs. Egg masses, each contain- ing ϳ 40 eggs per cluster (Fig. 1) were located near the base of the adult male’s ovigers. Egg diameter ϭ The postembryonic development of Tanystylum bealensis is characterized by a protonymphon larva with three pairs of appendages, the chelifores and two pairs of larval limbs, that are lost or modified over several molts. The larvae also only remain on the adult male for the first instar, after which they are found on a hydroid (in this case, Plumularia setacea ). Finally, the larvae acquire the paired walking leg limb buds individually over a period of several molts (see below). This mode of postembryonic development (Typical Protonymphon) is shared by all species for which larvae are known in the genus Tanystylum , in the closely related genus Achelia , and in the more distantly related Pycnogonum (Morgan, 1891; Okuda, 1940; Behrens, 1984; Tomaschko et al., 1997; Wilhelm et al., 1997; Bain, 2003a). Walking leg formation in Tanystylum bealensis (Table 1) follows a specific pattern: limb buds first appear in the second instar; at the fourth instar stage the limb buds have been modified into walking legs with six segments: coxae 1–3, femur, tibia, propodus; in the fifth instar the six-segmented walking legs have become eight-segmented in the following manner: the femur splits into two, forming a new femur and an additional tibia (Morgan, 1891; Bain, 2003a) and the propodus divides at its proximal end, forming the tarsus (Bain, 2003a). This same pattern has been described by Morgan (1891) for the closely related species Tanystylum orbiculare , but it differs radically from the mode of development of leg segmentation described by Okuda (1940) for the more distantly related Achelia alaskensis (Table 1). Postembryonic development in Tanystylum bealensis differs from that of T. orbiculare in the following respects: T. bealensis undergoes a total of nine molts from egg to adult, while T. orbiculare appears to have at least 10 (Morgan, 1891; Bain, 2003a); in T. bealensis , the first instar remains on the adult male for a short time after hatching, while all of the first instars of T. orbiculare were found living on a bed of hydroids; there is an extra instar in T. orbiculare (Stage 2 in Morgan, 1891), found between the first and second instars of T. bealensis (see below); instars three through seven are very similar in these two species, but more differences appear in the eighth instar: both the chelifores (one segment in T. bealensis , two segments in T. orbiculare ) and the palps (five segments in T. bealensis and six in T. orbiculare ) acquire their adult number of segments in this stage and the ovigers, while very similar in overall appearance in both species, are six-segmented in T. bealensis and appear to be unsegmented in T. orbiculare ; adult T. bealensis emerge from the molt at the end of the eighth instar, but T. orbiculare appears to have at least one more instar before the adult stage (Morgan, ...

Context 2

... DEVELOPMENT OF T. BEALENSIS 309 outside of a nudibranch or a polychaete and then was measured externally on eggs attached to male ovigers. All undergoes a modified series of molts in which all the measurements are Ϯ Standard Error. Specimens for scanning electron microscopy (SEM) were run through a chemical dehy- adult appendages appear at once as limb buds and dration series, first in a graded ethanol series (15%, 25%, 40%, then, at each subsequent molt, add more segments 50%, 70%, 80%, 95%, 100%) and then through a hexamethyldi- until the adult number is reached (Ohshima, 1933, silzane (HMDS) graded series (1:1 of 100% ethanol and HMDS, 1937; Salazar-Vallejo and Stock, 1987; Bain, 2003a). then 100% HMDS). They were then sputter-coated with gold The Attaching Larva, found only in the Families using an SEM coating system (SEM PREP 2), and viewed with a Hitachi S450 scanning EM at 10 –20 kV. All drawings were made Nymphonidae and Callipallenidae, hatches as a from SEM images. yolk-filled, nonfeeding larva that attaches itself to the male’s ovigers and remains there for several additional molts (Sekiguchi et al., 1971; Nakamura, RESULTS 1981; Bain, 2003a,b). First Instar or Protonymphon There are very few published descriptions of complete pycnogonid life cycles (see summary in Bain, The first instar (n ϭ 16) (Figs. 1, 2A, 3A, 4A, 5A) 2003a), and of these, only two, for Propallene longi- was always found either on the male’s ovigers or on ceps (Attaching Larva: Nakamura, 1981) and Pycno- the remaining unhatched eggs (Fig. 1). It has a gonum litorale (very highly modified Typical Prot- roughly oval-shaped trunk (Fig. 2A) (trunk length: onymphon: Behrens, 1984; Tomaschko et al., 1997; 0.167 Ϯ 0.004 mm), a pair of large, two-segmented Wilhelm et al., 1997), are known in sufficient detail chelifores on a one-segmented chelifore scape (Fig. because these two species were lab-reared, with all 3A), and two pairs of larval appendages (Figs. 4A, stages collected and described. The two most com- 5A). The ventral surface of the chelifore scape bears plete series of postembryonic developmental stages a single, stout spine (Figs. 2A, 3A), and the two pairs from field collections are for Tanystylum orbiculare , of larval appendages (the future palps and ovigers) described by Morgan (1891) and Achelia alaskensis , are each two-segmented, with a long terminal spine described by Okuda (1940), and both of these have (Figs. 4A, 5A). The first segment of each larval ap- the Typical Protonymphon type of development. pendage bears a long lateral spine that extends ap- Morgan (1891) described 10 stages (protonymphon- proximately half the length of the second segment. adult) that had been collected from hydroids, but The proboscis (Fig. 2A) is nearly as long as the was uncertain whether or not he had collected all chelifore scape at this stage and the mouth is tri- life history stages. Okuda (1940) described eight meric, formed by three bare lips at the tip of the developmental stages that had been found on Poly- proboscis. orchis karafutoensis medusae. The first stage was an advanced protonymphon (in which the posterior region had begun to elongate) and the last stage was a Second Instar juvenile with a seven-segmented oviger. The second instar (n ϭ 70) (Figs. 2B, 3B, 4B, 5B) During the summer of 1999, a large population of (and all subsequent instars) was found on the hy- Tanystylum bealensis specimens (Typical Protonym- droid Plumularia setacea . The body had nearly dou- phon), complete with all life history stages, was col- bled in size (trunk length: 0.294 Ϯ 0.003 mm) from lected in Barkley Sound, British Columbia, Canada. the previous instar and the posterior region had The large number of specimens collected (n ϭ 2,397) begun to elongate (Fig. 2B). The chelifores (Fig. 3B) contained many replicates of each of the life history were unchanged except that one dorsal and one lat- stages of this species, thus allowing for the first eral spine appeared on the scape. The two pairs of complete description of all life stages from a natural larval appendages (Figs. 4B, 5B) had grown slightly population. larger, while the long spine located on the first segment of each appendage appeared to have dimin- MATERIALS AND METHODS ished in size. The proboscis at this stage is two- thirds the length of the chelifore scape (Fig. ...